with water, HYDROGEN. Thanks to gas companies, every body 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 mass 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-seck. You have sometimes played at hide-andseek 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 quantities; 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 masses 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, wheth er carbon comes to him from a log of wood or from a building stone; whether the hydrogen comes from a candle or a glass 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 marrie 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 respira the oxygen within us unites itself to the hydrogen and carbon.

tion;

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 win

ter.

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, sweetineats, and all the good things people have learnt to make with sugar, fat, and flour. There is hy

drogen and carbon in everything we eat, as I have al ready told you; but sugar, fat, flour, and wine are the substances which contain them in the greatest quantities, 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 assert 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