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CONVERSATIONS ON CHEMISTRY.
FIRST STEPS IN CHEMISTRY.
Part I.
General Chemistry.
BY
Prof. W. OSTWALD.
AUTHORIZED TRANSLATION
BY
ELIZABETH CATHERINE RAMSAY. i2mo, viii -|- 250 pages, 46 figures. Cloth, $1.50.
CONVERSATIONS ON CHEMISTRY
FIRST STEPS IN CHEMISTET
BY
W. OSTWALD
Professor of Chemistry in the University of Leipzig
ATTTHOmZED TRANSLATION
BY
ELIZABETH CATHERINE RAMSAY
Part I GENERAL CHEMISTRY
FIRST EDITION, CORRECTED THIRD THOUSAND
NEW YORK
JOHN WILEY & SONS
London: CHAPMAN & HALL, Limited
1911
Copyriglit, 1905,
BY
ELIZABETH CATHERINE RAMSAY. Entered at Stationers' Hall.
THF SCIENTIFIC PRESS
nOBERT ORUMMOND AND COMPANY
BROOKLYN, N. Y.
-. .vA^.*iA*.'W» c. ■ . i^-V.
AUTHOR'S PREFACE.
The causes which led me to write this work lie partly in the past, partly in the future. The former spring from the feeling of thankfulness with which I even now regard the "Schule der Chemie" of Stockhardt, whose memory still lingers among us. By a stroke of good fortune this excellent work was the first text-book of chemistry which was placed in my hands, and it influ- enced the whole of my subsequent activity in science. Owing to the carefully thought-out directness in repre- senting the facts to the pupil, the skill in selecting experi- ments suitable to the physical and mental powers of the beginner, I have never lost touch with experiment, although I have been chiefly occupied with general questions of science. The request of the publishers, who used to issue this work, that I should write a modern Stockhardt, was both an honour and an opportunity of paying off an old debt of thankfulness.
So much for the past.
As regards the future, chemistry has undergone during the past century an enormous development, in which Ger- many has played an important part. Chemical science in Germany has been furthered by the work of thousands of diHgent hands and greatly aided by educational insti- tutions which have become a pattern for the whole world, which have brought about a constant interchange between
IV AUTHOR'S PREFACE.
science and its applications, and which have given an uninterrupted proof of a continued heahhy existence. It was almost entirely organic chemistry which developed in the direction of the discovery of new bodies and their systematic arrangement ; and even to this day, by far the majority of young chemists, after hurrying through a short course of analysis, are trained in these methods.
But hasty progress has its dangers, and it is the duty of every man who tries to look into the future to give a timely word of warning; for inorganic chemistry was a science before organic chemistry was thought of; more- over, the processes of inorganic chemistry form the basis of chemical technology, on which that of organic com- pounds is a superstructure.
The cry was first raised in manufacturing circles that the young chemist trained exclusively in organic chem- istry was unfit to cope with the solution of general prob- lems; with that reciprocity between science and tech- nology so characteristic of the German race, the teachers of our science have at once grappled with the problem.
Among the many proposals which have been made to escape, in good time, the pressing danger of chemical onesidedness, none appears to me more suitable than the encouragement of the growth which has developed upon the soil of science during the last ten years. I refer to general and physical chemistry. It deals with ques- tions which he at the base of organic and inorganic, of pure and applied chemistry; it forms a foundation for all real chemical education, and must be regarded as lying at the root of all chemical teaching, especially in its earlier stages.
By writing a series of text-books dealing with different stages of the subject I have tried to bring about the
AUTHOR'S PREFACE. V
knowledge of these principles as they at present exist, first among my colleagues in science, and next among students of chemistry.
The necessity of repeatedly revising the matter of these books, as well as daily experience in teaching, led to my early conviction that the very first steps of a young pupil must point in this direction; I also gained assurance that such an introduction was possible, and this book is the result of my efforts.
I must not omit to mention that it forms the first introductory volume, and that it will be followed as soon as possible by a second of about equal length, in which the system will be more developed.
I have chosen the form of dialogue, because after several attempts it appeared to me the most suitable; moreover, I have come to the conclusion that it occupies no more space than an ordinary description, while the impression it makes is much more penetrating and lively. I venture to hope that it will be found that it is at the same time the result of a varied experience in teaching, and not an accidental choice.
W. OSTWALD.
Leipzig, 1903.
CONTENTS,
PAGB
1. Substances i
2. Properties 5
3. Substances and Mixtures 10
4. Solutions 16
5. Melting and Freezing 23
6. Boiling and Evaporation 28
7. Measuring 36
8. Density 46
9. Forms 54
10. Combustion 61
11. Oxygen 71
12. Compounds and Constituents 82
13. Elements 92
14. Light Metals 103
15. Heavy Metals 113
16. More about Oxygen 117
17. Hydrogen 129
18. Oxygen and Hydrogen 139
19. Water 152
20. Ice 163
21. Steam 171
22. Nitrogen 182
23. Air 188
24. Continuity and Exactness 200
25. The Expansion of Air by Heat 208
26. The Water in the Air 218
27. Carbon 224
28. Carbon Monoxide 234
29. Carbon Dioxide 237
30. The Sun 244
vii
CONVERSATIONS IN CHEMISTRY.
1. SUBSTANCES.
Master. To-day we commence something quite new; you shall begin to learn chemistry.
Pupil. What is chemistry ?
M. Chemistry is a branch of natural science. You have already learned something about animals and plants and know that the study of animals is called zoology, and that of plants botany.
P. Then does chemistry teach about stones?
M. No, that is called mineralogy. Mineralogy is not the study of stones alone, but of many other things which are found in the earth, such as phosphorus, gold, or coal. But all these things, too, belong to chemistry. And other things, like sugar, glass, iron, which are not found in the earth, but are artificially obtained from other sub- stances, are also the subjects of chemistry. Chemistry is the study of all kinds of substances, whether artificial or natural.
P. Then does chemistry deal with trees ?
M. No, for a tree is not a substance.
P. But it is wood, and wood is a substance.
M. Yes, but a tree consists of more than wood, for its leaves and fruit are not made of wood, but of other sub-
2 CONyERSATIONS ON CHEMISTRY.
stances. All such substances taken alone belong to chemistry; but to get each alone, the tree must be de- stroyed.
P. But what do you mean by a substance ?
M. That is a long story. Let me see if you don't know it yourself, though you can't put it into words. What is this?
P. I think it is sugar.
M, Why?
P. Well, the sugar in the sugar-basin looks just like it. Let me taste it — yes, it's sugar, for it's sweet.
M. Do you know another way by which you can tell sugar ?
P. Yes, it makes your fingers sticky; so does this.
M. You can tell sugar, then, when some one puts it in your hand and asks you if it is sugar. And you knew it, first by its appearance, then by its taste, and lastly by its stickiness. These signs by which you recognize a sub- stance are called ''properties"; you know sugar by its properties. Sugar is a substance; one can tell substances by their properties. Do you think you could use all the properties of a substance in order to recognize it ?
P. Yes, if I knew them.
M. We will just see. Is there only one sort of sugar? No, you know loaf sugar, which is in large lumps, and sifted sugar, which is a powder, like sand. Both are sugar, because when you pound up loaf sugar it becomes like sifted sugar.
P. Yes: then they are both the same.
M. Both are the same substance, sugar, but one of its properties has been changed. The shape of a thing is also one of its properties; if you like you can change its shape, yet the stuff of which it consists remains the same.
SUBSTANCES. 3
This also applies to quantity. Whether the sugar-basin is full or almost empty, what is in it is always sugar. So you see form and quantity do not belong to the properties by which you recognize a substance. Is sugar hot or cold?
P. I don't know; it may be either.
M. Quite right. So neither heat nor cold is a property by which you can tell a substance.
P. No, of course you can't; for you can make sugar as coarse or as fine, or as hot or cold as you wish.
M. Now we have got to the bottom of it. Among the properties of a thing there are some which cannot be altered. You will always find that sugar tastes sweet, and that it makes your fingers sticky. But you can change its size and form, and you can heat it if you like. Every definite substance has its distinct unchangeable properties, and a thing bears the name of this substance when it has these fixed unchangeable properties, quite independently of whether it is warm or cold, large or small, or how its changeable properties may vary.
A thing has often another name, according to its use or its shape, different from that of the substance it is made of. Then it is said to consist of this particular substance.
P. I don't quite understand that.
M. What's this? what's that?
P. A knitting-needle and a pair of scissors.
M. Are knitting-needles and scissors substances?
P. I'm not sure — No, I think not.
M. If you wish to know, you have only to ask: What does the thing consist of, or what is it made of? Then you generally come at the name of the substance. What are knitting-needles and scissors made of?
P. Of iron. Then is iron a substance?
4 CONVERSATIONS ON CHEMISTRY,
M, Certainly, for a piece of iron is called iron, whether it is large or small, hot or cold.
P. Then paper is a substance, because a book is made of paper, and wood is a substance, because the table is made of wood, and bricks are a substance, because houses are made of bricks.
M. The first two examples are right, but not the last. Does a brick remain a brick when it is broken up and powdered? No: the name "brick" is given to a thing that has a definite shape, so it can't be a stuff. But what are bricks made of?
P. They're made of clay.
M. Is clay a substance ?
P. Yes — no — yes, it is, because if you break up clay it still remains clay.
M. Quite right. You can often help yourself out like that when you are in doubt. First you must ask: What is the thing made of? And when you have answered that, you must go further, and ask: Does it remain the same when I break it up? and if you can say Yes, then it is a substance.
P. Then there are many, many different kinds of sub- stances?
M. Yes, certainly there are many; far more substances than you can name. And chemistry has to do with all such substances.
P. Oh, then I shall never be able to learn all about chemistry — it's hopeless. I'd rather not begin.
M. Do you know the forest near here ?
P. Yes, rather: you could put me where you like in it, and I should always know where I was.
M. But you don't know every single tree in it? How can you help being lost ?
PROPERTIES, S
P. But I know the paths.
M. Now, look here, that is what we are going to do with chemistry. We will not learn about every single substance that there is, but we will learn the paths which divide up the countless substances into different groups, and by help of which we can find our way from one place to another.
When you know the principal paths you will know where you are in chemistry, and afterwards you can leave the chief paths, and find out more about the byways. And you will see that learning chemistry is just as good fun as walking in a wood.
2. PROPERTIES.
M, Let me hear what you learned last time.
P. Chemistry is the study of substances, and sub- stances are what things consist of.
M. The first part of your answer is right, but the second is not quite right. A piece of music consists of sounds; but are sounds substances?
P. Yes; for you can call the sounds music is made of, substances.
M. Yes, in a figurative sense you can. But in the language of science the name "substance" is limited to things that have weight.
P. What right has any one to limit the meaning of a name?
M. The right of necessity. In the language of ordinary life people are not generally very careful of the meaning of words, as you showed yourself just ngw. In science, however, we have to try to be as accurate as we can,
6 COhiyERSATlONS ON CHEMISTRY.
and that is why we have to give every- day words an exact and accurate meaning. These meanings are made as like as possible to those which they ordinarily have, and really mean the same thing to all intents and pur- poses, only the boundary-line of use and meaning is more sharply drawn.
Most things which are generally known as substances are called the same in chemistry; but no things that have no weight. Now correct the last part of your sen- tence: "Substances are everything" . . .
P. A substance is anything of which a weighable thing consists. Yes, but I don't know yet what a sub- stance really is.
M. What do you mean?
P. I know quite well what things to call substances, but that isn't all. It doesn't tell me any more than I knew before. I know nothing about the nature of a substance yet.
M. How should you know it? By giving a word a distinct scientific use, or defining a word, nothing more has happened than that I drew a circle round it so as to limit the meaning of the word within certain bounds. We have made a fence round our forest; but, of course, that doesn't teach us to know it. As you learn the prop- erties of the various substances, you will also learn their nature, and that will give you plenty to do.
P. But when I know all the properties of a substance, I'll only know — how can I put it? — the outside of it. I can't get through to its inner nature.
M. Don't you remember that there are different sorts of properties? What are they?
P. You mean what we spoke of yesterday ? There are changeable and unchangeable properties.
PROPERTIES. 7
M. And which help you to recognize the substance?
P. The unchangeable ones.
M. There now, you've found what you want. The unchangeable properties of a substance can't be taken away; when they aren't there, the substance isn't there either. These properties make the nature of the substance.
P. But that is only its properties. What I want to know is : What lies at the bottom of all its properties ?
M. You want to know what remains when you think all the properties of a substance are taken away. Now, just think, if you took away all the properties of a piece of sugar, its colour, form, hardness, weight, taste, etc., what would remain?
P. I don't know.
M. Nothing would remain. Because it is only by the properties I can tell that something is there; if no properties are present, there is nothing there that I can speak about. You must get rid of the idea that there is anything higher or more real to be found in a thing than its properties. Long ago, when science was little advanced, people thought something like that, and there are remains of it in ordinary speech, so that one uncon- sciously gets these ideas through the use of ordinary expressions. But once you recognize this error you can avoid it.
P. I see you are quite right, but I'm afraid it will take me a long time to get rid of the other idea.
M. You will be convinced when you have barned more chemistry that we really only speak of the properties of a stuff, and never of its ''nature.'' And you will forget your mistake later. — Anyhow, this talk has had its use, for now you see clearly that everything depends on our determining and knowing properties. Tell me
8 CONyERSATIONS ON CHEMISTRY,
some properties which help you to recognize a sub- stance. For example, what is the difference between silver, gold, and copper?
P. The colour; silver is white, gold yellow, and copper red.
M. Does colour belong to the changeable or to the unchangeable properties of a substance?
P. Generally to the unchangeable, I should think.
M. Why are you so uncertain about it?
P. I am not quite sure: the colours of gold and silver are unchangeable, but old copper doesn't look red, but dark, and often green.
M. Have you ever looked carefully at a piece of copper that has become green ? Is the copper green through and through ?
P. I think not; no, you can scratch off the green, and there is red copper underneath.
M. Quite right; and the green is, in other ways, not like copper; it is not tough like metal, but crumbly like earth. The fact is that another substance, green in colour, has been found on the copper, that was not there before, and it has only covered up the red copper, just as the yellow wood of the window-frame is covered with white paint.
P. How does the green come on the copper?
M. It is formed from the copper : you will learn later exactly how it comes. At first we will go back to the question of colours. Now we must take colour as an unchangeable property, by which we can recognize a substance. Only we must take care not to mistake the colour of a chance layer on the surface for the colour of the substance itself. We see that best if we break it into pieces, and so expose the inner part. Let us try it.
PROPERTIES. 9
Look what I have here. It is a blue substance, which is called copper sulphate.
P. Oh, please don't break it up, it has such a lovely shape, just Hke a cut jewel.
M. Those shapes are called crystals; they are not made by cutting, but form themselves without our help.
P. May I see that?
M, You will soon learn for yourself how to form crystals. I have a great many more, and we can quite well use this one, if we are going to learn anything by it. There, I have broken it: look closely if the blue coloui of this stuff is its own.
P. Yes, it is, because the stuff is just as bright a blue inside as outside.
M. Now we will break it up still smaller in this thick little porcelain dish, which is called a mortar. For that we will use this thick rod, which is called a pestle (Fig. i).
P. Why are you giving your- self so much unnecessary bother? We know already ^^^- ^•
what will happen.
M. Look at it carefully. When you have drawn a con- clusion you must test it properly, or else you won't know that you haven't overlooked or forgotten something. What do you see ?
P. The pieces don't seem to be quite so blue inside as the crystal was outside, for the broken bits seem to get lighter coloured, and now the powder is quite pale blue, almost white. I can't understand that, because before, the big bits looked quite dark blue. Perhaps something has been rubbed off from the mortar?
I o CON VERSA TIONS ON CHE MIS TR Y.
M. No, porcelain is hard, and is not affected by rubbing. But look at these broken bits of blue glass. Here it is even darker than the copper sulphate was, and here it is almost colourless, yet it is the same blue glass.
P. That is quite easy to explain : the glass is far thicker in one part than in the other. Ah, now I understand; the little pieces of copper sulphate are just as light blue as the glass in their thin parts, and the large pieces dark like the thick glass.
M. Right. When light penetrates a piece of the blue substance it gets reflected again and again inside, till it can come out somewhere, so that the further in it has to penetrate the bluer it becomes. That is why the larger or thicker pieces are darker than the smaller. In the same way the main mass of the sea is dark blue or green, and the small quantities of broken-up water seen on the foam of the waves or in the track of a ship look quite white. That is why, when you are talking of the colour of a substance, you must mention at once whether you are thinking of it in a state of powder or in big lumps. Generally, when we give the colour of a thing in chem- istry, we describe its colour as seen when it has been arti- ficially prepared. A great deal still remains to be said on the question of colour, but we have had enough for to-day.
3. SUBSTANCES AND MIXTURES.
M. Go over what you learned yesterday.
P. Substances are known by their properties. One of these properties is colour. This looks different, how- ever, according as the substance is in large or small pieces.
SUBSTANCES AND MIXTURES. II
M. Right. Do you know this stone? It is called granite. What is its colour?
P. Grey, and reddish, and black.
M. Why do you name several colours?
P. There are several in the stone; there are grey, and red, and black bits. You can't say that it has any one colour.
M. Is granite a substance ?
P. Of course; because all sorts of things are made of granite, for example, the street pavements. And a small piece of granite is still granite.
M. Let us see. Now, just imagine granite crushed into such small pieces that every separate piece is either black, red, or grey. Then we put all the grey pieces in a heap together, and the same with the red and the black. Would you call each of the three heaps granite, or only one, and which ?
P. Perhaps the red. No, that wouldn't do. Granite is only granite when it is all together.
M. Quite right. Could you do the same with a piece of sugar, and how many heaps would you have then?
P. No, it wouldn't work with sugar. Sugar always remains the same.
M. Right again. Now, notice well, you have discov- ered a very important difference. Substances like gran- ite, which can be divided up into different heaps after they have been broken up, are called mixtures. Those where it is not possible, as with sugar, are of the same kind through and through; we call them homogeneous. In chemistry we only concern ourselves with homogene- ous substances.
P. Why with these only ?
M, Because the number of the others is endless. Just
1« CONVERSATIONS ON CHEMISTRY,
think: You have two different homogeneous substances. Then you can make innumerable mixtures according to the proportions in which you mix them. If we had to take note of every single mixture, we should never come to an end.
P. But after all they are something; we can't leave them out.
M. Very good. You are quite right. But we don't need to know each mixture separately, and this is true for the following reasons: When we bring together two homogeneous substances into a mixture, all the prop- erties of the mixture are such as can be calculated from the properties of the two separate substances, according to the proportion in which they are mixed. For in- stance: A mixed colour is the result of the simultaneous and separate action of the single colours of which it is made; the mixing of colours in painting depends on this. For this reason we needn't examine very closely into the properties of mixtures.
P. Please explain that more clearly.
M. When a shopman has marked a yard of material at a certain price, he doesn't need to write down how much a half- or a quarter-yard costs ; and so you can easily find out the properties of the mixtures from those of the ingredients; and you don't need to look out and write down those of all possible mixtures. Everything that can be asked about a mixture can be answered by cal- culation if you know its ingredients and their relative amounts. Our silver money, for example, consists of ^Ya; of silver and ^/^^ of copper, and the value of a pound of this metal is made up of ^Ygy of the value of a pound of silver and ^/^^ of the value of a pound of copper.
P. I see that. But I can't always tell if it is really a
SUBSTANCES AND MIXTURES, 13
mixture. When I take my paint-box and mix blue and yellow, green appears, not a mixture of blue and yellow.
M. That is only because the grains of colour are too small for you to recognize singly when they are near each other. If you looked at the mixture through a micro- scope, you would sec the blue grains beside and on top of the yellow ones. Blue and yellow glass laid over each other make green. Therefore when the light from a yellow grain goes through blue, or vice versa, it be- comes green.
P. But supposing both stuffs were white, then I couldn't recognize them together even under a micro- scope, and I couldn't tell whether it was a mixture or not.
M. If I took a mixed spoonful of sugar and white sand, then I certainly couldn't see that there were two things in the mixture. But when I pour sugar into water, what happens then?
P. It dissolves, and later the water becomes quite clear again, and tastes sweet.
M. And what happens to sand?
P. It makes the water cloudy.
M. And doesn't make it sweet. Now, if I pour my mixture of sugar and sand into water, the water will become cloudy, and sweet like sugar. So I can tell them both together.
P. Yes, it is so.
M. Why is it so? Now, I will tell you. Colours are not the only properties which substances possess, and by which they can be recognized and distinguished. The behaviour with water is a special property, and this is different with sugar and sand, even though the colours are the same. Therefore when you want to distinguish between a great many different substances, you must
1 4 CON VERS A TIONS ON CHE MIS TR Y.
know not only one or two, but a great many of their properties, so as always to find out a difference even though other properties seem the same. That is why so many different properties of substances are examined and described in chemistry.
Now for another question. Looking at the ingredients of granite, we might think that we could separate them by their colour, so that we had them in different portions. Do you think that you could in any way separate the mixture of sand and sugar?
P. It ought to be possible, but I don't know how.
M. Just look at the glass in which I have stirred up the mixture with water. Now, the sand has sunk to the bottom, and the sugar is dissolved in the water.
P. Yes, now I see; you only need to pour off the water with the sugar, and the sand will be left behind in the glass.
M. Will they both be completely separated then?
P. No, you can't pour out all the water. The sand will be wet, and some sugar will still be in the water.
M. Now, g,ttend and see how it is possible to do it. I have here a round piece of a particular sort of paper, which is called filter-paper. It is something like blot- ting-paper, as it sucks up water, only it is made of a purer and firmer substance. I fold the paper in half, and then again in half, and pull it apart so that a sort of little trumpet is made, which is quite plain on one side, and on the other has three layers of paper. That is called a filter. I put my filter in a glass funnel and wet it with water. Now I can press the filter on to the sides of the funnel so that it quite covers it. The funnel is now put in a stand and a glass placed under it (Fig, 2).
P. What is the good of all this?
SUBSTANCES AND MIXTURES. 15
M. To separate the sand entirely from the sugar. If I pour the mixture of sand and sugary water into the filter,
Fig. 2.
the water will come through and the sand remain in the filter.
P. But the sand is still wet and some sugar is still there.
M. That we will soon wash out. I only need to pour some pure water into the filter, and this will run through and take the sugary water with it. Also, to rinse the last grains of sand that remain in the glass into the filter, I use fresh water. In case it wasn't completely through the first time, I wait till the water has run through, and repeat the rinsing out several times. So now we are ready. When the filter with the sand is quite dry, then we have completely separated it from the sugar.
1 6 CONVERSATIONS ON CHEMISTRY.
P. But how are we going to get the sugar?
M. We shall get that to-morrow. I pour the water that has run through the filter into a flat china dish, or a plate, and place it on the warm stove.
P. Why?
M. What does water do when you put it on a warm stove ?
P. It dries up.
M. Yes, it evaporates, it changes into water vapour, which disappears in the air, and nothing is left in the dish. Does sugar do that, too? Does it become less when it is on a warm stove?
P. No, it stays there till some one eats it up.
M. Quite right. If I put my water which contains the sugar in a warm place, the water will evaporate, but the sugar will stay behind, and when all the water is evaporated, only the sugar remains in the dish. In this way we shall at last have completely separated our mix- ture of sugar and sand.
P. I wonder what the sugar will look like to-morrow. At present you can't see it a bit, for the water is quite clear, and to-morrow it ought to be there still.
4. SOLUTIONS.
P. Is the sugar there ?
M. Here is the dish. Look at it.
P. Yes, I can see a white heap that looks like sugar. There is still some wet, though.
M. That is the rest of the water which remains with the sugar, and only goes away slowly. A great deal of sugar is dissolved there, and the fluid is much less mobile
SOLUTIONS.
17
than pure water, and the water takes far longer to evap- orate.
P. But it hasn't come out in powder as we put it in.
M. No, it has appeared in the form of crystals. The crystals in the dish are not large, neither distinct nor beau- tiful. But I have another sort of sugar here ; * do you know it ?
P. Yes, it is sugar candy.
M. Quite right; this kind of sugar candy is generally made this way. You dissolve it in warm water and let it slowly separate out or crystaUize. Only look care- fully at the sugar candy; every piece is a crystal.
P. Yes, now I recognize everywhere the smooth, even sides. Is ordinary sugar not made of crystals ?
^
^
//
U-^
//
Fig. 3.
M. Certainly, only the crystals are far smaller. Here is a magnifying-glass, a lens. Just look through it at the sugar in the sugar-basin.
P. It looks like sugar candy.
i8
CONyERSATIONS ON CHEMISTRY.
M, Loaf sugar also consists of crystals, but they are so grown together that you cannot easily recognize them. All this sugar is separated from solutions, and therefore it is always crystalline; that means it is made of more or less distinctly developed crystals.
P. Are crystals always left when you let a solution evaporate ?
M. In most cases. But to get crystals you needn't always let a solution evaporate; there are many other ways, one of which I will show you immediately. Here I have a glass with the copper sulphate we used lately. If I put some with water and shake it, it will dissolve, and the water will become blue (Fig. 3). P. Why do you do that in this little glass tube ? M. You will soon see why. A chemist uses these little tubes for most of his experiments, as long as he is
not working with great quanti- ties, and for that reason they are called test-tubes. Now I light my spirit lamp and heat the water with the copper sulphate (Fig. 4)-
P. Take care, the glass will crack! How extraordinary! it hasn't broken. M. This sort of glass doesn't break if you handle it properly. Now look at the contents; before there was copper sulphate with the blue water; now it has vanished and the solution is a darker blue. I can put more copper sulphate in now, and it will also dissolve. But if I add more and more, finally I can bring the solu- tion to the boil, and the remainder stays in the same condition. Now I add some more water to it and heat
Fig.
SOLUTIONS. 19
it up again, and it all dissolves. We will now put the clear liquid aside.
P. But why didn't the test-tube break before? Glass cracks when you heat it.
M. Not always. You know that you make glass by melting it, and to do that it must be very hot; every ves- sel or piece of glass has been made very hot, and yet has not cracked.
P. Yes, but mother scolded me the other day because I had poured hot wate» into a glass, and it had broken.
M. That is quite true. Here is a contradiction which we must try to unravel. In what other ways can you crack glass ?
P. By hitting or crushing it.
M. Yes, when you want to try to make the glass a different shape and at the same time try to strain differ- ent parts differently. Can heat also have an effect on the form of glass ?
P. Yes, heat causes all bodies to expand.
M. Quite right. Then a hot glass will be rather larger than a cold one. Have you ever seen that ?
P. No; it is so little that you can't see it.
M. All the same I will show you. I have here a fairly long glass tube. I fasten it with one end in a stand, so that it is horizontal, and put at the free end a measured ruler. Now notice the line where the end is pointing. So that you may see it better, I shall stick on a black needle with wax. Now I bring my lamp under the tube so as to heat it. What do you see?
P. The end first rises, and then goes slowly down again (Fig. 5). Extraordinary!
M. Why are you so astonished?
P. I thought the needle would go forward, because
20 CONVERSATIONS ON CHEMISTRY.
as the heat makes the glass tube expand it must get longer.
M. Instead of that it becomes crooked, and bends upward. How, I will explain to you.
P. Wait a moment; I know it myself. The lower part of the tube where the flame hits it has become hot- ter than the upper part, and so it has expanded more below than above, and has become bent.
M. Right; and afterwards the upper part got hot also.
Fig. 5.
and bent itself straight again. Then glass is slightly bendable. But if I bend it too roughly —
P. It breaks.
M, Now you can see why a glass breaks with heat. When you heat it unequally it bends, and when this hap- pens too quickly, it breaks. But if the glass is equally warmed this doesn't happen. The hot water warmed your glass in the inside while it was quite cold outside, and that is why it cracked.
P. But your tube was cold inside when you put it in the flame and heated the outside of it. Why didn't it crack too?
M. Because it is made of very thin glass. The heat passed quickly through the whole glass. You can also
SOLUTIONS. 21
bend thin glass far more than thick before it cracks. That is why all chemical glass apparatus needed for heating purposes is made of thin glass, and care is taken that it is not too quickly or unevenly heated, so that the warmth may spread itself equally over the whole glass. But now we will look how our copper sulphate solution is getting on, that in the mean time has become cold.
P. There is solid copper sulphate again in the glass.
M. I will pour the liquid part in another glass, and take out the hard part with a glass rod. To dry it, I lay it on a piece of filter-paper, that will suck up the liquid. Watch it carefully. What do you see?
P. There are crystals again.
M. Yes; these crystals have not come because the solution is dried up, but because it has cooled.
P Please explain that to me.
M. If you take a certain amount of water and dissolve copper sulphate in it, can you dissolve as much copper sulphate as you wish?
P. No; after a time it won't dissolve any more.
M. Right. A given amount of water can dissolve only a given amount of another substance. Such a solution is called ''saturated." If, however, you warm such a solu- tion, then it can dissolve more. But when you cool it again, the solution cannot contain the extra amount it has taken, and this separates itself in a solid form and takes the shape of crystals.
P. That is just the same as after evaporation; for the water went away, and there was no more there for the substance to keep in solution.
M. Quite so. Whenever there is more substance than a saturated solution can hold, it separates itself in solid form. Later on we will learn another condition
22 CONyERSATlOm ON CHEMISTRY.
that must be fulfilled by this. But I haven't yet asked you what you learned yesterday.
P. Yesterday we talked about mixtures and homo- geneous substances. Mixtures consist of different sub- stances.
M. And how can mixtures be recognized and sepa- rated ?
P. By the constituents having different properties; for example, we can pick them out if they have different colours, or one will dissolve in water and the other remain behind.
M. Yes, if the other doesn't also dissolve in water. But the solutions that are produced, are they mixtures or homogeneous substances?
P. Mixtures.
M. Why?
P. Because you can put them together out of different substances, and again you can divide them up into their ingredients.
M. That is right so far; but have solutions like other mixtures the same properties as the ingredients before they are mixed?
P. Yes, the solution of copper sulphate is blue like the copper sulphate and a solution of sugar tastes sweet Hke sugar.
M. Copper sulphate and sugar are soHd bodies, but their solutions are liquid like water. If you take another solid body like sand, and stir it up with water, it will make a thick mixture, and not a solution.
P. Yes, there is a difference there. But perhaps the sugar gets divided up into such small pieces that they can neither be seen nor felt.
M, You may believe it, but you cannot prove it. For
MELTING /iND FREEZING, 23
when you look at a solution even through the strongest microscope, you don't see any separate particles.
P. But perhaps the particles are still smaller?
M. It is useless to speak about it any longer, as we can't decide it.
P. There is something special about solutions, then, which can be distinguished from ordinary mixtures?
M, Yes; solutions are homogeneous mixtures.
5. MELTING AND FREEZING.
M. What did we speak about yesterday?
P. About solutions, but I didn't quite grasp it aU.
M. What is the difficulty?
P. That out of a solid substance or a liquid a real liquid is made.
M. Just think for a minute if you can't make liquids out of solid substances in any other way.
P. Oh yes, when ice or snow melts.
M. Does that only happen with ice or snow, or can other soUd substances melt ?
P. Yes; on New Year's eve we melted lead.
M. Through warming or heating you can make solid things melt, or turn them into liquid. And when the liquid is cooled?
P. It becomes soHd again.
M. Then we can change ice into water, and water into ice, if we warm the ice, or cool the water. At what temperature will ice become liquid?
P. At 0°.
M. And when does water freeze to ice ?
P. Again at 0°.
2i
24 CONVERSATIONS ON CHEMISTRY.
M. Does the ice become liquid when it is warmed to 0° ? P. It ought to.
M. You have forgotten what you learned about that in your Physics lessons. We will just try it for our- ^ ^ selves. I have here a thermometer. This sort I, ^^ \ is made out of a narrow glass tube, with a bulb at the bottom containing mercury (Fig. 6). As mercury expands with heat much quicker than glass, it rises higher in the tube, and the higher the temperature is, the higher it rises. A row of equidistant strokes, with numbers, a scale, makes it possible to read the height of the mercury, and conse- quently the temperature. I now dip the buib of the thermometer into the crushed ice here in the beaker. In a short time it sets itself opposite the stroke with the mark 0°.
P. Why does the mercury stand at the 0° ? M. The thermometer-maker arranged that. When he had the instrument so far ready that only the scale remained to be put on, he put it in melting ice, and marked the place where the \ / mercury was. After that he placed the scale so I that the zero came exactly on this place, i P. Then there is no heat there.
^°* * M. No, it is a temperature that we have called 0°. It is quite an arbitrary choice, because you know that in winter the temperature falls far below zero. The lowest temperature that has been reached sc far lies about 260° below 0°.
P. Why did they hit upon this choice? M. That you will soon see. I surround the beaker with my hands, and try to warm it. Look at the thermometer.
2i;o
MELTING AND FREEZING, 25
P. It is Still at o^.
M. Now I pour some water out of the bottle that has stood in the room for this purpose. About what tempera- ture is this water?
P. In a room it should always be about 17° or 18°. The water will be about the same.
M. Look at the thermometer.
P. It is at 5°.
M. The warm water has raised the temperature then. Now stir it carefully round.
P. Now the thermometer is getting lower; now it is again at 0° and is remaining there. How is that? The room is warmer, and the thermometer ought tt) rise.
M. When ice and water are together, the temperature always remains at 0° as long as both are present. If you try to raise the temperature by adding heat, so much ice melts as to use up the whole added heat. If you take heat away, so much water freezes as to replace the heat removed.
P. Is heat made when water freezes?
M. Certainly; when water freezes to ice, exactly the same amount of heat is formed as is used when the ice is melted again.
P. How is it that it is exactly the same?
M. Just suppose for a moment that the two quantities were different; suppose that on freezing, the resulting heat was represented by the number 80, and on melting only 60 was used. If we freeze water, and then let the ice melt, it is exactly the same at the end as at the begin- ning; but of the heat, 80 parts have been produced, and only 60 used, so that 20 remain over. Now this can be done as often as you Hke, so that you could produce any quantity of heat from nothing. But that is not possible ,
26 CONyERS^TIONS ON CHEMISTRY.
and therefore in melting, exactly as much heat is used as was given out on freezing.
P. Is it quite impossible to make heat out of nothing ? Rubbing makes heat.
M. But not for nothing. To rub, you must work, and you cannot create work out of nothing. But let us leave this subject, for I will explain to you later what a quantity of heat is, and how it is measured. We will go back to our water and ice. You saw that when both were together, the thermometer always remained at a particular temperature, which is generally called o°. Therefore there is quite a definite temperature when solid ice changes into liquid water, or melts. Now, do you think that there is always a particular temperature when a solid substance melts ?
P. There must be something of the sort, as lead is easily melted, and silver is difficult to melt.
M. Now we come to a general law, that every substance melts at a particular temperature and freezes at the same temperature. The melting-point and freezing-point of a substance are always the same. It is that temperature at which the solid substance and the liquid substance can exist together, and at which heat added or removed is used only in changing the liquid or the solid from one into ,the other. The melting-point then is as much a property as its colour or solubility.
P. Who made this law?
M. The name law is only used figuratively. People found that it was the case with substances, and have consequently compared them with obedient pupils, who always do what they are told. In science, people under- stand by a law something that applies to many things, and can be expressed in a general form.
MELTING ^ND FREEZING. 27
P. Are there many laws li'ie that?
M. Yes, a great many. To know such laws makes the task of noticing and using individual facts much easier.
P. Please explain that more distinctly.
M. Let us take the law that a mixture of water and ice has always a dofmite temperature. If a thermometer- maker in London has made his thermometer so that a mixture of ice and water shows a temperature of 0°, he can be perfectly sure that wherever in the whole world ice and water are brought together the temperature will be 0°. Were this not the case he couldn't sell a thermometer, and we couldn't use a bought one for our purpose.
P. It is really nice of the law to help the thermometer- maker so much.
M. A law of nature is not a being who either does some- thing, or leaves it undone. People have discovered that ice and water together have always the same temperature. Therefore, in this case, the thermometer-maker is placed in such a position that he can always make generally useful thermometers. But with one point, the point of zero, the thermometer is not finished; all the other lines have to be marked.
P. Aren't these just ordinary millimetres, like a ruler?
M. No, that wouldn't work. For sometimes the tube is narrow and sometimes wider, sometimes the bulb with the mercury is large, sometimes smaller. The mercury would then rise to different heights if the thermometers were equally warmed, and so they wouldn't agree.
P. That is true. Then you must warm all thermom- eters the same amount, and mark the place of the mercury, and then put on equal numbers of marks till you come to 0°.
M. Good. To what temperature should you heat them ?
28 CONVERSATIONS ON CHEMISTRY.
P. To any.
M. That wouldn't be right. Of course all thermome- ters would agree that had been made at the same time, but at another place no one would know what the com- mon temperature was where the top mark was made.
P. Then I can't think of anytl>ing better.
M. It would help us if we could only find a tempera- ture that was as easy and certain as the ice-point.
P. Ah, now I remember; it is the boiling-point of water.
M. Yes, it is the temperature at which water boils. That is what we shall speak about to-morrow.
6. BOILING AND EVAPORATION.
M. What did you learn yesterday?
P, I learned that melting ice always shows the same temperature, which never alters whether much or little water or ice is present.
M. And what about the freezing of water ?
P. That shows the same temperature. But what hap- pens when all the water is frozen? •M. Then we have only ice, and this we can cool as much as we like. In the same way, when we melt ice all the ice becomes Hquid ....
P. Then we have only water, and this we can warm as much as we like.
M. That is nearly right, but you jumped to a too rapid conclusion, because it doesn't hold in all cases- We will speak about this shortly. But first let us go over again what we spoke about. What is the condition that gives the temperature of o°? Try to explain this as quickly and generally as possible.
BOILING AND EVAPORATION, 29
P. Let me think a minute. Ice is at 0° when it melts, and water when it freezes. But when ice is melted, or water is frozen, it isn't at 0° any longer. There must be ice in water, or water along with the ice. Oh, now I know; when ice and water are together, then the temperature is at 0°.
M. Right; that is the condition. Can you see exactly why this condition must be fulfilled?
P. It seems to me it must be quite simple, only I can't get it out.
M. It is really quite simple. What happens when you try to warm a mixture of ice and water?
P. You explained that to me yesterday. It only melts some ice, and that uses up the heat that has been put in.
M. And when you try to cool it ?
P. Then some of the water freezes to ice, and gives . . .
M. And gives out exactly the same amount of heat as has been taken away. You see the thing is like the height of water in a pond that always remains at the same level. If you take water away, more flows in from the spring; if you pour water in, it runs over the dam, and the height of the water is still the same.
P. I understood that, but I haven't got it quite clear yet. Does a lot of water with a little ice give the same temperature as a lot of ice with a little water?
M. You have not been attending. We learned all this yesterday as a law of nature; that is to say, as a thing that is always the same.
P. Oh, now I remember; now I see it all. Why, it is ridiculously easy; I thought it would be far more difficult.
M. That will often happen. When you have got a thing quite clear, it always seems very easy. But the YpXing it clear is not always so simple and easy. But
30
CONVERSATIOhlS ON CHEMISTRY,
now let us go back to my first remark. Can you really heat water without ice as much as you want? What happens when I put water in a pot over the fire? P. First it will get hot, and then it will begin to boil. M. Right. We will make the experiment. I have
here a flask made of thin glass which I can put over the flame without its cracking. In it is some water, and I shall put it over a tripod which stands above my lamp (Fig. 7)-
P. Why is that wire gauze on the tripod?
M. For one thing, so that I can put large and small vessels on it. Again, the metal spreads the heat of the flame, and prevents the glass from breaking so easily if it is a little thicker. Now I put my thermometer in the water. P. Do you see? The water is getting warmer. M. Wait a bit.
P. Now the water is boiling, and the mercury has risen quite high; it is already at ioo°. Now it will soon fill the whole thermometer. What will happen when the mercury has no more room to expand?
M. The thermometer will break, for it exerts very strong pressure.
P. Then take the lamp away at once. M. Look at the thermometer first.
Fig. 7.
BOILING AND EVAPORATION, 31
P. It is Still at 100°.
M. And will stay there as long as you like. I am making the flame bigger. What do you see?
P. The water is boiling harder.
M. And the thermometer?
P. That is still at 100°. Oh, now I am beginning to notice something. It seems to be exactly the same here as with the melting.
M. Quite right. Now try to trace the resemblance. Then the temperature was unchangeable when two things, ice and water, were together. What is it here?
P. There is water here too, but what is the second? Wait a bit, I've got it now; it is steam. Is that right?
M. Yes. When I supply heat by means of a flame, it doesn't heat the water any more, but changes it . . .
P. Into steam!
M. Now we must reverse this relation. We had the same temperature before, whether we started with water or with ice; now . . .
P. Now we must get the same temperature whether we start with water or steam. We have got the one when we started with water, but how do we get the other? We must take a vessel with steam, and try to cool it. That isn't easy to do; we must have a boiler for it.
M. We can do it in a much easier way. Look, I take the thermometer out, and let the water boil quickly for a minute or two. The thermometer has now cooled a little, and it has fallen to under 50°. Now I put it again into the flask; not into the boiling water, however, but hold it above in the upper part of the flask. What do you see ?
P. Water is dropping from the thermometer. How did it get there? I know; the steam in the upper part of the flask has condensed on the cold thermometer.
32 CONVERSATIONS ON CHEMISTRY,
M. Right. Read the temperature.
P. It is at 1 00° again.
M. Now we have made the experiment for which you wanted a boiler. The upper part of the flask contains steam, which rushes upwards and makes clouds outside. By the cold of the thermometer a part of the steam is made into liquid water, and also in the upper part of the flask too. You have thus steam and water together. Steam condenses to water on the thermometer till the lost heat is supplied again, and the temperature has risen to 100°.
P. Is there really steam in the upper part of the flask? It is quite clear.
M. Steam is as transparent as air.
P. Is that so? I thought steam was always misty and untransparent. When a steam-engine blows out steam, you see it like a thick white cloud, and in the same way the clouds in the sky are steam.
M. No, what you see isn't steam, but liquid water in very small drops that have been made out of steam by cool- ing. If you could look into the boiler of a steam-engine, you would see that the inside is quite clear, just as if it was full of air. Also in the clearest air there is always a large amount of steam; and mist and clouds are made up by the cooling and building up of liquid water in the shape of tiny drops. So you see these things behave in very much the same way as water and ice. Water and steam only exist together at a definite temperature, and when they are present together that must be the temperature.
P. How does it happen that it is exactly 100° ?
M. In every thermometer the 100° is marked just where the mercury rises to in boiling water.
P. How can they do that?
BOILING AND EyAPORATtON.
33
100-
-212
50-
M. Don't you remember how we left the thermometer- maker? He had only been able to put one mark on his tube, and had written o° there when the mercury was in melting ice. Now, he must have another distinct tem- perature to have another mark, in order that he may divide up his instrument. This second temperature is that of boiHng water, and people came to the conclusion that the portion between the two marks was to be divided into a hundred parts. As the lowest mark is called o°, the top one must be ioo°.
P. Now I understand. But how can higher or lower temperatures be measured ?
M. As many equal divisions are marked below the zero-point and above the loo- point as there is room for. According as the thermometer is required for high or low temperatures, more or less mercury is put in, so that there is enough space over on the required part (Fig. 8).
P. But our window-thermometer is not divided up to ioo°. It stops at 50°. How could they make the right division in that case?
M. First a thermometer is made with great care from 0° to 100° and correctly divided. That is called a normal ther- mometer. Then the short thermometer is brought into the same medium as this — for instance, both are dipped into a rather large quantity of water. Since obviously both thermometers must now register the same temperature, we have merely to mark on the small one the number at which the mer- cury stands in the large one.
— 100
Fig. 8.
34 CON VERS A TIONS ON CHE MIS TR Y.
P. Is that how it's done? Now, I don't think I've anything more to ask. Yes, I have though ; on the left of our window- thermometer is a C and on the right an F., and it is divided differently on both sides.
M. F. means Fahrenheit. Fahrenheit was a German who made the first comparable thermometer; he lived in the eighteenth century. He wanted to divide his thermometer from the lowest temperature that there was; so he put it in a mixture of snow and sal ammoniac, and marked the point to which the mercury sank as o°. The piece between this point and freezing-point of water he divided into 32 parts, and found that 180 of these parts were contained in the space between freezing-point and boiling-point. People used the division of Fahren- heit for this reason alons, because the freezing-point was 32° and the boiling-point 32° -f 180°, or 212°.
P. Why don't they still use Fahrenheit's plan?
M. Because the mixture of sal ammoniac and snow is very difficult to bring to a definite temperature, while the freezing- and boiling-points of water are much surer.
P. Does every one use these thermometers?
M. The English and Americans do. They use them only in ordinary life, however, mostly for open-air ther- mometers. In all scientific work they use the centigrade thermometer. Give me the equation between Celsius and Fahrenheit, and use the letter / for Fahrenheit, and c for Celsius.
P. /:c=i8o°:ioo°, or 5/ = 9C.
Mo That is not right.
P. Why not?
M. The freezing-point of Celsius is zero. If you say c = o°, then your equation comes out to / = o°. But the
BOILING AND EVAPORATION. 35
freezing-point of Fahrenheit is not o°, but 32°. What must you do so as to make / = 32° when c = o°?
P. I must put the 32 on the other side.
M. Well, let me hear the equation.
M. Put the c = o in here. Now what happens?
P. 5/ = 32. No, that is not right; the / must stand alone on the left. How can I do that? Now I know: First, I must write ]=^/ f,c and then add 32 to the right; so /=V5C+32. Now, I put c = o° and that comes right;
M. Yes, now the equation is right.
P. I have read about a thermometer called Rdaumur that was quite different.
M. Yes. For rather more than a hundred years the thermometer of a Frenchman, Reaumur, has been used. In his, the space *between freezing- and boiling-point was divided not into 100, but 80 parts. On the other hand, the Swede Celsius introduced the division of 100. In Germany the Reaumur thermometer came into use, while in France the centigrade one was used. Presently people grew accustomed to register all temperatures by the centigrade thermometer; in science no other is now used. What is the relation between the degrees of Reaumur and Celsius ?
P. 100° C. are 80° R.
M. Simplify the proportion.
P. 10° C. are 8° R., or 5° C. are 4° R.
M. You can write this as an equation too. Take c for centigrade and r for Reaumur degrees — that makes c:r::5:4, so c=^/^r^ or r^*/^c. The first equation you
36 CONVERSATIONS ON CHEMISTRY.
use when you wish to change Reaumur into centigrade, and vice versa.
P. Has the mixture of ice and the other thing really —
M, Sal ammoniac ?
P. And sal ammoniac the lowest temperature that there is?
M. Far from itl It is sometimes colder here in winter. Think how many degrees of Celsius there are to the zero of Fahrenheit.
P. I must put / = o; then 0 = ^/^0+32; that makes
M. Yes, not quite 18° under 0°; but in America it is often 20° to 25° below zero.
P. What is the greatest cold that there is?
M. Up to the present 259° below 0° has been attained.
P. What do you mean?^ Will they get further?
M. Not much. Probably — 273°C. is the lowest tem- perature there is.
P. Why do you think that ?
M. I can't explain to-day, but you will soon discover and believe it as well.
P. Oh, I wish I knew!
7. MEASURING.
M. What did you learn yesterday?
P. How thermometers are made.
M. Yes. As a thermometer is a sort of measuring instrument we will speak a little about measurement. What can be measured?
P. All sorts of things: Lengths, weights, surfaces. I think almost everything can be measured.
MEASURINC. 37
M. Not all, but a great many things. What is used for measuring?
P. A measure.
M. What is that?
P. There are different sorts; it depends on what you want to measure.
M Give me an example.
P. Well, the length of the table can be measured in feet and inches.
M. Although feet and inches are used in England and America, all scientific people measure in what is called the metric system.
P. What is that?
M. We are going to learn it. Here is a centimetre rule. Measure the length of the table.
P. The scale is 50 centimetres long; I see that on the last figure. I lay the measure so that its end is against the end of the table, and notice to where it reaches. Then I put the measure at that mark, and again make a scratch where it ends. My measure comes beyond the table, now that I have put it at the second mark, and I look at which number the table ends. It is at 22. So the table is 50+ 50-f 22^=122 cm.
M. Quite right. You went on adding centimetres together till you had got the same amount as the length of the table. The measure only helped you to count the centimetres.
P. Yes, so it did.
M. And how do you set about measuring weights?
P. I put the thing in one pan of a balance, and add weights to the other, till they are both the same weight.
M, And how can you notice, or tell the weight ?
38 COhiyERSATIONS ON CHEMISTRY.
P, The number of ounces each one weighs is marked on the weight; I add the figures all up afterwards.
M. Let us use grams. You see it is the same as before; you add grams together till their weight is the same as that of the object. The weights only help you to count the grams.
P. So they do. I never noticed that both were so like.
M. You will soon see that all real measuring is based upon the same principle. But now for another question: Why didn't you measure the length with grams and the weight with centimetres?
P. It wouldn't work.
M. Why not?
P. However many centimetres I put together they would never make a weight.
M. Quite right. Put this in a simple form.
P. Length can be only measured by length, and weight by weight.
M. It could be said still more simply. Every quantity can be measured by a like quantity.
P. Yes, I understand that.
M. You measured length in centimetres, hre centi- metres the only measure of length?
P. No, there are millimetres, kilometres, inches, miles, fathoms, and a great many others.
M. How far do these differ ?
P. A centimetre has a different length from an inch, and so on.
M. Yes; these definite lengths, such as a centimetre, inch, and mile, are called units of length. In every state- ment of measurement we get the kind of unit which has been used, and the number- of units which are contained in the thing measured.
MEASURING.
39
Fig. 9.
P. Then why are there so many sorts of units for the same sort of quantity; for example, length?
M. That is because the choice of the units is arbi- trary. At first different groups of people who required a unit of length chose one without troubling themselves about what other people were using. Finally these differences grew so un- bearable that in France, at the end of the eighteenth century, the State determined to abolish the old meas- urement and to use a new one in its place. It was determined to protect the standard against acci- dental destruction, and so it was decided to use the world itself as a measure. The length of a quadrant of the meridian, that is, the length from ^ to A^ (Fig. 9), was divided into ten million parts, and these parts were called metres, and were to serve as a common unit of length. A centimetre is a hundredth part of this length, so that it is a thousand- millionth of the earth's quadrant.
P. But how can the earth's quadrant be divided, when no one has been to the north pole ?
M. Only a part of it is measured, the relation of which to the whole is determined by the angle which lines at right angles to two tangents form with each other. But it turned out that this measurement was far less accurate than the comparison of two metre scales. Accordingly the metre was taken to be the length of a standard kept in Paris, made of the most indestructible material which could be found — an alloy of the noble metals platinum and iridium.
40 CONyBRSATlONS ON CHEMISTRY.
P. But supposing this scale was lost or got destroyed?
M. Care is taken about that. Twenty similar scales have been made, all carefully compared with each other, and there is one at Berlin, London, New York, St. Peters- burg, Rome, and many other places, so that any one of them might be lost without the loss of the standard. Then, again, many other scales made of different materials have been compared with them, so that the permanence of the unit is about as certain as that of the human race.
P. But the metre is quite an arbitrary measure. Why hasn't one been chosen which is free from man's control?
M. Because there is practically none.
P. But with angles it is different. I have learned in my geometry class that a right angle is a natural measure which cannot be altered. Why can't that be done with lengths ?
M. Tell me any natural measure of length.
P. . No, I am afraid I can't. But why is there a
difference ?
M. It depends on the fact that an angle cannot be made infinitely great. If you rotate a straight line round a point in another straight line, the angle between both increases at first, but it can't become larger than four right angles, for that angle is equal to the angle o, and afterwards the same angles come as before. The largest possible angle has consequently a finite value,^ and that value is the natural unit. But with length it is different, for you cannot think any length so great that it could not be made greater.
P. So nothing which can be made infinitely great can have a natural unit?
M. Quite right. You will soon become convinced that for all such magnitude arbitrary units must be
MEASURING. - 41
chosen. The best proof is that no one has been able to find a natural one. Now we will go back to the metre. It is not convenient to measure all magnitudes of the same kind by means of the same unit. You can measure the length of the table in centimetres; but if you measure the height of a hill or the length of a river in centimetres, your numbers will be far too large, and for such great lengths larger units are employed.
P. Yes, I know; metres and kilometres.
M, Right. People have used such different units for a long time, but they did not stand to one another in a sufficiently simple relationship. At the same time as the metre was introduced, it was decided only to admit such measures of the same kind, as stand to each other in the proportion 1:10:100:1000, and so on; that is, in powers of 10.
P. Why was that done ?
M. Because in reducing from one measure to another there is hardly any work to be done ; you need merely add zeros, or alter the position of the decimal point. Thus you have:
I kilometre (km. for short) = 1000 metres (m. for short).
I m. = io decimetres (dcm.) = ioo centimetres (cm.) = 1000 millimetres (mm.).
P. What is the meaning of kilo?
M. Kilo is the Greek word for a thousand. It was agreed at the same time that the multiple of each unit should be expressed by Greek prefixes (deca-, hecto-, and kilo-) ; while the fractions are expressed with Latin prefixes (deci-, centi-, milli-).
P. Now I understand the meaning of the words kilo- gram and milligram.
M. You see the unit of mass is Jled the gram. It is
42 CONFERS A TIONS ON CHE MIS TR Y.
derived from the centimetre; it is the mass of a cube of water at 4° C. The multiples deca-, hecto-, and kilo- gram are derived from it, but only the last (kgrm.) is in use. A kilogram is equal to two pounds. The deci- and centi- gram are also not often used; but the milligram (mgrm.) = 0.00 1 gram is much used in scientific work.
P. You said that the gram is the unit of mass. I thought it was the unit of weight, for people weigh with grams and kilograms.
M. Mass and weight are related to each other. Mass is the property of bodies which keeps them in motion when they are once moving; and mass is measured by the work which must be expended in order to produce equal velocities. Now weight or the force with which bodies are drawn to the earth are at any given place exactly proportional to the mass, so that when two weights are equal the masses are also equal. And for that reason masses can be measured by help of weights.
P. Why do we require to know masses? Surely we buy bread and iron and gold by weight.
M. Yes; hy weight, but not on account of weight. In science weight is derived from mass, and not mass from weight, because the mass of any body is unchangeable although its weight may be altered.
P. But if I keep a thing carefully shut up, so that nothing is lost, surely its weight remains unchanged?
M. I don't mean it in that sense. Of course, if you take anything away from a body, its mass will be de- creased in the same proportion as its weight. No; a body has a smaller weight on a high mountain than in a valley. And weight is less at the equator than at the pole.
P. I remember learning that in my Geography lesson;
MEASURING. 43
it had to do with the attraction of the earth. Because the earth is flattened at the poles, a body there is nearer the centre of the earth than at the equator.
M. Quite right; but you must add that the attraction decreases with the distance from the centre of the earth; moreover, near the equator the centrifugal force increases, so that a body near the equator is more swung off from the earth than if it is near one of the poles, and it conse- quently weighs less.
P. If I weigh a kilogram of sand here, and carry it up a high mountain and weigh it again, would it really weigh less ?
M, Not if you were to weigh it on an ordinary balance with arms; it would counterpoise exactly as much weight there as here.
P. But you said —
M. Don't you see that your weights become lighter in the same proportion as your sand?
P. How can that be? Oh, I see; I hadn't thought of it. But I can't understand how it can be proved that the weight has become less.
M. By determining the weight, not by help of counter- poises, but by another method. A spring balance, in which weight is measured by stretching a spring, would show that your sand weighed less on the top of a hill \an in a valley. The most exact measurement is made
ith a pendulum, for it swings more quickly the greater ihe attraction.
P. Why?
M. You will learn it in your Physics lesson. We must go back to our old subject. I told you that things are bought by weight, not because of weight. Why do people buy bread?
44 COr^VERSATIONS ON CHEMISTRY.
P. To eat it.
M. Do you eat it in order to grow heavier?
P. Ha! hal ha! No, because I Hke it and because it makes me strong.
M\ The last is the important reason. And coals are bought, not because they are heavy, but because they make you warm.
P. But I can't understand the use of weight.
M. Which would you rather have, a small piece of cake or a large one?
P. Of course, a large one.
M. Why?
P. Because there is more of it. A little one wouldn't satisfy my hunger.
M. And which weighs more ?
P. The larger one, of course.
M. Now you see the use of weight. The properties and uses which make us buy things increase or decrease with the mass or the weight. The power which bread has of keeping you alive increases proportionally to its weight, and the greater the weight of the coal you buy the more heat you can get from it, and just as with these marketable properties, so a great many scientific prop- erties are dependent on the mass and on the weight The balance is therefore a very important piece of chemical apparatus, not so much because we want to know the weight of things, for often we do not care to know it, but because of the other properties which are connected with weight.
P. So weight is like the paper of a book, which is worth very little in itself, but becomes valuable for what is printed on it.
M. That is a good comparison even though it doesn't
MEASURING. 45
quite fit. Let us take a better example. As you know, liquids are bought and sold both by measure and weight. Wine and beer are sold only by measure, that is, by the space which they occupy; paraffin-oil is sold both by weight and by measure; sulphuric acid is sold only by weight.
P. Why?
M. Convenience and custom are the reasons. Measur- ing is much quicker than weighing, and a measure is much more easily made than a balance; and so this plan is preferred. But sulphuric acid is a somewhat dangerous liquid, and people don't like to pour it; therefore they prefer to weigh it. But for the purpose of determining quantity by measurement, for any one substance volume and weight bear a constant proportion to each other. Hence the actions and uses of liquids are proportional to their volumes, just as they are to their weights. The purchaser of paraffin-oil is not interested in the volume it occupies or in its weight; he buys it because of the amount of light or heat which he can get from it. But these amounts are proportional to the volume, and so the volume becomes a measure for the amount of light which the paraffin-oil will produce. Now tell me what you know about measures of volume.
P. The unit is called a litre.
M. That is only half right. The real unit of volume is derived from the unit of length, and is a cube, the side of which is one metre long — a cubic metre. But this measure is far too large for most purposes, and therefore one has been chosen nearer in volume to the old pints and gallons. It is a cube, the side of which is. Yio of a metre; its capacity therefore is Yiooo of ^ cubic metre. It is called a cubic decimetre, or a litre (1.).
46 CONyERS/l TIONS ON CHE MIS TR Y.
P. You have surely made a mistake in saying that a cubic decimetre is a thousandth of a cubic metre. A deci- metre is only a tenth of a metre.
M. Think a minute!
P. What a stupid I was! The volume of a body is pro- portional to the cube of its side, and 10X10X10= 1000.
M. Yes, that is right. In science we use as a measure one-thousandth of a litre. How large is that cube?
P. I won't make another mistake. The side is ten times less, y^o dcm. is Yioo i^i. It is a centimetre.
M. The measure of volume is called cubic centimetre (ccm.). Now write me down a table of measures of volumes.
P. I cbm. = 1000 ]., and i 1. = 1000 ccm.
M. Quite right. Now we have had enough for to-day, although there is a great deal more to say about measure- ment.
8. DENSITY.
M, Yesterday you learned how to measure and to weigh; to-day we will talk a little more about measure- ment. Which is the lighter, a pound of lead or a pound of feathers?
P. You can't catch me with that old joke. Of course they are the same weight.
M. But which is the lighter, lead or feathers?
P. Hm! Well, feathers are really lighter.
M. That is a contradiction. It depends upon the fact that the words light and heavy are used with a double meaning. When you say lead is heavier than feathers, you mean that a handful of lead has a greater weight
DENSITY. 47
than a handful of feathers; if equal volumes of feathers and of lead are compared, the lead weighs more. If we say wood is lighter than iron, we attach the same meaning to the word lighter, although you could easily choose a given piece of wood heavier than a given piece of iron.
P. I understand that.
M. But in science it doesn't do to use such indefinite expressions. The property which is greater with iron and lead than with wood and feathers is called density^ and we say iron is denser than wood and lead denser than feathers. How is density determined?
P. By weight and by volume.
M. Yes. And as the density is greater the greater the weight in a given volume, and smaller, the greater the volume of a given weight, the density is made propor- tional to the weight and inversely proportional to the volume; so that if w is the weight and v the volume, the density d is expressed by the formula
d=~,
V
P. What is the use of this formula ?
M. To measure the density. Let us take an example: What is the density of water?
P. It depends on what weight and what volume you take.
M. No, it doesn't depend upon that. We choose once for all the gram as unit of weight and the cubic centimetre as unit of volume. Now, if we take an arbitrary quantity of water, say a litre, what is its weight ?
P. One litre of water weighs looo grams.
M. And what is its volume in cubic centimetres?
48 CONVERSATIONS ON CHEMISTRY.
P. looo c.c. make a litre.
M. So we have w= looo and v = looo; how large is </?
P. J= 1000/1000=1; the density is i.
M. Now make the same calculation for 20 c.c. of water.
P. ^^=20/20=1. It is I again. Oh, I see; because the volume and the weight always become larger and smaller to the same degree, the fraction must always have the same value whatever quantity of water is taken.
M. Now you understand it. Here I have a little lead cube; what is its density?
P. I must first find its weight. Let me weigh it myself. It weighs 38.84 grams. And now I must find its volume. But how can I do that?
M. As it is a cube you have only to determine the length of one side. Here is a rule.
P, The side is 15 mm. long, and so the volume is
i5' = 3375-
M. Equals 3375 what?
P. 3375 c.mm. Oh, I should give the volume in cubic centimetres. I'll be right this time. The volume is 3.375 cc. %
M. Quite right. Now calculate the density.
P. 38.84/3.375 = 11.51-
M. So the cube has the density 11.51. I can go further and say that lead has the density 11.51, for if I had taken any other cube of lead, or indeed any other piece of lead, I should have found the same number. Tell me why.
P. I can see that you would have got about the same number, but I am not sure that you would have got exactly the same number.
M' You have forgotten what I told you before (page 2)
DENSITY. 49
about properties. Density is a property; for all samples of the same substance it will have the same value. Now ordinary lead is really a very pure substance, and con- tains hardly anything mixed with it, and so the properties of different samples have the same value.
P. But all bodies expand with heat ; so that the volume of the lead cube will be larger when it is warm than when it is cold.
M. Quite right. Is weight changed by heat ?
P. Not so far as I know.
M. Weight is quite independent of temperature. So it follows that the density of lead becomes smaller as the temperature rises, because while the nunierator remains the same, the denominator increases.
P. Then density isn't quite a definite property.
M. Yes, it is, for at a definite temperature it has a definite value. The same holds for every other sub- stance. Water, too, changes its volume with temperature; and therefore 4° has been chosen as the temperature at which the weight of i c.c. is called i gram.
P. Why was that temperature chosen?
M. Because water has its greatest density or its smallest volume at 4°. What are you thinking about?
P. I am thinking how it would be possible to determine the density if the thing wasn't a cube.
M. That is a very sensible question, for very few sub- stances can be made into that shape. Look here, I'll show you how it can be done. Here is a glass tube which is divided into tenths of cubic centimetres by little lines. I pour water into it and read where the level stands; I find 5.33 c.c.
P. You have read off hundredths, and there are only tenths marked upon the tube.
50 CONVERSATIONS ON CHEMISTRY.
M. Every one who makes measurements must learn to do that. As a rule, the level of the water does not lie neatly on a line, but between two. I divide the distance between two lines into tenths with my eye, and so I get my hundredths.
P. I couldn't do that.
M. It isn't difficult to learn, and you must try it after- wards. But now we will go on. I have here a glass with shot. They are made of lead; weigh it.
P. It weighs 43.58 grams.
M, Now I shake some of the shot into the tube. Weigh the glass again.
P. It weighs 28.42 grams.
M, What is the weight of the shot that I have shaken into the tube?
P. 43.58-28.42 = 15.16 grams.
M, And now I read the level of the water in the tube. It stands at 6.66. That is 1.33 c.c more. What con- clusion can I draw?
P. Oh, now I see. The volume of the water has risen so as to tell the volume of the shot. The volume of 15.16 grams is 1.33 c.c, and so its density is 11.40. It is almost exactly the same number that we calculated before. But it is not exactly right.
M. Because you didn't measure with sufficient accu- racy. You gave the side of the cube as 15 mm.; measure again.
P. Yes, it is a little smaller.
M. And measure the other sides of the cube.
P. They are not quite equal.
M. You see, then, that your former measurement con- tained errors, and therefore the result cannot be quite accurate. To measure exactly is a very difficult thing;
DENSITY. 51
and therefore we must rest contented at present with what we have found; the right number is 11.4. I will let you use the balance and the measuring-glass, and you can determine for yourself the density of various sub- stances. But take care that you always remove the bubbles of air, or you will measure them along with the volume of the body, which will appear too great, and you will get too small densities.
P. Yes, I will draw up a table. What shall I meas- ure?
M. You had better find the densities of your minerals. But now to another question: Have liquids also definite densities ?
P, I think so. Yes, water has the density i.
M. Right. Now think; how can you determine the density of a liquid?
P. By determining its weight and its volume. Wait, I know. I shall pour it into the measuring-glass and read out its volume.
if. And how will you find its weight?
P. Exactly as you did with the shot. I shall first weigh the flask which contains the liquid, then pour it into the measuring-glass, and then I shall weigh the flask again.
M. It can be done in that way, but it is possible to do it in a much simpler manner. Weigh the measuring- glass once for all, then pour in liquid and weigh again, and you need only subtract the weight of the measuring- glass.
P. That gives me one weighing less.
M. You can lessen your work still further by not measuring out an arbitrary quantity of liquid, but a definite volume. This is not easy with solid bodies,
52 CONVERS/iTIONS ON CHEMISTRY.
but is quite easy with liquids, because they fill a given volume completely. For example, if you pour exactly i c.c. into your measuring-glass, and determine its weight, what will your equa- tion be?
P. Then d^g/i. That i?> d= g\ the weight is the same as the density.
M. Do you see you don't require to divide. It is often said that the density is the weight of unit volume. This expression is not wrong, but doesn't cover enough, and so I didn't tell you it before.
P. I have just tried to pour i c.c. of water into the measuring-glass, but it is very difficult to get exactly the right amount. I have found either too much or too little.
M. Pour in a little too much, and then remove the excess with a small strip of blotting-paper. It sucks up such small quantities that it is quite easy to obtain the right volume. FiG."io. P. Yes, that works.
M, It is still easier with this apparatus (Fig. lo), which is called a pipette (this is a French word and means little pipe). I suck the upper end while I hold the lower in the liquid, until the level rises above a mark on the stem; then I close the end quickly with my forefinger, and while the point touches the side of the vessel, I can easily let so much liquid run out that it stands exactly at the mark.
P. But I must put the liquid in another vessel to weigh it.
M. No. You can lay the pipette itself upon the scale. If you have determined its weight, when empty once for all,
DENSITY. 53
you need only subtract that from the total weight and you have the weight of a cubic centimetre, or the density. It is still simpler to make a counterpoise of wire of the same weight as the pipette. Such a counterpoise is called a tare. Then the remainder of the weights on the pan will give you the density.
P. I'll certainly do that.
M. In that manner you can determine the densities of various liquids, such as spirits and salt water. You will find the first lighter, the second heavier, than water.
P. Then I can make a table of densities of liquids as well.
M. Now you know how to determine densities of solids and liquids, what about gases?
P. Can't their densities be determined in the same way by measuring their weight and their volume?
M. Of course they can, but it is not so easy. In the first place the weight of a large volume of air is very small; i litre of air weighs only a little more than i gram, as you have seen already. Then the volume of gases is very easily changed if the temperature or the pressure alters. And so very different densities are got for the same gas if it is measured at different temperatures or pressures.
P. But that happens too with solids and liquids.
M. The changes are much smaller with them, so that they need only be taken into account if great accuracy is required.
P. Then how is the density of a gas determined ?
M. That is a rather difficult thing, which I shall explain to you later. To-day I will merely say that people have determined upon a standard temperature and a standard
54 CON VERSA TJONS ON CHE MIS TR Y.
pressure at which to measure the volumes of gases, and so uniform results are obtained.
P. I should never have thought that measuring was such a difficult matter.
9. FORMS.
M. I am not going over to-day what you learned yes- terday, because it was really just a rep^titioi of what you had learned before. We will go back to what we spoke about in the lesson before last. You learned two very different properties of water. What law is at the bottom of the melting of ice and boiling of water?
P. That both happen at a definite temperature.
M. Yes, but not only water; every substance has these properties.
P. Really all?
M. All substances that are really pure substances. Mixtures and solutions have changeable melting- and boiling-points.
P. How changeable?
M. If a solution is brought to boiling point, we notice, as the boiling proceeds, that the temperature doesn't re- main unchanged, as with pure substances, but gradually rises, in proportion to the amount of steam that goes away. In the same way, when a mixture fuses or melts, it begins to liquefy at a definite temperature; this does not remain stationary, however, but rises higher as more heat is added and more of the mixture becomes liquid.
P. May I see that ?
M. Later. At present we will stick to pure sub- stances. You have seen that liquid water can be changed
FORMS, 55
into solid ice and into gaseous steam. Do you know what these two conditions are called?
P. Yes; states of aggregation.
M. Quite right; that is the usual name. What does it mean?
P. Aggregate means assembled, but I don't know what that has to do with liquid or steam.
M. The name is given because it is taken for granted that all bodies are made up of tiny particles which are able to lie on each other, or arrange themselves in various ways. They are called atoms. According as these atoms are near or far from each other, they make solid, liquid, or gaseous bodies.
P. Can you see these atoms with a glass?
M. No, not even with the strongest microscope. People take for granted, because of that, that they are •smaller than the smallest thing that can be seen through a microscope.
P. But are they really there ?
M. It is true I cannot guarantee them. There is no proof of their existence.
P. Then how can you say that it depends upon them whether a body is solid or liquid ?
M. Real things behave in many respects as if they were collections of atoms, if atoms exist. If it be assumed that bodies consist of atoms, it may be deduced that they must behave as they really do.
P. That is very awkward. Why do people not simply say: They behave this way or that way, and be done with it ?
M, Because, starting with the assumption of atoms, there can be deduced several conclusions which agree with fact. Such an assumption is called a hypothesis.
S6 CONVERSATIONS ON CHEMISTRY.
P. But I can't see what is gained if there is no proof that the hypothesis is true.
M. The hypothesis serves to make the real relation- ship more easily noticeable. If you have to keep in mind three names, Alfred, Arthur, and Anthony, it will be easier for you to remember them if you notice that they all begin with A. Moreover, a hypothesis serves as a stimulus to research. People imagine how a number of atoms would behave under given circumstances, and find out whether the actual bodies behave in that way.
P. Do they always agree?
M. No, I am sorry to say, not always.
P. But after such a conclusion has been drawn, people ought to see whether it is right or not.
M. Certainly; but this gives an opportunity of putting definite questions to nature and of making suitable experiments or observations. And so our knowledge increases, and that is always an advantage.
P. But if they don't agree?
M. Then there is nothing for it but to wait and hope that the contradiction may be explained.
P. But that is a very uncertain way of doing things.
M. So it is; yet the use of hypotheses for learning and investigation is so great that people will always make use of them.
P. Couldn't they do without them?
M. Of course they could; but people are so much in the habit of using hypotheses, like the atomic hypothesis, that they find great inconvenience when they try to realize things without their help. And therefore they will not give them up.
P. Then please explain to me how solid, liquid, and gaseous bodies are built up of atoms.
FORMS. 57
M. Ah, you put me in a difficult position if you wish me to show you the use of the atomic hypothesis, for up to the present it has not been entirely satisfactory. However, we needn't delay over that at present; I only mentioned the subject in order to explain the derivation of the name "state of aggregation." In talking over these things with you I prefer to consider these relations without its help; and for that reason I will not use the term, but rather speak of forms.
P. What does the name mean?
M. It points to the chief differences of these states. How does a solid body behave in relation to its form?
P. I don't know anything particular to say about that; it can be broken, cut, or bent.
M. But if it is left alone ?
P. Then it keeps its form.
M. Right. Have you ever thought how important that is?
P. I don't see anything very important about it. Sometimes it is a great nuisance; for example, if I want to break sugar.
M. Think for a minute. If the stones and rafters of this house were to change their shape, it might fall to pieces at any moment; none of our tools would be usable; you couldn't cut .with a knife if the blade didn't keep its shape; your morning milk wouldn't stay in its can if the shape of the can kept changing continually.
P. Yes, now I see, but I can't think it out to the end. The whole world would go to bits.
M. Now I see you are beginning to grasp it. Have all bodies the property of keeping their shape ? For instance, how does water behave in this respect?
58 CONVERSATIONS ON CHEMISTRY.
P. Water does not keep its shape; you may pour it into any sort of vessel you like.
M. Is water the only thing that has this property ?
P. No, all Hquid bodies are like it. Yes, now I see the great difference. But how is it that solid bodies keep their shape?
M. That is a senseless question. How do you know when a body is solid?
P. I catch hold of it.
M. And you are convinced that it keeps its shape. The word solid is merely the name for the common properties of many bodies of keeping their shape.
P. But that must have a cause.
M. I don't understand you.
P. Why is this silver coin not liquid ?
M. Well, when you heat it, it melts and becomes liquid. Here is a piece of thin silver gauze; if I hold it in the flame it will liquefy, and a drop will form on the end. See, the drop has fallen.
P. So it has.
M. The question whether a body is solid or liquid depends solely upon its temperature. Below its melting- point it is a solid, and above its melting-point it is a liquid.
P. Is that the case with all bodies ?
M. Yes.
P. Then, by cooHng, any liquid can be made solid, and all solids become liquid when heated?
M. Generally. If substances do not decompose they behave in that manner. Only there are liquids the freez- ing-point of which is very low, and solids which melt at a very high temperature. There are melting- and freezing- points in all regions of temperature.
P. Why does a solid freeze at a definite temperature?
FORMS. 59
M. That is another senseless question. You can only ask : What is the freezing-point connected with ? It is just as if you were to ask: Why are there camels? Whereas one can only ask: What properties have these animals, and how do these properties compare with those of other animals? In the same kind of way, melting- points are phenomena of nature, and have definite rela- tions to other phenomena.
P. What sort of relations?
M. If I were to answer that question you wouldn't understand, for you would first have to know those other properties.
P. Yes, that is true. I see you would require to know the other properties before you could compare relations between them.
M. Yes. So we must begin our work by collecting facts, by writing them down, and then by comparing them with each other in order to find out in what they agree. That is how laws of nature are discovered.
P. I never thought of it in that way. I supposed that some very clever man must have discovered them all by himself.
M. Nobody does anything all by himself, as you call it. But think a minute. One law of nature tells us how certain things will behave under definite condi- tions. The thing must be known under those condi- tions before such statements can be made.
P. Yes, that is so ; but then every one must be able to discover laws of nature.
M. And so any one can, if he finds things in conditions which have not yet been sufficiently investigated. But that is rather difficult, because the common and ordinary conditions of things are already discovered ; and it is very
6o CONyERSATIONS ON CHEMISTRY.
hard to acquire enough exact knowledge to find undis- covered spheres to examine. For instance, it would be quite easy to discover the north pole if you could only get to it. The difficulty is not to see the north pole, but to get a place from which it can be seen.
P. Then I will really learn thoroughly, and perhaps later I may discover something.
M. Yes, do so. You know the end in view, anyhow. But now we will return to our subject. Do you under- stand now the meaning of the name forms?
P. Yes, solids have forms, but liquids haven't.
M. That is partly right; but what about gases?
P. They haven't any form, either.
M. How do they differ from liquids?
P. They are far lighter and thinner.
M. Yes, that is right, but you haven't come to the main point. If I pour some liquid into a vessel, it falls to the bottom, and fills the vessel according to the amount. But if I put gas in an empty vessel, what happens then?
P. I don't know; a gas can't be seen.
M. It fills the whole vessel, however much or httle there is.
P. That is extraordinary. How do you know that?
M. Only a definite amount of any sort of liquid can be poured into a given vessel, that is, as much as there is room for. If less is put in —
P. A part of the vessel remains empty.
M. Right. If you attempt to put more in, it doesn't work, for a liquid doesn't allow itself to be pressed together, or, to be exact, only slightly. A great quantity of gas can be put into a given space, and it is always possible to put in still more.
COMBUSTION, 6l
P. Does that go on forever?
M. No; more and more pressure is needed for it. We shall soon go into these things more particularly. At present the difference between liquid and gaseous bodies is important to us. It is true that liquids have no definite form, but they have a definite space or vol- ume, which is unchangeable whatever form they may be made to take. So a litre of petroleum is always a litre, whether it is in a can or a jug, or anything else it may be kept in.
P. And gases?
M. Gases have neither a definite form nor a definite volume, but spread themselves out through all the avail- able space until they entirely fill it.
P. Then the name "form" isn't suitable for gases?
M. Not at all. Liquids take the form of whatever contains them, but only so far as they fill it. Gases take the form of whatever contains them, because they always fill it completely.
P. Then "form" is the way in which bodies take their form!
M. You may describe the word in that way.
10. COMBUSTION,
M. Now you know something more about all the three states, and can have a more complete idea that nearly all substances are known in these three forms.
P. Why not all?
M. With some the melting- or boiling-point is so high, or the freezing-point so low, that it is not possible to reach them,
62 CONVERSATIOl^S ON CHEMISTRY,
P. Oh, I wanted to ask you about that a long time ago : are these changes of one form into another chemical or physical reactions ?
M. You know that such a classification is more or less arbitrary. If we define a chemical process as one in which most of the properties of the substances con- cerned alter, then we must consistently define change of state as involving a chemical process.
P. But we spoke about melting and boiling in my Physics lesson, so they belong to physics.
M. Ice can be changed as easily into water as water into ice. But in chemical changes, only one change, generally, is easily made; the other causes great diffi- culty. Formerly, because of this difference, people did not consider change of form as a chemical change.
P. You said "formerly"; is it different now?
M. Now people have learnt that many changes which are generally called chemical can be reversed, and are subject to the same laws as physical changes. — But now we will turn to things which have always been looked upon as chemical. Have you ever looked at a candle burning? Yes? Then describe to me what you saw.
P. When you light a candle it burns down till it is all gone, and during this it has a hot, bright flame.
M. Right. What is necessary for burning?
P. Well, the candle.
M. Nothing else?
P. Not that I know of.
M. If you put the burning candle in water —
P. It goes out.
M. Why? What is different from before?
P, It has no more air,
COMBUSTION,
63
M. Right. For burning, then, it is necessary to have the candle and air. I will show you now that a candle can burn under water if it is only put under together with air. I let a little board float about on the water in this large glass, place a bit of burning candle on it, turn a glass upside down over it, and now I can dip the whole thing under; the candle is burning (Fig. 11).
P. Oh, that is pretty! Please hold it a little longer like that. Ah! the flame has gone out. Some water must have got on the wick.
M. We will make the experi- ment again, and hold the glass quite still.
P. The flame went out again, after it had burnt a little.
M. Now we will leave out the water altogether. I put the little candle on a smooth plate of glass and place a glass beaker firmly over it.
P. The flame is going out again.
M. What- must you conclude from this experiment ?
P. That the candle can't burn for long in a glass beaker.
M. That would not be right. I put my beaker up- right, and put the candle in. You see it burns, perhaps a little unsteadily, but it doesn't go out.
P. Cover it with something. May I? Do you see now the flame is out again.
M* How can you express that knowledge?
Fig. II.
64 CONVERSATIONS ON CHEMISTRY.
P. A candle can only burn for a short time in a covered glass.
M. Must it only be a covered glass?
P. I don't think so.
M. It needn't be any one thing. An extinguisher puts a candle out, as you know, and it is made of metal. But why does a candle burn in a lantern?
P. Because it has air-holes.
M. What have they to do with it?
P. Fresh air always comes in at them, and the used-up air gets out at the top air-hole.
M. Now just try to put together all that we have spoken about.
P. The candle requires air to burn. In a closed space a candle can burn only a short time. If the air in this space is changed, the candle can burn longer.
M. Good. But this room is a closed space, and yet the candle can burn here as long as it will last.
P. Yes, because the room is so big.
M. There you have discovered something. So you think that the larger a closed space is the longer a candle will burn in it?
P. Yes.
M. It is so. But there are many important conclu- sions to be drawn from it. Can you give me any reason why that is the case?
P. No.
M. We will look for resemblances. A short candle will only burn a short time, a long candle, a long time. Why?
P. Because in burning the candle gets used up. Should the air get used up by burning?
M' Look for a moment. I have here a candle
COMBUSTION.
6S
attached to a wire, and lower it, burning, into a bottle (Fig. 12). When it has gone out I take it carefully out and light it. If I put it at once in the bottle again —
P. It goes out immediately.
M. It follows from that that the air in the bottle is used up.
P. How? There is some there still.
M. That isn't air. Air has the property that a candle can burn in it. What is in the bottle has not this property.
P. But it looks just like air.
M. Quite so; what is in there is a colourless gas like air, but not what we call air. A chemical change has taken place with the air, and it has other properties.
P. Other properties? Yes, the candle doesn't bum any longer. But beyond that I don't see any other properties.
M. That is because nearly all gases look very much like each other. The difference of their properties is only brought to light after careful searching. I have here a large flask with ordinary mortar shaken up with water and left for a while. Most of the mortar has sunk to the bottom, but a little has dissolved in the water. It seems also as if the water had kept its properties; it doesn't look any different. But still it has changed. Taste it!
P. How unpleasant! lik^ soap! It isn't poisonous?
M. No. I pour some of the lime-water into a bott)e
Fig. 12.
66 CONyERSATlONS ON CHEMISTRY.
Vfhich has ordinary air in it, and shake it. What do you see?
P. Nothing much.
M. The Hme-water remains unchanged. Now I do the same with the bottle where the candle burnt.
P. The water is becoming quite milky.
M. So you see that the gaseous contents of the bottle in which the candle burnt has a property that ordinary air doesn't possess. The air then has gone through a chemical change.
P. So you can see by means of lime-water what you can't see with your eyes?
M. Yes. If we could see the new things in the air without help, we wouldn't need to make use of lime- water. A substance, which in such a way makes known something present, is called a reagent, and the event which is called forth by it, a reaction.
P. A reaction means that one thing acts on another ?
M, Yes; the changed air and lime-water work upon each other, and so the white substance is formed which makes it cloudy. But now we will go still deeper into the subject. What happens to the candle by burning ?
P. It vanishes.
M. Do you mean it goes quite away?
P. Yes. Nothing remains over from it.
M. But if your book or your apple goes away, then you ask where they can be. And in the same way with everything else.
P. Yes, they can't vanish.
M. But the candle?
P. H'm! But" where can it be? It really vanished before my eyes.
COMBUSTION. 07
M. Yes, it became invisible. Can't it have changed into something invisible?
P. There isn't anything invisible.
M. Oh! isn't there?
P. No, there aren't any spirits or ghosts.
M. It seems that even they are sometimes visible. But can you see air?
P. No. But air is changed by burning. I can't
understand that.
M. It is quite simple. The candle and air transform each other mutually by burning, and a gaseous substance is the result, which, owing to this state, cannot be seen.
P. Gaseous substances that aren't air?
M. So that is your difficulty? You know that many liquids look like water that aren't water. In the same
way there are many gases that
look like air, but are something quite different. On that account in the earlier developments of chemistry there was great diffi- culty, till characteristics like that with lime-water taught people to distinguish between the different gases. But now we will try another experiment. I light a candle again, and hold a large empty glass over it (Fig. 13). What do you see?
P. The glass is becoming cloudy, as if it had been breathed on.
M. What is the cloudiness that ap- pears on a glass when breathed upon ?
P. I know that; it is made by drops of water which come from warm breath, and are laid on the cold surface.
68 CONIFERS A TIONS ON CHEMISTRY.
M. Right. What appears in the glass consists also of drops of water.
P. How do they get there?
M. The candle in burning changes itself partly into water.
P. That is extraordinary! I never thought of that. But water won't make the lime-water cloudy?
M. No, water never does that. Two new substances are formed when a candle burns. One is water and the other is that which makes lime-water cloudy.
P. What is it called?
M. Carbonic acid gas.
P. That is a funny name. What does it mean?
M. You can find that out later on.
P. The whole thing is becoming more and more mud- dhng!
M. You are right. We will examine simpler cases first; if you understand them, you will understand the others. We are going to burn iron.
P. Can you?
M. Quite easily. You know what iron filings are?
P. Yes, they are little specks of iron which have fallen down in filing.
M. I sprinkle some iron filings in the flame.
P. How pretty! Just like Httle stars!
M. That is burning iron.
P. Why didn't the iron gauze burn when I held it in the flame?
M. It wasn't hot enough, for the heat is conducted off by the gauze. But the little iron specks, on the other hand, are quickly heated, and lose none of their heat by being conducted off.
P. Then large pieces of iron ought to burn, if they are made hot enough.
COMBUSTION.
69
M. And so they do. Later on we will burn iron gauze itself. Iron burns also on melting, if it is glowing. The' burnt iron breaks off with hammering.
P. But you don't see any flame.
M. There is such a thing as burning without flame. The little stars from the iron filings were not flames. We will make an experiment like that now. This black powder is also iron, only in far smaller pieces than ordi- nary iron filings. I put a small wire tripod on the bal- ance, lay a narrow piece of wire gauze on it, and shake out several grams of iron powder on it (Fig. 14). The whole thing is now made to balance. Then I hold
Fig. 14.
a flame to the edge of the little heap. Now it is begin- ning to burn.
P. I only see it glowing.
M, That is how powdered iron burns. Charcoal can only glow when it burns.
P. That is true. But why have you put it all on a balance ?
70
CONVERSATIONS ON CHEMISTRY.
M. You will soon see. What do you think? will iron become lighter or heavier by burning?
P. I should think lighter. The scale with the iron will rise. M. Notice carefully.
P. It is sinking! Perhaps it was only a draught. No, it is getting heavier. That is very odd. M. Why?
P. One time a thing becomes lighter by burning, another time heavier.
M. In the case of the candle the thing that was made by burning went away, with iron it remains. If it stays, it increases the weight.
P. With the candle as well? I'd like to see that. M. To do that, you have only got to keep hold of what is made by the candle burning — water and carbonic acid gas. P. That must be rather difficult. M. Not very. There is a sub- stance called caustic soda which has the property of binding together every trace of water and carbonic acid gas with which it comes in contact. I put some loose pieces of it in the top part of a lamp-funnel, which I place over a burning candle (Fig. 15), and put the whole on the scale and balance it. We won't need to wait very long.
P. Yes, the scale with the candle is beginning to sink. M. And the proportion will be more, the more the candle is burnt.
Fig. 15.
OXYGEN. 71
P. Is it the same with all burning substances?
M. Yes ; you may try to burn, after this, oil, petroleum, or matches, or whatever else you like, instead of a candle, under the cylinder with caustic soda. The weight will always be increased.
11. OXYGEN.
M. What did you learn last time?
P. That all bodies become heavier by burning.
M. That is not quite complete. Think of the candle.
P. That all bodies become heavier by burning if you take w^hat is formed into account.
M. Think of the candle again! If it was entirely burnt ?
P. Ah, yes! That which is formed by bodies in burning is heavier than the body was itself.
M. Now it is right.
P. But can iron burn, too, so that nothing remains?
M. So that no iron remains? Certainly. Look, for yourself, what the iron powder that we burnt yesterday has become.
P. It is a dark mass, which looks rather like the powdered iron, only it is caked together.
M. Take some, and crush it in the mortar.
P. There is a black powder.
M. Now grind some powdered iron after you have cleaned out the mortar.
P. It is bright like iron.
M. Now you see the difference. The burnt iron isn't iron any longer, but a substance with other proper-
72 CONVERSATIONS ON CHEMISTRY.
ties, and the iron has vanished in the same way that the burnt candle vanished.
P. But the air, which helped with the burning?
M. The same thing happened to it which happened to the iron. In the same way as the solid iron changed to the solid substance, smithy scales, the vanished part of the air used by burning a candle becan^e part of another gas.
P. Has another gas been made by iron as well?
M. No.
P. Then air must vanish when iron burns in it.
M. We will make the experiment. I put my tripod with powdered iron on a little floating board, hght it, and cover it with a large glass, so placed that it stands on the bottom (Fig. i6). As the experiment is rather
slow, we must wait till the glowing iron is extinguished and has become cold. What do you see now?
P. It really seems as if air had vanished; but only a part, less than a quarter.
OXYGEN. 73
M. If you measure it more closely, it is about a fifth.
P. Perhaps you took too Httle iron.
M. No. If I had taken morC; no more air would have been used. -
P. But this is quite different from a candle, and also from iron. You can burn them completely.
M. Can you bum wood entirely?.
P. Ash remains.
M. It is the same with air. Wood is a mixture of combustible and non-combustible substances; when the former are burnt, the latter remain behind. Air is a mixture of two gases ; the one gets separated by burning, and is called oxygen, the other is left unchanged, and is called nitrogen. Oxygen only takes up about a fifth part of the space of air.
P. If you had pure oxygen, would it entirely vanish in burning?
M. Certainly, if there were no other gas. We will make pure oxygen.
P. Can we?
M. It has been possible for rather more than a hundred years. This white salt is called potassium chlorate. When it is heated a great deal of oxygen is formed.
P. What sort of brown powder are you mixing with it ?
M. That is heated iron-rust. When some is put with it, the oxygen forms more easily and regularly. I shake the mixture in a Httle round flask. Now I must make my apparatus. To do that, I take a cork which just fits into the neck of the flask, and cut a piece of glass tubing.
P. How can you cut glass?
M. It isn't exactly cut, but broken. But so that the fracture comes at the right place, and is even all round,
74
CONVERSATIONS ON CHEMISTRY.
I must scratch, and divide the tube at the place I wish.
P. What sort of tool is this?
M. This is an old three-cornered file whose teeth are ground down so that three cutting edges are made. If I saw the glass tube with this sharp edge, it will crack. Then I break the tube apart with the crack furthest from me, so that it breaks off evenly (Fig. 17).
Fig. 17.
P. That is clever! Can I do it too?
M. I will give you a piece of glass tube afterwards and you can practice on it. Now I will bend the tube.
P. You can't; it will break.
M. Heat will make glass soft, so that it can be bent. I put the part where the bend is to come in the flame and turn the tube continually, so that it shall be evenly heated, otherwise it would crack. After a time the glass will become so soft that it will bend with its own weight. I help it a little till it is the right shape, and let the glass get cold and hard, then it will keep its new shape.
P. That looks so easy. Can I do it too?
M. It isn't difficult, but it needs practice. The main thing is, not to apply the heat to any one point, and only to use very light pressure in bending it. Otherwise
OXYGEN. . 75
the bend may easily come uneven. Now the other end of the tube must be bent a Httle, and finally I turn each end in the flame for a little, so that the sharp edges become round and can't cut or scratch any more. That must never be forgotten.
P. How is that managed?
M. Soft glass behaves like liquids. You know that instead of having corners or points, their surfaces are always rounded.
P. Why do liquids do that?
M. On account of the surface tension. Through this the surface endeavours to become as small as possible, and as a ball is the form which contains the most contents with the least surface, all liquids try to form themselves into the shape of a ball.
P. But liquids take the form of the vessel that contains Inem.
M. Quite right. This comes from gravity, as they always try to get as low down as possible. Both causes work simultaneously on the liquid, but the gravity is generally far the stronger, and the shape of the water is most dependent on that. Now we must make a hole in the cork. For that I bore a hole with a steel point, an awl, and then I file it with a somewhat large round file till I can with a little force stick the glass tube through. Now everything is put together, and I fasten the apparatus, so that I can slip a piece of wire gauze below the flask and put the lamp under it (Fig. i8).
P. Why do you put the end of the tube in a dish of water ?
M. To collect the gas. If I were to stick the tube in an empty flask, that is, one filled with air, and were tc lead the gas into it, it would mix with the air, and I
76
COhiyERSATlONS ON CHEMISTRY.
shouldn't be able to see when the flask was full. So I fill the flask with water, and allow it to be displaced by the entering gas, holding the mouth of the flask over
Fig. i8.
the glass tube. As the gas doesn't mix with water, I get it pure.
P. Bubbles of gas are rising; hold the flask over them!
M. At first it is only the air which was in the appa- ratus.
P. Then how do you know when the new gas comes?
M. I take the tube out of the water and hold a glowing splinter at its open end. What do you see ?
P. It goes on glowing.
M. Then it is only air. And now?
P. Oh, it begins to burn of itself!
M. Not of itself, but with the oxygen which comes out. Now I bring the tube below the water again, and hold the flask over it. But in order not always to have to hold it, I place it in a little lead stand, below which the tube ends, so that the bubbles rise in the flask and expel the water (Fig. 19). In the mean time I will
OXYGEN.
77
fill some flasks with water so that we may afterwards fill them with oxygen.
P. Please show me the ex- periment with the glowing splin- ter again.
M. It is the test for oxygen. Whenever a glowing spHnter is put into oxygen it catches fire. I can repeat the experiment often with the oxygen in this flask. But at last it gets used up, and the experiment won't succeed any more.
P. What is the reason of that? ^iG- ^9-
M. I will first show you some other similar experiments. I tie a piece of charcoal on to a wire, light it at one corner, and place it in the oxygen. It soon begins to glow all over, much more brightly than in air. A bit of sul- phur in a little iron spoon which you can hardly see burning in the air gives a bright blue flame. A piece of phosphorus in a similar spoon, which burns in the air with a yellow flame, looks as bright as the sun. A thin spiral of iron wire, on the end of which a little tinder has been fastened and made to glow, catches fire and burns, throwing out sparks, and the smithy scales fall, while hot, into the water which covers the bottom of the flask. It is better to put a little sand
Fig. 20.
78 CONVERS/iTIONS ON CHEMISTRY.
at the bottom of the flask so as to keep it from breaking (Fig. 20).
P. Oh, that is a beautiful firework!
M. We won't forget what the firework means. What can you say in general about these experiments?
P. Things burn much more brightly in oxygen than in air.
M. Quite right. But they burn in air too, at the expense of the oxygen that is present. Where does the difference lie?
P. They give off more heat when in pure oxygen.
M. Your answer is right or wrong according to what you mean by the word heat. If you mean to say that the quantity of heat that i gram of carbon or iron gives out is greater when it burns in oxygen than when it burns in air, that is wrong. The quantity of heat is the same. But if you mean that the temperature rises higher, that is right.
P. I mean the temperature.
M. Of course you do! This is the reason. The same amount of heat which is given out in both cases has only to heat the resulting product of combustion when the substance burns in pure oxygen; but when it burns in air it has to heat the nitrogen which is mixed with the oxygen.
P. Is the brighter light connected with the higher temperature ?
M. Certainly. The temperature can even be estimated from the brightness of the light. But, besides that, the higher temperature makes the burning take place more quickly.
P. What has that got to do with it ?
M' It has been generally found that chemical changes
OXYGEN. 79
take place more quickly the higher the temperature. But we will go back to oxygen again. The phenomena you have seen are all chemical changes, for the burning substances and the oxygen have disappeared, and new substances have been produced instead.
P. Are the heat and the light which have been pro- duced, new substances too?
M. No, these things are not called substances, because they possess neither weight nor mass.
P. But all the same they are really there.
M. Certainly, because they are real things. They behave something like substances, for they change into one another, and new quantities of them can never be produced except by such change. Only they have no weight like substances.
P. Then these must be what are called forces?
M. People used to call these things forces, but that led to a misunderstanding, since the word force had already been used for something different. Now they are called energies. Heat is one kind of energy, and light another.
P. Yes; people are said to be energetic who can do something and carry it through.
M. The scientific use of the word energy is pretty nearly the same. Energy is what causes things to change.
P. So when substances change by chemical action, is that energy too?
M. To be sure; only we express it somewhat differ- ently. We say that substances possess chemical energy when they are able to act on each other and produce new substances. At the same moment that the substances change, a change of a part of their chemical energy
8o CONVERSATIONS ON CHEMISTRY.
takes place, and this assumes the form of heat or light, and sometimes of electrical or mechanical energy.
P. That strikes me as very curious and mysterious.
M. The change of one kind of energy into another is not more mysterious than the change of one substance into another; indeed, it is even simpler. To tell you a little more about energy : you must know that the ordinary work which a man or a horse or a steam-engine does is also energy.
P. Then I ought to be able to make heat or Hght or electricity by my arms.
M. So you can; when you rub your hands together they grow warm, and if you turn a blunt drill with all your force in a hole, it soon becomes so hot that you could burn your finger with it. And you know already that people can make fire by friction.
P. Yes, that is true. So I can make as much heat as I like?
M. Not as much as you like, but as much as you can. When you turn the drill for some time you can't go on any more; you are quite exhausted; that is, you have used up the store of energy which you possessed.
P. Where did I get that energy from?
M. From your food. It is chemical energy which you have taken in with your food, and in your body there is a kind of apparatus, the muscles, which change chemical energy into work.
P. How do they do that?
M. I wish I knew! Investigators haven't found out yet how it is done. But there is no doubt that chemical energy is used up in doing work, for you see that a hard worked horse must be well fed in order to do its work.
OXYGEN. 8i
P. But I always have a good appetite, even when I do nothing.
M. Then you waste the chemical energy of your food. Of course you always want a certain quantity in order to keep your temperature up to 37° C; for as your body is considerably warmer than its surround- ings, it is continually losing heat which must be replaced by means of food. That is a second way in which you can make heat, although it is beyond your control.
P. Can I make light too?
M. Yes; if you rub two pieces of sugar together in the dark, they will make light.
P. Don't they make light in the daytime?
ikf. Yes; only the Hght is so weak that it can't be seen by daylight. This experiment shows that the work of your muscles can be changed into light.
P. But can't I make light without anything?
M. You can't; but glow-worms and little animals, which are the cause of the phosphorescence of the sea, can. These change the chemical energy of their food directly into light.
P. And can I make elec^trical energy too?
M. Certainly ; you have only to rub a piece of sealing- wax with a cloth.
P. Oh, yes, I know. But I must use my muscles again to do that ; I don't do it directly.
M. Electric currents run through your body when- ever you exert yourself, indeed whenever you think. But they stay in your body and it isn't easy to conduct them outside.
P. I never dreamt that I could do all these things!
M. Well, you needn't be conceited about it, for every animal can do the same.
S2 CONyERSATlONS ON CHEMISTRY.
P. Still, it's very queer. Where does the energy of food come from?
M. From the sun.
P. I don't understand that.
M. Where does food come from? Either from plants or animals. Plants grow only in sunlight, for they use the energy of hght to build up their structures; they store it up in this form. And we consume the energy of the sun in the plants. And the animals whose flesh we eat subsist upon plants, that is, upon the energy of the sun.
P. I shall think of the sun quite differently after this.
M. If you only think of what we have been speaking about, you will understand more of the world than you did.
12. COMPOUNDS AND CONSTITUENTS.
M. Last time you learned a great deal that was new to you. Tell me the principal things.
P. First I learned how oxygen is made and collected; then I learned that substances burn far more brightly in it than in ordinary air, and that is because air contains only a fifth part of oxygen. Then I learned something about energy. But that was so much and so unfamiliar that I can't say it in a few words.
M. We will try together. Wherein does energy resem- ble substances, and wherein does it differ from them?
P. Resemble? Yes, it can change into many kinds, and when one kind is formed, others vanish.
M. Right. Where do they differ?
COMPOUNDS AND CONSTITUENTS. ^Z
P, Energy can't be weighed, and it comes from the sun on to the earth. Substances don't come from there.
M. No, at any rate not in detectable quantity. Now, first of all, be sure of these points; the others will be much more comprehensible if we are careful over these. Now we will return to oxygen. There is still here a flask which was filled yesterday. What noticeable properties has it?
P. Oxygen looks like air; it is colourless.
M. What does it smell like?
P. I can't smell anything; it has no smell.
M. You should have been able to tell me that without opening the bottle. Just think, a fifth part of the air is made of oxygen.
P. Oh, yes, because air doesn't smell, oxygen can't.
M. These are the noticeable properties of oxygen. Besides these it has others, which can only be found out by measurements or experiments. The burning phenom- ena that I showed you are also such properties. They are called chemical properties, because they depend upon chemical processes. Also the reaction of oxygen, the bursting into flame of a piece of glowing wood, is one of these chemical properties. Now we will learn another way of telling oxygen. This brick-coloured powder is called mercuric oxide. I put some into a test-tube made out of a particular kind of glass that has rather thicker sides, and is more difficult to melt than ordinary glass, and attach a gas-delivery tube as before. Then I make the glass hot with a lamp. What do you see?
P. The red powder is becoming black. It is becom- ing charred.
M. No; if I let it get cold it will become red again.
P. Then how does it get black?
84
CONVERSATIONS ON CHEMISTRY.
M. There are many substances which change their colour with heating. Colour depends to a great extent on temperature.
P. Now there are bubbles coming (Fig. 21).
Fig. 21.
M. Again it is, first of all, air which has expanded by heat.
P. But now the bubbles are much more frequent and continuous.
M. We will collect some of the gas in a little test-tube and try it with the glowing splinter. It is still air that is coming out of the test-tube. But the second time it is filled—
P. The splinter has caught fire; it is oxygen.
M. Perhaps. We will collect some and see if it is colourless and scentless. Try it!
COMPOUNDS AND CONSTITUENTS. 85
F. Yes, it is scentless, and one can't see any colour. But why was it necessary to prove it in this way as well ?
M. Before one can say that anything that one has is a definite substance, one must have made sure that all its properties are the right ones.
P. But then, one can't look for all its properties; there would be no end to it.
M. There you are right. But several properties must always be tested for, because it often happens that dif- ferent substances have one common property, whereas other properties are different.
P. Really just the same property?
M. One can't be absolutely certain, even when no difference can be seen. Since no property can be noticed, or measured with absolute exactness, one can't be certain that a seeming resemblance will not turn out to be a difference on closer inspection. But just to make these difficult researches unnecessary, people examine several properties. For it is very seldom that two different substances have several properties in common.
P. Look what has happened to the experiment mean- while. The test-tube looks just like silver at the top.
M. Yes, and the greater part of the mercuric oxide has disappeared. I heat it a little longer and now it is all gone. I take the delivery-tube out of the water and let everything cool.
P. Why don't you leave it all as it is?
M. The hot oxygen gas in the tube in cooling would contract, and the water might enter the test-tube. Now look closely: the silvery stuff in the tube can be brushed together with a feather, and changes into bright liquid droplets.
P. They look just like mercury.
86 CONVERSATIONS ON CHEMISTRY.
M. They are mercury.
P. But how did it come there?
M. It came out of the mercuric oxide.
P, And has the oxygen been made from it too?
M. Yes, these two substances, and nothing else.
P. But why isn't the mercury where the mercuric oxide was?
M. Because the mercury with the heat from the lamp became volatile, that is, it changed to a vapour. Then, when the tube was colder, the vapour changed back again to liquid mercury. I will now take some more mercury in a test-tube, and heat it; look, the first drops are forming, it is becoming thicker, and now it looks like a silver looking-glass. I repeat the experiment with the liquid metal that I made before; you see it behaves just the same; it is mercury too. But take care of the vapour; it is poisonous.
P. I shouldn't have thought so!
M. Why not?
P. Mercury is a metal, and metals don't boil.
M. Certainly they do, only the boiling-point of most of the best-known metals lies so high that it can't be reached by ordinary means. But, for example, in the flame of the electric arc all known metals turn to vapour. Mercury, however, boils fairly easily at 350° C. But now we will go back to our experiment. You saw that by heating, the red powder changed into mercury and oxygen. Out of mercury and oxygen red mercuric oxide can be made again. You can, so to speak, reverse the reaction.
P. That is wonderful. Can I see it?
M. Unfortunately I can't show you. Mercury oxide is fonned out of mercury and oxygen, if they are left in
COMPOUNDS AND CONSTITUENTS, ^7
contact together at a temperature something over 300°. But that takes place so slowly that it would take a week to get a couple of grams. But if it is done it shows exactly the same properties as mercury oxide.
P. Isn't it made in that way, then ?
M. No, it is made in quite a different way, which you wouldn't understand yet.
P. Then it doesn't matter which way it is made?
M. Certainly; there is an important general law, that a definite substance always has the same properties in whatever way it may have been made.
P. I shouldn't have thought so!
M. You have just had an example of it: the oxygen made from mercuric oxide had exactly the same properties as that made from potassium chlorate.
P. Yes, so it did. I never thought of that; I took it for granted.
M. You see again: People take things for granted when they don't think about them. Now notice some new names; because from one single substance, mer- curic oxide, two different substances, mercury and oxygen, can be made, and vice versa; from the two latter, again, a single substance, mercuric oxide, can be made; the latter is called a compound and the former the constituents. So mercuric oxide is — ?
P. Mercuric oxide is a compound of mercury and oxygen.
M. Yes, and mercury and oxygen are the constituents of mercuric oxide. — Now we are coming to an important question about the proportions by weight in chemical processes. In this closed flask there is oxygen, and there is a piece of charcoal hanging from a wire in it. I weigh
88 CON VERS ATIOhlS ON CHEMISTRY,
the flask on the balance. Now I will light the charcoal without opening the flask.
P. How will you do that?
M. I could do it in several ways. If I had put a second wire through the stopper, and bound both wires together with a thin piece of iron wire, I could make it glow with an electric current, and it would light the charcoal. But since we have sunshine, I can do it in a far simpler way : I shall light the charcoal with a burning glass.
P. Good; that's splendid. Hurrah! the charcoal is burning already.
M. And now it has gone out again, since the oxygen is used up. Now what do you think; will the flask have become heavier?
P. Of course.
M. You have taken it for granted again! But we will look. What do you see?
P. The pointer is going backwards and forwards over the middle. The weight seems to have remained the same. Perhaps the increase is so little that it can't be noticed?
M. No, even with the most careful weighing it would always be the same.
P. But that can't be right! I learned and saw that weight increased with burning.
M. The weight of what?
P. Ah! so it was. The product of combustion weighed more than the burnt body weighed.
M. Well, and here?
P. Here it weighs the same.
M. That is a false conclusion. It really weighs more.
P. But then, how is it that the weight didn't change?
M. It is because the oxyen disappeared. The product
COMPOUNDS AND CONSTITUENTS. 89
of the burning weighs just as much more as the used oxygen. So the gain and loss have balanced each other.
P. That is extraordinary.
M, Yes, it is an example of one of the most important laws, which holds for all chemical, and also for all physi- cal, processes; whatever changes take place between defi- nite substances, they never change their combined weight,
P. But the separate weights change?
M. Certainly; but what the one side loses, the other gains. The law only refers to the sum of all the weights.
P. You always taught me, in cases like this, never to ask why it is so, but with what it is connected. Is any- thing known about it?
M. Certainly. You know that weight and mass are in every place proportional. So also the law of the unchangeableness or conservation of mass holds.
P. What is the use of this law?
M. It makes it possible to account for proportions by weight in chemical changes even when you cannot, or do not want to, weigh each substance separately. For example, if I weigh the amount of mercuric oxide I take, and the amount of mercury I get from it, then I know how much oxygen was there too. Because there must always be this equation: Mercuric oxide = mer- cury + oxygen, where the name of the stuff denotes its amount by weight.
P. Has oxygen a weight? It is a gas!
M. Do you think that gases have no weight?
P. I can't believe it.
M, The density, or the relation of the weight to the volume, is small with gases, several hundred times smaller than with water. But they certainly have weight. One litre of ordinary air weighs more than one gram.
90
CONVERSATIONS ON CHEMISTRY.
P. I'd like to see that.
M, I can show you quite easily. Here is a flask of strong glass which I close up with a stopper, in which there is a glass stop-cock. So that it shall not get pulled out, I tie it firmly down with wire or string. Now I shall weigh it all. I can pump air into the flask through the open stop-cock with a bicycle pump. After pump- ing twice, I close the stop-cock, put the flask again on the balance, and it has become distinctly heavier.
P. Can you see how much air you have pumped in?
M. Yes; with the aid of a Httle rubber tubing I con- nect the dcHvery tube from the oxygen apparatus with the stop- cock, put a flask filled with water over the end, and now, if I open the stop-cock, the air which I pumped in will come out, and collect in the flask (Fig. 22). If you
Fig. 22.
had weighed the flask exactly before, and weigh it again now, the loss of weight is the same as the air that has just come out. And if you know the capacity of the flask, you can measure the volume of the air.
COMPOUNDS AND CONSTITUENTS. 91
P. Yes, so you can!
M. After this you may try to measure like this; you will find that air is about 800 times lighter than water. Now we will go back to our experiment. Have you noticed the amount of oxygen I got from potassium chlorate and mercuric oxide?
P. Yes. There seemed to be far less from mercuric oxide.
M. Yes. One gram of potassium chlorate gives far more oxygen than one gram of mercuric oxide. But if I make the experiment twice, each time with one gram of mercuric oxide, what will be the result?
P. Each time it will be the same.
M. And with potassium chlorate?
P. The same.
M. You think, then, that if a substance is changed into another, this always happens according to definite proportions by weight.
P. I don't know whether it is quite definite, but it must be so, more or less.
M. It is exactly so. You could have thought of that before. For a definite substance has always quite defi- nite properties; its capacity, in certain cases, to change into another substance, is one of these properties; it follows that the ratio of the weights of the original stuff and of the product of change must be definite.
P. I should never have had the courage to draw such a conclusion.
M. How can it be proved that such a conclusion is right?
P. By experiment.
M. Right. Now experience has shown for several hundred years that between the substances which take
92 CONyERSATIONS ON CHEMISTRY.
part in any change, roughly speaking, a definite ratio must exist; from a pound of fat an unhmited amount of soap cannot be made, but somewhere about the same, and so on. But it is only in the last one hundred years that this question has been carefully tested and the law found to be quite exact.
P. Does it apply to all substances?
M. To all pure substances; that is, to those that are neither solutions nor mixtures.
P. It is strange. The laws which you have taught me up till now are all very simple and easy to under- stand. But I'm afraid I will never be able to under- stand and use them at the right time.
M. That is only natural. A law is like a tool: if you have had no practice, it is of very Httle use having the tool, even if you know what it is for. But what we are going to talk about later on will give you the necessary practice.
13. ELEMENTS.
M. Last time you learned two important laws, which .show the relation of the proportion by weight of such substances between which chemical changes take place. The one was called the law about the conservation of weight; just say it over!
P. If chemical changes take place between given sub- stances, the combined weight is not changed by it.
M. And what is the other law about?
P. About the proportion of weight in chemical changes. If one substance changes into another, the weight of the one has a definite ratio to the weight of the other.
ELEMENTS. 93
M. Right. It is called the law of constant proportion.
P. But what connects these ratios?
M. That is a sensible question ! I can give you a very wonderful answer for it. But to do that I must first make a new idea clear to you : that of chemical elements. You remember the equation: Mercuric oxide = mercury + oxygen; what sort of quantity was -concerned?
P. That of weight.
M. Now, if you split up a definite quantity of mercuric oxide by heat, and collect the mercury, will it weigh more or less than the mercuric oxide?
P. Let me think a minute. It must weigh less.
M. Why?
P. Because it weighs as much as the mercuric oxide, with the oxygen, and oxygen has weight too.
M. Right. Then if mercury is made into mercuric oxide, or oxygen into mercuric oxide, in each case weight is added: in the one case, the needed amount of oxygen, in the other. case, of mercury.
P. I understand that.
M. You remember also that we called oxygen and mercury the constituents of mercuric oxide, and the latter a compound of the former.
P. Yes.
M. Then it follows that a constituent must always weigh less than any of its compounds.
P. Because something is added each time. •
M. Quite right. Now you can believe that all sorts of chemical experiments have been made with oxygen, like the one you have seen, and that every time the weight of the new substance, which was the result of the con- sumption of the oxygen, was determined. No single instance has been found in which one of the resulting
94 CO^yERSATIONS ON CHEMISTRY.
substances weighed less than the oxygen it contained. All weighed more.
P. Then oxygen can only form compounds ?
M. Yes, and no constituents of oxygen are known. Substances of this sort are called elements. What is an element?
P. A substance, all the products of change of which weigh more than it does itself.
M. Quite right. It can also be said that an element is a substance of which no constituents are known. But this definition is not so clear, because it must first be known what a constituent is.
P. But I learned before that an element was an unde- composable substance'
M. It means the same thing. The changing of a substance into its constituents is called decomposition. Because, out of a single thing, several different ones arise, such a process is called decomposition.
P. Now I understand. But to decompose means to separate what is already there, not to change it.
M. It is like this: If a definite amount of mercury and oxygen has changed, or united into mercuric oxide, it is true that the mercury and oxygen have vanished, but they can be obtained out of it again at any time. And exactly the same amount of each constituent is obtained as was originally there. You can look at it in this way : as if both the constituents in the compound were still really present, and had hidden themselves, as it were, when they combine with each other. Hence the expressions decompose and combine.
P. Yes; which is true then? Are the constituents really in the compound still, or not?
M. You asked that question without thinking. A
ELEMENTS. 95
compound isn't a bag or box in which something can be *'in." If you understand by "in" that by suitable means they can always be taken out of the compound, they are in it. But if you mean that they are hidden away somehow in the compound with all their properties, that wouldn't be clear and would be misleading. You know now what I mean when I say oxygen is an element.
P. Are there more elements?
M. Certainly, mercury is one too. Sulphur, iron, tin, lead, and copper are also elements. There are altogether about seventy- five different elements. Here is a table of elements (see on the next page); if you look through them you will see some friends. But most of them are unknown to you. A great many of them also are very rare, that is, the substances out of which they can be procured are rarely found.
P. Can't the rare elements be made out of other sub- stances which are more frequently found?
M. No, that can never be the case. A given com- pound can only be divided up in one way into elements, that is, from every substance only definite elements can be obtained, and however one may try, the same elements are always found in the same proportions. And to make this substance artificially, just the same elements must be taken in the same proportion, or compounds must be made use of from which these elements can be got, or in which they are "contained."
P. Is that another law of nature?
M. Yes, it is the law of the conservation of the ele- ments.
P. Please explain it a little more.
96
CON VERS A TIONS ON CHEM IS TR Y.
Aluminium Al
Antimony Sb
Argon. Ar
Arsenic As
Barium Ba
Beryllium Be
Bismuth Bi
Boron B
Bromine Br
Cadmium Cd
Caesium Cs
Calcium Ca
Carbon C
Cerium Ce
Chlorine CI
Chromium Cr
Cobalt Co
Copper Cu
Erbium Er
Fluorine F
Gadolinium Gd
Gallium Ga
Germanium Ge
Gold Au
Helium He
Hydrogen H
Indium In
Iodine I
Iridium Ir
Iron Fe
Krypton Kr
Lanthanum La
Lead Pb
Lithium Li
Magnesium Mg
Manganese Mn
Mercury Hg
Molybdenum Mo
Neodymium Nd
Neon Ne
Nickel Ni
Niobium Nb
Nitrogen N
Osmium Os
Oxygen O
Palladium Pd
Phosphorus P
Platinum Pt
Potassium K
Praseodymium Pr
Radium Ra
Rhodium Rh
Rubidium Rb
Ruthenium Ru
Samarium Sa
Scandium Sc
Selenium Se
Silicon ■ Si
Silver Ag
Sodium -. Na
Strontium Sr
Sulphur S
Tantalum Ta
Tellurium Te
Terbium Tb
ThalHum Tl
Thorium Th
Thulium Tu
Tin Sn
Titanium Ti
Tungsten W
Uranium U
Vanadium V
Xenon X
Ytterbium Yb
Yttrium Y
Zinc Zn
Zirconium Zr
M. You know that some time ago there were chemists who gave their whole hfe trying to make gold or silver out of lead or other cheap metals, without one of them succeeding; they were called alchemists. Now, the whole of alchemy was built upon the hope that it was possible to change one element into another, perhaps lead into gold. It could not be foretold that this was not possible; it was only by resultless efforts continued
ELEMENTS. 97
through centuries, that it was found to be impossible in the case of gold and silver, and, later, in the case of all other elements.
P. Then the gold-making wasn't so mad and useless in the long run?
M. Neither the one nor the other. It wasn't mad, because it became known that it couldn't be done. Only the gold-makers didn't work scientifically, that is, in an orderly manner, because they only tried things on chance. And the final result — that elements could neither change into each other, nor the compounds of definite elements into the compounds of other elements — was an important scientific discovery, which made the study of chemistry far easier.
P, I don't understand that.
M. Just suppose that if we provide each element with a definite sign, then we can mark every compound by putting the signs of their elements together. Just as you make the word "hat" out of only the signs h, a, and t put together, and it can only be divided up into these signs, and you can never build up the word "rose" out of these signs, so compounds and elements act in the same way. In the table of elements (page 96) there is a sign like that, against every name, that is made from the first letter of the name, and generally a second letter as well. Every substance that is on the earth can be represented by placing together such signs, for however many substances there are, every one of them can be decomposed into elements only in its own par- ticular way.
P. I see, it is again one of those laws which are really very simple, only you must be accustomed to them first.
ikf. You will soon get accustomed enough to them.
gS CONyERSATIONS ON CHEMISTRY.
In the mean time we will take our table of elements and see how much chemistry you know already from daily life. Oxygen you know already; it is a colourless gas. Hydrogen is a colourless gas too, but combustible.
P. What is hydrogen?
M. An element that can be obtained from water.
P. Then isn't water an element?
M. No, it isn't in the table. It is a compound of oxygen and hydrogen. You know something about nitrogen too; it is the other ingredient in the mixture of ordinary air. It is also a colourless and tasteless gas.
P. Yes, because air is.
M. Right. Now comes carbon. It isn't a gas, but a solid body. Ordinary charcoal consists of carbon, but not in the pure state. These four elements are always in all living things, plants as well as animals, and as such form a definite group. That is the reason I named them first to you. Moreover they are the type of four different groups of other elements.
P. What does that mean?
M. Among the other elements there are a number which behave in the same way as oxygen, while others are more like hydrogen, others like nitrogen, and again others like carbon.
P. What do you mean by "like"?
M. They have to some extent similar physical prop- erties in an uncombined condition as so-called free elements. In many cases also the compounds which are formed with a third or fourth element are like in their properties.
P. That doesn't appear to me a definite reason foi classifying them.
M. Neither it is. But by taking into consideration
ELEMENTS. 99
all the properties of all the compounds which can be produced from an element, so many resemblances and differences turn up that a chemist who knows the rela- tionships doesn't find the choice difficult. As you don't know them yet you must simply accept my classification.
P. But it appears to me to be unscientific to accept anything that I can't prove.
M. You will be able to prove it when you have learnt enough chemistry. Besides I won't use the classification for any scientific conclusion, but only for your own con- venience, so that you can learn the facts more easily; besides, such arbitrary things can be treated in science in an arbitrary manner.
P. Yes I see.
M. Now impress the following names on your mind:
|
* Hydrogen |
* Oxygen |
* Nitrogen |
* Carbon |
|
* Chlorine |
* Sulphur |
* Phosphorus |
* Silicon |
|
* Bromine |
Selenium |
Arsenic |
Titanium |
|
* Iodine |
Tellurium |
Antimony |
Later on we will study carefully only those elements marked with an asterisk.
P. Why only these?
M. The others are either too seldom found in nature, or their compounds have too little importance in their applications. As we can't learn nearly all that has been found out up to the present in chemistry, we must be satisfied with a selection. I arrange this so that at any rate you learn the substances which on account of their uses, or on account of their sources, come most frequently before our notice.
P. Then I am only to learn a little part of chemistry?
M, There are very few people who know every fact that has been proved in chemistry up to the present. I shall try to teach you those parts of chemistry that wiU
loo CONVERSATIONS ON CHEMISTRY,
give you the best conception of the most important relations. Later you can take up a special branch, which you can learn as thoroughly as you wish. But now we will speak about the elements we have chosen. I have already told you about hydrogen, that it is a colour- less, combustible gas; but its flame is quite pale and gives very little light. It is the lightest substance there is, and for thai* reason is used for filling air balloons.
P. Is there hydrogen in the little red india-rubber balloons that children play with?
M. Certainly, and if one of these freshly filled balloons is set fire to, the hydrogen burns with a puff.
P. I will try that next time.
M, But don't hold it too near your face, or you may burn yourself, for the flame is hot, and it often goes off with a tremendous bang. — Chlorine is a greenish gas, with a very unpleasant, pungent smell. Perhaps you have already smelt it, because a white powder called chloride of lime is often scattered on unpleasant-smelling decomposing matter; its smell is that of the greatly diluted chlorine.
P. Yes, I remember; our boy always strews it at the street corner. Why do people do that?
M. The chlorine destroys the bad-smelling substances and kills the offensive little germs or mould or bacteria. — Bromine is at ordinary temperature a deep reddish- brown coloured liquid, and has a yellowish-red vapour which smells the same as chlorine.
P. Ah, then that is one of those resemblances of which you spoke.
M. Yes. Iodine smells like it too, only at ordinary temperature it is a solid, shiny, black substance, the vapour of which is violet.
ELEMENTS. loi
P. I remember that my throat was painted with tinc- ture of iodine once. Has that anything to do with the element iodine?
M. Yes, it is a solution of iodine in spirits of wine. With that we finish the first group. Of the second you already know oxygen. And sulphur is familiar to you too.
P. The yellow stuff?
M. Sulphur is a solid substance of a yellow colour, and burns with a blue flame.
P. And in doing so gives off a very bad smell. Why do most substances in chemistry smell so queer and un- pleasant ?
M. The bad-smelling substances are mostly those which have a corrosive effect upon the inner skin of the nose. If they didn't smell badly, we wouldn't notice anything, and we would always have a sore skin and a cold in our noses. Chemistry would be a far more dangerous thing to work with than it now is.
P. Ah, that is good. Do all poisonous substances smell nasty?
M. First of all, we can only smell those substances that change into gas or vapour, because otherwise they would never reach our noses. Fortunately most poison- ous substances have a bad smell, especially the corrosive ones. Still there are some poisonous gases and vapours which have none, or only a very faint smell. They are especially dangerous. We will learn about one of these gases later on.
P. Then I'll take care.
M, We will go now to the nitrogen group. You know a little about this already. It is not poisonous, because we breathe it together with the oxygen in the air. But in pure nitrogen, without any oxygen, animals must die,
I02 CONVERSATIONS ON CHEMISTRY.
because they require oxygen to live. You know some- thing about phosphorus too.
P. Yes, it is in the heads of matches.
M. Right. From that you know one of its properties. It catches fire very easily; even the heat resulting from friction makes it do that. That is why it is used in matches.
P. I saw in the dark lately, that the heads of matches shone; there was a pale-green light, and the cook told me it was because the matches had become damp. How is that possible?
M. Phosphorus bums slowly if it is left in the air, and in doing so shines as you saw. So that the small quantity of phosphorus, which is contained in a match's head, shall not burn of its own accord, the phosphorus is mixed with gum, or hme, which dries, and forms a covering that keeps oxygen out. In the damp this covering is dissolved, and the phosphorus comes in contact with air.
P. Yes; but when I wet a match in the room later, it didn't shine.
M. That must have been a so-called Swedish match; they have no phosphorus in their heads.
P. What does phosphorus itself look like?
M. Almost like wax. It is kept under water, because it burns slowly away in air, as I said before. Since it is very poisonous, it is better I should not give it to you in your hand.
P. How is it made?
if. You think you could make it for yourself without my permission! No, that isn't so easy. It is one of the ingredients of bones, and is separated in a rather comphcated way.
LIGHT METALS. 103
P. How can it be in bones if it is so poisonous?
M. Phosphorus as a free element is poisonous, but its compounds are not. There you have another exam- ple of how different elements and their compounds can be. — ^Now we come to the last group. Besides carbon, which you already know, you must learn about silicon.
P. Does silicon come from the Latin silex, flint?
M. Yes; flint consists of a compound of silicon and oxygen; it is usually called silicic acid. Quartz, sand- stone, rock crystals, and flint consist of it. Finally, almost all rocks are compounds of sihca, so that the element silicon is one of the substances that are found in the greatest quantity on the earth's surface. — Now that will do for to-day. I will only say that the elements mentioned now go under the name of non-metals. They form the larger division of the elements; the other consists of metals.
P. I think I've learned a great, great deal to-day.
M. That was only a walk through our future work. The real learning comes later.
14. LIGHT METALS.
P. How many different sorts of metals are there?
M. About sixty. As we do not know enough about some, the number is rather uncertain.
P. But how can you find your way among such a large number?
M. In the same way that you can find your way amongst the much larger number of animals and plants: they are divided into groups, in which those which resemble each other are put together.
tC>4 CONVERSATIONS ON CHEMISTRY.
P. They do that with animals and plants according to their shapes and organs; that can't be done with metals.
M. That is not quite right; the crystals which form when different elements are in their solid state show some resemblance, like the shapes of plants and animals. But metals have other properties which are remarkably different among each other, while organized beings resemble each other pretty closely; those are their chemical properties or their capacity to form compounds with other substances. Besides that, their physical properties, lustre, colour, density, hardness, and so on, are very different.
P. Then I must know the properties of all the elements I am to learn about, if I am to understand and remember their classification.
M. You need to know first of all only those which lead up to, and complete, the classification. At present you only need to know that the elements which I place in one group possess definite resemblances in their properties.
P. Yes, that is true. What properties are the basis of classification?
M. They are very different. It happens that the groups which have been placed together because of one definite property are almost always those which would be made because of other properties. So the present usual grouping is the result of quite a number of these selections of properties. Those which, in each group, have similar properties will be explained to you separately later.
P. So there is a perfect order?
M. Almost, to the same extent, as there is order among plants and animals. There, too, there are doubtful
LIGHT METALS. 105
points, either because the difference is too little or because different methods of classifying lead to varying classi- fication.
P. But it can't be that in such unchangeable things as the properties of elements there can be contradictions ?
M, There are no contradictions in the properties, but the irregularities of the somewhat arbitrary arrange- ment that we have made —
P. Yes; then why isn't everything simply arranged as in arithmetic or geometry ?
M. For this reason: we have only incomplete know- ledge of the properties of the elements. Most of our experiments, for example, are made at temperatures which are not very different from that of a room, and under ordinary atmospheric pressure. Our conceptions of the properties of the elements would be quite different if we knew how they were affected by all sorts of pressures and temperatures.
P. Then the imperfection of the classification is only due to the incompleteness of our knowledge?
M. That is quite possible, for experience has shown up till now that a department of science becomes clearer and more easily surveyed, the more exact and all-embrac- ing our knovdedge becomes. But now we will go back to our subject. We will divide metals into light metals and heavy metals.
P. What is the meaning of light metals? All sub- stances have weight, and so are heavy.
M. Quite right. Those metals whose density is less than four times that of water are called light.
P. Why was four made the limit?
M. Because the other properties of metals are such, that by making a limit here, it made their differences
106 CONVERSATIONS ON CHEMISTRY.
most clear. This is a case of the mutual help of dis- tinguishing marks that I mentioned before. — Light metals fall into three groups: the alkali metals, the metals of the alkaline earths, and the metals of the earths. These groups contain the following important elements:
|
Alkali Metals. |
Metals of the Alkaline Earths. |
Metals of the Earths. |
|
Sodium |
Magnesium |
Aluminium |
|
Potassium |
Calcium |
P. But those are very few.
M. They are by no means all. But I won't mention the others just at present, because either they are so seldom found, or have so little importance in their uses, that you needn't bother yourself about them just now.
P. Is the aluminium which you have named the well- known white metal?
M. Yes, and if you have had a piece of it in your hand you will remember that it is extraordinarily light. It is, in fact, only 2.7 times heavier than water.
P. Yes. Aluminium really is a light metaL But is it true that it is made out of earth?
M. It is half true; only earth is not a definite chemical substance, but an accidental mixture of all sorts of rocks and their products from decay and time. But in nearly all rocks and earth aluminium is found in the form of a compound with oxygen. The different sorts of loam and clay especially contain the element aluminium.
P. Ah, that is why it is called a metal of the earth. But if it is so common, why is it so expensive?
M. It isn't so very expensive now; one pound costs about twenty-five cents; that it is so much more expensive than the substances it is obtained from is because it requires a great deal of work to separate it from its
LIGHT METALS. 107
compounds. It was hardly known before the electric cur- rent began to be used. The difference of price between aluminium and its compounds, then, shows the greater amount of work or energy which is contained in the element aluminium, than in the compounds from which it is prepared, and as you know work is never given as a present.
P. Can you get the work out of the aluminium again?
M. Certainly. Here is a mixture of aluminium with a compound of iron, iron oxide, which you already know. If I light this mixture an immense amount of heat is given off, the mixture glows white hot, the metal iron is set free, and all sorts of welding and melting can be done with the hot mass.
P. That is a pretty experiment. How was the mix- ture made?
M. Aluminium powder and iron oxide are mixed in the proportion of i to 3. Both substances must be thor- oughly dried beforehand by heat. The lighting is done with a small piece of magnesium ribbon (you will soon learn about magnesium itself), which is made to burn by means of a match. The mixture is placed in a clay crucible, or in a cavitj, which you can make in a dry brick.
P. What happens exactly?
M. Iron oxide, as you knew, is a compound of iron and oxygen. If aluminium comes in contact with it when hot, it unites with the oxygen, and the iron is separated; as through the uniting of oxygen with alumin- ium much more work is set free than is necessary to separate oxygen from iron, a great amount is left over, which appears as heat.
P. Is work the same thing as heat?
toS CONVERSATIONS ON CHEMISTRY,
M. In so far as the one can be changed into the other. Vou can tell that work changes to heat because, by fric- tion, heat is obtained. And to overcome friction, work is necessary.
P. Yes, now I know. And a steam-engine makes work from heat.
, M. Right. But now we must go back to our light metals. Of the metals of the alkaline earths you prob- ably already know magnesium.
P. Isn't it magnesium that burns so brightly?
M. Yes, magnesium is a light white metal, which can be lighted, and burns with a bright flame. It is used when a bright light is required and no electrical current is at hand. For that purpose it is generally made in the form of a narrow strip or ribbon. Here is such a piece of magnesium ribbon that is brought in this form into commerce. I light it, and you see how brightly it burns.
P. What is the white ash and the white smoke that comes from it?
Af. That you ought to know for yourself. What is combustion ?
P. A combination with oxygen. Then is the white stuff an oxide of magnesium?
M. Yes. And the strong light is another example that in this combination between oxygen and magnesium there is a great deal of surplus work which shows itself as light and as heat.
P. Then is light a sort of work?
M. Yes, certainly. You know that plants grow and increase in light and form wood, leaves, and so on. The wood you can burn and obtain heat from, as a sign that there is work in it. This work has come from the Hght of the sun, because plants can only grow in light.
LIGHT METALS, 109
P. Where is magnesium found?
M. Like aluminium it must be obtained from its compounds by means of electric work. In nature, compounds of magnesium, generally with oxygen, occur in very large masses. Dolomite, which forms large mountains, is rich in magnesium compounds; they also occur in nearly all rocks.
P, What is magnesia that is used as medicine? Has it anything to do with the metal magnesium?
M. Yes, it is magnesium oxide, the same substance which is formed on burning the metal. The medicine, Epsom salts, is also a compound of magnesium. All these substances you will learn more about later on.
P. I should really like to have heard more about magnesium: there are so many sorts of things connected with it.
ilf . You will find the same thing with other metals. Calcium, for example, as a metal, is very little known, because it takes far more work than magnesium does to separate it from its compounds, and it bums far more easily.
P. Why should I learn about it now?
M. Because its compounds are extraordinarily exten- sive; it belongs to the elements in which the earth's surface .is richest. Limestone, of which large mountains and countries consist, is one of its compounds; chalk and marble are the same compound in rather different forms.
P. But chalk, marble, and limestone are surely different !
M, Yes, in their outward appearance. But if I take a small piece of the three substances, and pour hydro- chloric acid over them, they behave in the same way;
no CONyERSATIONS ON CHEMISTRY,
they froth up and let a gas escape. And the resulting solutions give, in the same way, a white precipitate if I add dilute sulphuric ^cid. And there are a great number of other reactions which always occur which- ever of the three minerals I use. Their difference is only that dialk consists of far smaller particles than the other two, and that limestone contains additional impurities, which make its colour appear grey. But marble also often contains impurities and appears red, sometimes black. So the three are only different physically; chem- ically they are the same.
P. Are there other compounds of calcium?
M. Innumerable. By pouring water on the burnt lime which is got by strongly heating limestone, it heats itself and swells up, and with more water forms milk of lime, which, mixed with sand, is used as mortar. Gypsum and cement are also compounds of calcium.
P. I'd like to learn more about them too.
M. Again, you must wait till later on for them; other- wise we won't get through our talk. Now we have still the first group — the alkali metals — to consider. Look, in this glass there is sodium.
P. It looks white like silver. But why is the glass sealed ?
M. Because sodium combines even at ordinary tem- perature with the oxygen of the air. As no air can get in through the glass the metal remains unchanged, and its white colour and silver appearance can be recognized. These grey pieces are also sodium.
P. But they look quite different!
M. That is only on the surface, where the compound of oxygen has formed. If I cut off this layer with a knife, the shining metal will be exposed.
LIGHT METALS, III
P. But it will soon be grey again!
M. Yes, it will combine with the oxygen in the air.
P. What sort of liquid is the piece of sodium in?
M. It is ordinary petroleum. I told you before that it was made of hydrogen and carbon; it contains no oxygen. That is why sodium can be kept in it, and is protected from forming a compound with oxygen.
P. Then can sodium take oxygen out of a compound?
M, Certainly. I throw a piece of sodium into water. It becomes hot, melts, and the ball dances about on the water, always getting smaller. Now take care, a little explosion will follow. See, now it is over, and all the sodium has vanished.
P. Where has it gone?
M. It has united with the oxygen in the water, and has become an oxide, which has dissolved in the water.
P. Is this oxide found in nature?
M. No, it can only be artificially made. But there is another compound that is found in nature. Ordinary salt is a compound of sodium.
P. With what?
M. With chlorine.
P. That can't be true, surely.
M. Why not?
P. Sodium is such an acrid stuff, and chlorine too, and yet their compound makes common salt which we can eat.
M. You have guessed wrong again, as if the ele- ments were contained as such in their compounds. That common salt is a compound of sodium and chlorine tells you no more than that salt can be made with both, and vice versa, both elements out of salt.
p. Is that really possible?
112 CONVERSATIONS ON CHEMISTRY.
M. You shall see it for yourself later on.
P. I can hardly wait to see and learn about all these wonderful things.
M. At present we must speak about the last light metal potassium. Here is a glass tube with potassium.
P. It looks just like sodium.
M. Yes, and behaves in a similar way. If I take a piece out of the oil where it is kept and throw it into water the effect is so strong that a violet flame is the result.
P. Then potassium won't appear as a metal in nature ?
M. No! if there had been any to be found, it would have seized all the water to be had, and changed into a compound with oxygen.
P. What are the compounds of potassium ?
M. There are a great many. Among the substances that you know I will name saltpetre. Further, potassium is an ingredient of many minerals. Ordinary red felspar contains potassium. There are potassium compounds in the earth from rocks, and they are taken up by plants, which need potassium to enable them to live. For that reason potassium compounds are found on the ashes of plants. They remain behind on burning, as they are not volatile. They can be separated from the ashes with water, and 'by evapoilating the water, are obtained in solid form. The white, salty-looking substance which is so obtained is called potash.
P. I would like to make that.
M, It is quite easy ; you only need to stir up wood-ash with water and pour it through a filter (see page 15). Then a clear liquid runs through, which tastes like soap, and leaves a white or grey salt behind, if it is put in a saucer in a warm oven. But take care that you use
HEAVY METALS. 1 13
only the ash of wood, and not that of coal, because that doesn't contain potash.
P. I have learned so much to-day that I'm afraid I shall never remember it all.
M, All that we have been speaking about will come again later on when we learn the compounds of separate elements. To-day I only showed you that you know quite a lot of chemistry, that is to say, many substances which you have noticed in daily life. You must certainly gain first orderly knowledge of substances and their behaviour, that is, real scientific knowledge.
P. I shan't fail for want of diligence and attention.
15. HEAVY METALS.
M. To-day we begin to talk about heavy metals. Among these are the ones that have been longest known, such as copper, gold, tin, lead, and iron.
P. Why were just these known first?
M. Gold is found as such in the earth. Copper, tin, and lead are very easily separated from their ores, so that it was possible to obtain them at an early epoch without any great experience or skill. Iron came into use much later, as it was more difficult to obtain. But we will make a table again. And here also I will only bring before you the most important metals:
|
Iron |
Nickel |
Copper |
Silver |
Gold |
|
Manganese |
Chromiutn |
Leac |
Tin |
Platinum |
|
Cobalt |
Zinc |
Mercury |
P. I know almost all of these.
M. You won't know much about manganese. It is a metal that is very like iron, and you have learned
114 CONIFERS AT IONS ON CHEMISTRY.
about its compound with oxygen in one of our earlier experiments. It is the substance which we used when preparing oxygen from potassium chlorate to make it come off more easily.
P. Cobalt is blue; is it an element too?
M. No, the blue colour is that of a compound of the element cobalt. Cobalt is also like iron, but keeps better in air, and doesn't rust like iron. You know nickel?
P. Yes.
M. Some coins are made of nickel. Besides, cook- ing-utensils are made of it. The metal is far whiter than iron, almost like silver, and remains bright in damp air without rusting. It is hard, and is difficult to melt. For that reason it is a fairly valuable metal.
P. What happens to iron when it rusts?
M. It combines with the oxygen of the air and with water. Therefore iron keeps much better in dry air than in damp air.
P. What does nickel-plating mean?
M. It means covering over with nickel. With the help of an electric current the metal can be deposited from solutions of nickel compounds on to any sort of metal object. As nickel keeps so well in air, these covered or nickel-plated objects keep better than without this covering.
P. I don't know chromium at all.
M. I won't tell you much about this element yet. It is whiter than iron, very hard, and melts with great diffi- culty. Many of its compounds are brightly coloured and so are used as colours for pictures and painting. But you know zinc ?
P. Is it the white or light-grey metal of which roof- gutters and whole roofs and bath-tubs are made?
HE/tyy METALS. 115
M. Yes; it is much softer and more easily melted than the other metals which were mentioned before. — We now come to the copper group. You already know that metal quite well.
P. Yes, and I know lead too ; it is so heavy.
M. Its density is*ii.4. It melts very easily, and is soft. Most metals with low melting-points are soft.
P. And vice versa ?
M. No, it doesn't hold the reverse way. Gold and silver are fairly soft, but have a high melting-point. But it holds again for tin: tin is rather soft.
P. And it can be very easily melted. We did it on New Year's day, and poured it into water. What made the crinkled shapes that we got?
M. You should be able to answer that for yourself. Tin melts at 235°. What will happen if you pour melted tin into water?
P. The water will begin to boil. Now I understand it: the water makes steam, and swells the Hquid metal.
M. Right. And it hardens when it comes in contact with the remaining water. — ^What do you know of mer- cury?
P. That it is liquid at the ordinary temperature.
M. It is the only metal that has this property. It isn't, however, the only liquid element, for bromine at ordinary temperatures is also liquid. — You know silver too?
P. Yes, from silver coins and teaspoons.
M. Mercury and silver are counted as precious metals, and so are gold and platinum in the next group.
P. Why are they called that? Because they are so expensive ?
Il6 CONVERSATIONS ON CHEMISTRY.
M' Not exactly for that reason, as there are other much rarer elements, which are much more costly, and yet are not called precious. No, they are called so because they remain bright when heated, and don't become black and ugly like other metals.
P. But why?
M. That you must answer for yourself. I have already told you what happens to iron when it is heated in air.
P. Yes, it combines with oxygen, and the other metals will do the same. Can't the precious metals combine with oxygen?
M. Certainly. Their oxides are also known. But they have the property that when heated they decom- pose into metal and oxygen. I showed it to you once before with mercury.
P. Oh, so that is why their oxides can't be formed by heating the metal, as they would at once decompose.
M. Right. It requires work to make these metals combine with oxygen, and heat alone can't perform this work.
P. Do the precious metals form no compounds?
M. Yes, some can be obtained if the precious metals are treated with substances which yield work on com- bination. Sulphur, for example, does so with silver and mercury.
P. Can I see it?
M. Certainly. I put a drop of mercury in a mortar and add some sulphur to it. Then I rub both together. What do you see?
P. It is all becoming black. Now there is a fine black powder, like soot. What is it?
M. It is a compound of sulphur and mercury. In
MORE ABOUT OXYGEN. li7
the same way silver can be combined with sulphur. Rub some sulphur with a cork on a silver coin.
P. The silver is becoming brown and blackish grey.
M. There again is another combination of both elements. Both metals unite directly in the same way with chlorine, bromine, and iodine.
P. Aren't these precious, then?
M. No. But gold and platinum are still more precious, as they don't combine with sulphur by rubbing them together.
P. Don't they combine with anything?
M. Yes, they can combine with chlorine. But* this compound decomposes into elements again on heating, just as you saw with mercuric oxide. We will stop with that for to-day.
P. Chemistry is a tremendously large subject.
16. MORE ABOUT OXYGEN.
M. To-day we will learn more about oxygen.
P. I know about it already.
M. Only ver)^ superficially, for you know only a very small part of what is known about it. And what I am going to tell you is only a little part of what is known.
P, But you know all about it?
M. No, I don't think there is a single man who really knows all that is known about oxygen.
P. 1 don't understand that. If no one knows it, then it isn't known.
M. One man knows one part, another man knows another, so that the knowledge is present in somebody's
1 1 8 CONyERSA TIONS ON CHEMISTR Y,
mind, but not all in the same person's. Besides, almost all is to be found in books, and is accessible for every one who wishes. From time to time a man is found who discovers as much as possible about it, and puts it all together in a particular book, to save others the trouble of searching. But he can only give extracts, and so one who for some reason wishes to learn exactly what is known of the subject must look over the books himself, or by experiment arrive at the desired knowledge.
P. Is everything right that is found in books?
M. Most of it ; and when there is anything wrong, it is no intentional error, but the author for some reason made a mistake. A most remarkable and praiseworthy thing in scientific literature is that almost every word is written conscientiously.
P. But if someone has made an oversight and written something wrong, the error would remain there forever.
M. Only until it is contradicted by some other fact that is found. Then it is seen on which side the fault lies, and after that one can generally find out how the error came. But now we will go back to oxygen. You remember how we made it before?
P. Yes, from a white salt. What is it called?
M. Potassium chlorate. It contains about two fifths of its weight of oxygen, which it gives up when gently heated, especially if a little oxide of iron or of manganese be added.
P. You told me that already (page 114) but it strikes me as so remarkable that I should like to see it. Can you show me how iron oxide makes it easier for the oxygen to come off?
M. Certainly. I am melting a Httle potassium chlorate in a test-tube. What do you see?
MORE /iBOUT OXYGEN. 1 19
P. It melts; now it has become as clear as water; now quite small bubbles are rising.
M, These are traces of oxygen. Now I take the lamp away from the glass, and add a little oxide of iron to it.
P. It froths like soda-water. Does the salt begin to boil?
M. No, oxygen comes off suddenly. If I put- in a burning splinter, it catches fire. You know that is the test for oxygen. You see that even though the salt has cooled a little on taking away the flame, the oxygen comes off much more quickly on adding the oxide of iron.
P. That is really very curious. Why does it happen?
M. The oxide of iron has acted like oil on a rusty machine, or like a whip on a horse.
P. I don't understand that.
M. You are not the only one. It is a fact that many chemical processes which go very slowly of themselves can be accelerated by adding other substances to them, even though the added substances undergo no permanent change. The investigation of the question why these accelerations, which are ascribed to catalytic action, really take place is a difficult scientific problem, and perhaps in a few years I may be able to give you an answer. In the mean time we will use this fact as a convenient help.
P. When I know more I'll try to find out the reason of catalytic actions.
M. That is a good plan. But now we will make some oxygen. You know already how it is done. First I will place this flask filled with water here, for we must first expel the air from the flask by oxygen before I col- lect it.
120 CONyERSATlONS ON CHEMISTRY.
P. But you will lose some oxygen in that way.
M. That doesn't matter ; if we want it pure, we must make up our mind to that. You will always meet with that same difficulty in future. Now I begin to heat, and you see that soon bubbles rise out of the glass tube. Now place the flask on the stand, but take care that you always keep its mouth under water or else air will enter.
P. How quickly it's going!.
M. Yes; it will be better to take the flame away for an instant. Now fill an empty flask with water and have it ready.
P. But how can I turn it upside down without letting the water run out?
M. Hold your thumb on the mouth.
P. My thumb is too small.
M. Then take your hand or a piece of cardboard, or anything flat. The best thing is a cork that fits it.
P. Now the first flask is full of oxygen.
M. I close it under water with a cork, and can take it out and put it aside.
P. Why do you put it upside down?
M. Because generally the cork doesn't fit tight, and the water then prevents the oxygen from coming out. Now the second flask is nearly full; get another flask ready.
P. I didn't think that so much oxygen could come out of so small a quantity of salt ; the sixth large flask is half full, but it has stopped coming now.
M. Yes. Now we will take the tube out of the water; if we didn't, the water would rise up into the flask and break the hot glass.
P. What a lot of things there are to think about!
M. Yes, the art of experimenting is not to require
MORE ABOUT OXYGEN. 121
to think about such things, but to do them involuntarily. Now we will do what we had to put off before; we will calculate the density of oxygen.
P. Calculate? But we must first measure it.
M. The measurements are already made. I used lo grams of potassium chlorate; it contains about 4 grams of oxygen; more correctly 3.9. Our flasks are each half a litre or 500 cubic centimetres in capacity, as you can see by the 500-mark which is stamped upon the bottom of each. We have thus collected somewhat less than 3 litres of oxygen, so each litre weighs in round numbers 1.3 grams, and each cubic centimetre 0.0013 gram, so (see page 48) the density of oxygen is equal to 0.0013.
P. I shouldn't have thought the calculation could have been made so easily.
M. It was easily made, but it was not exact. I wanted to show you how to arrive at a knowledge of such values. It wasn't my intention to make an accurate measure- ment.
P. One thing more. You said that the weight of the oxygen from 10 grams of potassium chlorate was 3.9 grams, but not how you found it out.
M. That's not difficult. You weigh the test-tube with the chlorate before heating, and then afterwards.
P. I see it now. The loss of weight is equal to the weight of the oxygen that has come off.
M. Yes. Here you have an application of the law of the conservation of weight.
P. So I have used a law of nature without knowing it. What is the use of stating these laws of nature when you use them without knowing them?
M. It was an accident that you used it rightly. It is just as easy to make a wrong use of them, and in order
122 CONVERSATIONS ON CHEMISTRY.
to avoid that the law must be expressed and used inten- tionally. This is troublesome at first, but later on, if my teaching makes the right impression on you, whenever you learn anything new, you will find it necessary to state it as a law of nature.
P. I don't think I shall ever get as far as that.
M. We mustn't forget that we are speaking of oxygen all the time. When we collected it over water, did you notice anything strange?
P. I don't think I did.
M. The bubbles of oxygen rose through the water without diminishing in size. That is a proof that oxygen is insoluble, or very sparingly soluble in water.
P. Can gases dissolve in water, then?
M. Certainly. You have an example in soda-water. As long as it is in the bottle it looks quite clear, but when you pour it out a quantity of gas escapes which was dissolved before.
P. Yes, I have seen that. But why does the gas escape when you pour it out?
M. Gases dissolve iri water and other liquids more readily at high than at low pressures. In the bottle the solution is at pretty strong pressure, and when the bottle is opened the pressure is reheved, so that the gas escapes.
P. Ah, that is the reason why it pops and foams. What sort of gas is it?
M. It is carbonic acid gas, the same gas which is produced when carbon burns in air or oxygen. We shall get to know more about it afterwards.
P. Then we ought to be able to make carbonic acid gas out of smoke.
M. That doesn't work; for in smoke the gas is mixed
MORE ABOUT OXYGEN. 123
with much nitrogen of the air, and besides, it contains disagreeably-smelling stuff from the coal.
P. I only meant it as a joke.
M. But the proposal is quite a sensible one. If carbonic acid gas were an expensive gas, it would be worth considering whether it couldn't be separated from the mixture and purified. But because such a separation costs trouble and money, the question is asked, can it not be made in a cheaper way? The answer to that question is the foundation of the chief part of chemical industry. But we will go back to oxygen. It is very sparingly soluble in water; while water dissolves its own volume of carbonic acid gas, it dissolves only about a fiftieth of its own volume of oxygen.
P. But if it is more strongly compressed?
M. It remains the same. If a gas is compressed more strongly, more goes into the same volume, and water dissolves exactly as much more. On the other hand, the proportion varies with the temperature; the higher the temperature the less gas dissolves. What do you notice when fresh spring-water is allowed to stand in a room for some time?
P. Do you mean the little bubbles of air that stick to the side of the glass ?
M. Yes, that is what I mean. When the cold water which is saturated with gas warms up, part of it must escape, and it does so in the form of bubbles, which gradually grow larger, and finally separate and rise. We have learnt something about the behaviour of oxygen when it is kept in a flask by itself, and when it is brought together with other bodies. Now we will get to know about it in the free state.
124 CONyERSATIONS ON CHEMISTRY.
P. I am curious to hear about that.
M. You know that it is a constituent of air, and indeed the most active. The other constituent is called nitrogen, and animal life cannot survive in it, nor can flames burn in it, but go out. As air penetrates everyv^here, so oxy- gen can penetrate everywhere, and it