Now tell me, when you set fire to a bit of paper, how long does it take to burn?

Half a minute, at the utmost, you answer.

Very good. And how long does it take to produce that rust-stain, even though it is probably not a hundredth part the size of the paper?

Two or three days, is your reply, for so I told you my self.

Here is a strange difference indeed; but from it you may discover why you have not seen any signs of rejoicing or illuminations at the iron wedding. These are always in proportion to the quant.i.ty of oxygen which is being married at once--and this was--oh, such a slow affair! When the quant.i.ty is very small indeed, the festal illuminations are very small indeed too, and in fact escape observation altogether. In the same way that you would not be conscious of little bits of thread laid delicately one after another on your back, whereas you would plainly feel a large sheet, were it to fall on your shoulders. Yet what is the large sheet but a great quant.i.ty of little bits of thread? Only in that case they would all come upon you at once, like the marriage illuminations of burning paper.

Wait a little longer and we shall finish.

What is there, then, in the paper which pleases the oxygen so much that he unites himself to it so readily, and in such large quant.i.ties?

What is there? Two substances of high degree, who have actually risen to the dignity of a royal alliance, by the important part they play in the world; one of these, charcoal or _carbon_, we know quite well already; the other I have only mentioned to you in connection with water, HYDROGEN. Thanks to gas companies, everybody in these days knows _hydrogen,_ at least by name. But before proceeding, I will just tell you that it is by far the lightest body that is known. It is forty and a half times lighter than air, which is not very heavy itself, although in the ma.s.s it has its weight, as we have seen.

The true province of hydrogen is water, where it keeps house with oxygen, in proportion of one to eight pounds, as you may remember I stated in my last letter. But beside this, _hydrogen_ and _carbon_ are in a manner inseparable friends, whom one invariably meets side by side in all animal and vegetable substances. In wood, coal, oil, tallow, and spirits of wine; in everything in short that we call _combustibles,_ because the name of _combustion_ has been given to this marriage of oxygen with other bodies, hydrogen and carbon keep themselves shut up very discreetly and very quietly; like two children playing at hide-and-seek. You have sometimes played at hide-and-seek yourself, no doubt? Now, if some naughty child had come behind you with a lighted candle, what would you have done? You would have had to turn out, whether you liked it or not, and be caught. Well! this is what happens to our two friends, when you bring the paper to the fire. The heat forces them out, and the oxygen, which is always at hand, seizes upon them. In a twinkling they are married, and a beautiful flame springs up into the air, which lasts till everything has disappeared.

Hydrogen and carbon! These, then, are the two great combustibles, the two parents of fire; and as nature has lavished them upon us in what we may call inexhaustible quant.i.ties; when you hear people lamenting and saying that wood is disappearing, that coal is diminishing, and that the human race will end by not knowing how to warm themselves, do not disturb yourself in the least.

There is more hydrogen in a bucket of water than is wanted to cook a large dinner. There is as much and more carbon in our stone quarries than in our coal pits, and when all the woods in the world are cut down (which I trust will never be!) do you know what we shall do? Why, we shall take to burning the mountains. The Jura mountains in Switzerland, for instance, (to take the most favorable case) are great ma.s.ses of carbon, without its ever being visible. Everything depends upon knowing how to make it come out of its hiding place; but that will de done when it is wanted: more difficult matters have been accomplished already. As to oxygen, whether carbon comes to him from a log of wood or from a building stone; whether the hydrogen comes from a candle or a gla.s.s of water, is a matter of perfect indifference to him. He only considers persons, not their origin, and marries as willingly in one case as in the other.

So we have returned to the subject of _respiration_, on which I always seem to be turning my back; but now the question is, what brings us to it again? And this is the explanation.

When the oxygen picked up in the lungs by the blood has traveled with it to the organs, he finds there two well-known friends--hydrogen and carbon.

You smile, and exclaim at once, "Then he marries them, does he?"

Yes, my dear child; and it is only for that purpose he enters our bodies at all. And this is why I could not make you understand the nature of respiration until I had explained that of fire to you. As I have told you before, it is the same thing. Invite air into your body by the bellows of your chest, or drive it into the fire by the kitchen bellows--it is always king Oxygen whom you are sending to his wedding.

LETTER XXII.

ANIMAL HEAT.

Now, then, we have got hold of the secret of respiration; the _oxygen_ within us unites itself to the _hydrogen_ and _carbon._

And for what purpose, do you suppose?

Unquestionably it must be to make a fire, since they never come together without doing so.

But what do people make fires for? I ask next. Well! surely to warm themselves, do they not?

And this is the history of your body being warm exactly like a dining-room stove, where the oxygen in the air forms an alliance with the hydrogen and carbon of the wood. Nature warms little girls inside, on precisely the same plan by which men warm their houses in winter.

Imagine, then, a little stove, furnished with little arms for helping itself out of the wood-basket as it is wanted, and with little legs to run and refill it when it is empty; the fire must be always burning there, and the stove must be always warm.

Just such a little stove is your body; your mouth being the little door, by which there constantly enter--not wood, that would hardly be pleasant--but--hydrogen and carbon under the forms of bread, mutton broth, cakes, sweetmeats, and all the good things people have learnt to make with sugar, fat, and flour. There is hydrogen and carbon in everything we eat, as I have already told you; but sugar, fat, flour, and _wine_ are the substances which contain them in the greatest quant.i.ties, and consequently they are our best _combustibles._

You are surprised, perhaps, at _wine_ being a combustible; wine, which you think would put out rather than make a fire.

And it would. But that is only because in it, what is good for burning is mixed with a great deal of water, which prevents our being able to set it on fire. But if part of this water is withdrawn, you have _brandy,_ which lights easily enough; and if part of the remaining water is withdrawn from the brandy, you have _spirits of wine_, which takes fire more easily still. If you have ever seen a _spirit-of-wine_ lamp, you must know something about this. Judge from that what a fire spirits of wine must make in the body, even when it has a good deal of water with it; for it is right to tell you that your little stove is very superior to the one in the dining-room, and that it hunts out for consumption the smallest portions of combustible matter, in places where the other would be a good deal puzzled to find them.

This is not all, however. I have much greater wonders to tell you yet.

What should you say to a stove, which, summer or winter, night or day, in rain or sunshine, amid the ice of the pole, or under the sun of the equator, was able to keep itself constantly in the same condition; neither hotter nor colder one minute than another, whether you gave it much or little fuel, at a given moment, and sometimes when you gave it nothing for whole days together? It would be worthy of a fairy tale, would it not? Yet the human body is a stove of this description.

But this requires a little explanation.

It is rather bold in me, you may think, to a.s.sert so freely, that all the year round, from one end of the earth to the other, the human body is never colder nor hotter than mine is, for instance, at this present moment. "Hot" and "cold" is soon said, you argue: but the exact varieties of _more_ or _less_ are not so easy to measure, and especially not easy to remember, with reference to so many bodies, scattered over the face of the whole earth. What may be warmth for one in one case, may not be equal warmth for another; and even supposing that the same individual learned man could go and inspect every part of the globe in succession, how could he possibly recall, while touching the body of a negro in Senegal, in July, the exact amount of animal heat he had found in a Greenland Esquimaux in January?

Be content. I should not have settled the question so cavalierly, if people had not discovered an infallible method of estimating accurately, and always in the same manner, the degree of warmth, in other words, the _temperature_ of the body.

Let us first see, then, what this method is, though it will oblige us to digress a little; but you are accustomed to that now, surely; and besides, if I were to go straight ahead, you would not be able to follow me.

Do you ever recollect being very cold? Let mammas look after their little girls as much as they please, to prevent it, it is sure to happen to every one some day or other. Now does it not seem at those times as if the whole body were contracting itself--and when people are shivering with cold, have they not a shrunk, shrivelled look? When the weather is very hot, on the contrary, our bodies feel as if they were swelling and stretching, and one seems to take up more room than before. This is the case with all bodies. Heat swells, or, as learned people call it, expands, them: cold shrinks or contracts them.

Furthermore, _mercury_ is one of the things most susceptible of this action of heat and cold, and we have had recourse to it accordingly, in the construction of the _thermometer_, [Footnote: _Thermometer_ comes from two Greek words: _thermos_, heat; and _metron_, measure. The degrees in the Thermometer about to be described are marked on the _Centigrade_ principle. [Not the one (Fahrenheit) in general use in the United States.]] a very useful instrument, which you will hear spoken of all your life.

The _thermometer_, or _heat-measure_, consists of a little hollow ball filled with mercury, out of which rises a small tube of very thin gla.s.s, in which the mercury can move up and down. When the thermometer is exposed to heat, the heat causes the mercury to expand, so it goes up the tube; when the thermometer is exposed to cold, the mercury contracts and sinks again.

Now suppose you were to melt some ice in the palm of one hand, and try to dip a finger-tip of the other in a saucepan of boiling water; you would find a great difference of temperature between the two, would you not? Which difference of temperature people have succeeded in measuring with the thermometer, as accurately as your mamma measures a piece of cloth with her yard measure.

This is how it is done:

You surround the ball of mercury with pounded ice, and while it is melting make a mark at that point in the tube where the mercury has stopped in its descent. Then plunge the thermometer into boiling water.

Whereupon the mercury goes up, up, up, till at last it reaches a point beyond which it will not pa.s.s. Here a second mark is made, and the s.p.a.ce between the two marks is divided into a hundred perfectly equal parts, indicated by so many small lines, which are called _degrees_. But this word _degrees_ has a double meaning in some languages. It means _steps_ as well as the degrees of measurement we are talking about; steps being, as you know, the perfectly equal parts into which a staircase is divided. Fancy the mercury-tube a staircase, then, rising from the cellar where the melting ice is, up to the garret where the boiling water is, and let it consist of 100 steps. The mercury goes up and down this staircase, according as the temperature it encounters approaches that of the boiling water or of the melting ice; and if you wish to know exactly how far it is from the cellar or from the garret, you have only to count the _steps_. Hence arise those expressions which you so often hear--high temperature and low temperature. These mean, temperature according to which the mercury goes up or down this staircase.

On the actual floor of the cellar where the ice melts, there are yet no degrees (a floor is not a _step_, you know), so there you find the word _zero_, which means a cipher or nought. Then you begin to count 1, 2, 3, 4 degrees, marked by lines up to 100, where you reach the garret, _i.e._ the boiling-water height.

Of course, if the thermometer be exposed to an amount of cold greater than that of melting ice, the mercury will sink below the cellar.

Accordingly the staircase is carried below it, with steps (so to speak) of precisely the same size as those above, and you count as before, 1, 2, 3, &c., as it descends; adding however, to distinguish these degrees from the others, "_below zero_." You may go on in that way as far as 40; but there you must stop. At that point the mercury freezes. He sits down there on his last step, and will not go any further!

In the same way if the thermometer is exposed to a heat greater than that of boiling water, the mercury will rise higher than the garret.

So the staircase is made to go up higher, and always with steps of the same size, counting from 101 upwards, as far as 350 if you choose; but no further, observe! If the temperature were raised beyond that, the mercury would begin to boil, and then, indeed, good-bye to steps and measured degrees! The gentleman would dance so fast that there would be no possibility of seeing anything, to say nothing of his flying away!

Now nothing is easier than to use the thermometer. You place it in the situation where you want to measure the heat, and the mercury goes up or down of itself until it reaches the degree which corresponds with the temperature of the place. It is much more convenient than your mamma's yard measure, which has to be moved about over the stuff, and which is very apt to slip if you do not hold it carefully. Dressmakers would be delighted to have a measure which only wanted laying upon the material, and which would unroll itself and stop short just at the proper point. And this kind of office the thermometer really performs.

We will suppose to-day to be the 30th of November. I have just carried the thermometer out of doors; the mercury has fixed itself at the second degree _below zero_. This tells me that it is freezing cold. My fingers have told me so already; but exactly to what extent they could not say. Just now in the room, the mercury was at the 15th degree _above_ zero, thanks to the stove in which we have a good fire. In summer-time it rises to 25, 26, or 28 degrees. I once saw it climb as high as 33 degrees: in the shade of course, you understand; in the sun it would have been quite another affair. Well! there was a universal outcry against the heat. Grown-up young ladies whom I try to teach all sorts of things as I do you, pretended that it was impossible to work. Yet I should find a still greater heat inside my body, if I could get the thermometer there. Have no fears, however; I am not going to make a hole in it: luckily there is one already. I put the ball of mercury into my mouth. And now I can almost tell without looking. The mercury was on its way up the staircase as soon as I took the ball in my hand--and now it has reached the 37th step.

You can try the experiment on yourself, but I forewarn you that it ought to be rather hotter with you than with me: the mercury will probably rise a degree higher. I will not promise that in your grandpapa's mouth it may not sink a degree--but that will be all. In different mouths it has, between the 38th and 36th degree, room for the play of a little variation, but it can no more go beyond these than a tethered cow can get beyond the circle made by her cord as she turns round the stake. Go round the world with your thermometer, pop it into everybody's mouth, wiping it if you choose as you proceed, you will always find the mercury on guard. Its tethering cord is somewhat elastic, like everything else about us; but if by any accident it should exceed its limit by even one degree above or below, it would be quite as extraordinary as meeting a giant of eight feet, or a dwarf of three--which one does see occasionally, although the standard of human height varies generally round the centre of five feet.

Since there is a fire always kept burning within us, there is no difficulty in comprehending why our bodies always keep warm. Of course, however, the fire must be kept brighter in winter than in summer, but people have no need to be told so. Nature provides for the necessity.

She gives us more appet.i.te in cold than in hot weather; not that we can perceive much difference in ourselves in this respect from winter to summer; for our bodies stick to their accustomed habits, and call out pretty loudly for the same daily rations, though without having the same need of them. In order to estimate fairly the connexion which exists between the internal need of food--_i.e.,_ of combustible matter--and the external temperature, we must compare the Hindoo, who lives on a pinch of rice a day, between the tropic and the equator, with the Esquimaux, who, to keep up his 37 degrees of heat, beyond the polar circle, in a country where European travellers have seen mercury freeze, sometimes swallows from ten to fifteen pints of whale-oil at a sitting! Just fancy _whale-oil!_ which is much nastier than even cod-liver oil, if you ever tasted that; but, on the other hand, it is a thorough _combustible_, and the poor people are not so very particular: come what will, the fire must be kept up, and that briskly. But without going thus into extremes, a friend of mine once told me that in Portugal, the land of oranges, it is not uncommon to see gentlemen and ladies (that is to say, those who can eat and drink what they please) dine standing, in five minutes, on a bit of bread and whatever else may be handy. Propose this system to the inhabitants of our colder and damper climate, whose very young ladies, fair and delicate-looking as they are, need a helping of good roast-beef for dinner to keep life in them, and they would only laugh at you. But those who were well instructed could go on to inform you that the chilly atmosphere of northern countries creates the necessity for a more active internal fire than is ever needed under the burning sun of Portugal, and that a mouthful of bread per day will not, in their case, suffice to maintain the appointed thirty-seven degrees of heat.

For the same reason, Spaniards drink water, and are satisfied; whereas English wine-merchants add brandy to a good many foreign wines, or they would be quite unacceptable from being deficient in combustible.

It is for the same reason, also, that Russians can swallow, without wincing, b.u.mpers of brandy which would kill a Provencal outright: and that the Swedish Government has no end of trouble to keep the country people from converting into brandy the corn that ought to go to the miller; whilst the Mohammedan Arabs accept without difficulty that precept of the Koran which forbids the use of wine and spirituous liquors. It is easy for the Arabs, who are kept warm by their climate, to do without brandy. It is less easy for the Swedes, who are surrounded by cold.

All this comes as a matter of course, and we do the same thing ourselves, without being unusually sagacious. In January, when the thermometer goes down to twelve or fifteen degrees below zero, I put more fuel into my stove than I am doing to-day, with only two degrees of cold to bear with. There is nothing surprising in all this.