chemistry and physiology by proving what it was that really took place when a fire burnt, when iron rusted, when animals breathed. Before his time many fancies reigned supreme in men's minds about these matters, fancies which we now look upon as foolish, because we are able to view them in the light of his wisdom. He tried these fancies, literally, in the balance, and found them wanting. He shewed that all burning is an oxidation, a union of the thing burnt with the oxygen of the air in which it is burnt, and without which it cannot be burnt; that the rusting of iron is only a slow combustion, and that the breathing of animals is only a hidden combustion. He said, what no one had said before him, that our food is made up in great part of two things, called carbon and hydrogen, and that these unite within the body with the oxygen of the air we breathe, and are then burnt off, the carbon and oxygen uniting or burning to form carbonic acid, the hydrogen and oxygen uniting to form water. And this grand truth, which Lavoisier worked out with the balance and the retort in the quietude of his laboratory, while France was heaving under the sense of coming troubles, still remains and always will remain as the basis of our knowledge of the chemical doings of the animal body. Oxidation is the fundamental fact in the existence of every animal. From the highest to the lowest every creature is busy all its life long in getting oxygen, from the atmosphere if it live on land or in the air, from the air which is held dissolved in all natural waters if it live in the water, and in binding that oxygen to the carbon and hydrogen of its food.

At the present day, however, we do not accept this truth exactly in the form in which Lavoisier put it. He thought that the carbon and hydrogen of the body and of the food were, by help of the mysterious processes of the body, distilled as a combustible liquid, drop by drop, into the lungs, and then and there burnt off by the oxygen which came in with the breath into carbonic acid, which went out with the breath, and into water, which went out partly as the vapour of breath and partly in other ways. He proved that there was oxidation; but his combustible liquid was only a supposition, and, like many other suppositions, has been cast to the winds. We now-a-days deny, not only that any such liquid exists, but also that the lungs are in any way the place where the oxidation or burning goes on. admit that oxygen comes into our body through the breath that we draw in, and that carbonic acid goes out through the breath that we send out; but we hold that oxygen meets carbon, that the fire which is born of their encounter burns, not in the lungs only, or even chiefly, but in every part and parcel of our body that enjoys the privileges and fulfils the duties of living matter.

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To Lavoisier the lungs seemed, as it were, a kind of furnace, where the fire of life was ever kept burning; to us they appear as a kind of double-acting flue, bringing to the vital fire within the air deprived of which it would soon cease to burn, and carrying away from it the carbonic acid by which it would otherwise soon be smothered and choked.

The modern theory, however, raises up a difficulty which was not felt during the reign of the old one. Oxygen is a gas and carbonic acid is a gas, but our bodies are made up of solids and liquids, of flesh and blood, and, as long as we are alive and well, there is not a bubble of air to be seen anywhere from the crown of our heads to the soles of our feet, save only in the lungs and stomach. Indeed, a few bubbles of air in the blood bring instant death. How, then, are these gases enabled to travel all over the body? How does the fire that is burning in our finger-ends or in our brains, ever so far from the lungs, separated from its chimney by many layers of muscle and of bone, of fat and of sinew, get its inward draught of oxygen and its outward draught of carbonic acid? By means of the blood, which is the great gas-carrier of the body, and which, as it rushes along the path allotted to it, from the nooks and corners of the body to the heart and thence to the lungs, from the lungs to the heart and thence to all parts of the body again, may be said to act the part of a great animal ventilator.

Not that the blood carries to and fro these gases as gases. That is not at all necessary. A fish breathes with its gills quite as truly as a man does with his lungs; it takes in oxygen and gives up carbonic acid; but if you watch it you will see no bubbles of gas going to and from its gills. The oxygen and carbonic acid which it gives and takes have passed from their gaseous state and are said to be dissolved in the water in which the creature is swimming. And as the gases outside the fish are dissolved in the water, so the gases inside the fish are dissolved in the blood; and when water and blood meet in the gills, with only a very delicate skin between them, they compare accounts, as far as their gases are concerned, and strike a balance. The blood, as it comes to the gills from the body, is loaded with carbonic acid, and has but a scanty supply of oxygen, while the water, if it be fresh and good, is rich in oxygen but poor in carbonic acid. So blood takes up oxygen from the water, and water takes up carbonic acid from the blood, and then both move on; the blood to carry its burden of oxygen to the burning of the fish's slow fire, and the water to carry its carbonic acid away to where further duties await it. It is not necessary, however, that the oxygen should be dissolved in water before it can enter into the blood, or that there should be water present for the carbonic acid

to get into, in order that it may escape from the blood; the one may be taken from and the other given to the air without there being any water to act as a go-between. When a fish is taken out of the water it dies, not because the blood that passes through its gills cannot barter gases with the air, but because, through the drying-up of the exposed tender gills, the minute bloodvessels get shrivelled up and otherwise altered, and the circulation of the blood in them is brought to a stand-still. The business of respiration, so to speak, is closed, not because the agents have become unfit, not because of any lack of goods, but because the premises have been ruined, and traffic can no longer be carried on in them. In man and air-breathing animals, the arrangement of parts is just suited for carrying on in the open air this interchange of carbonic acid and oxygen. The blood-vessels, instead of being exposed on branched and tufted gills, are hidden in the moist cavities of the lungs, where the air, though it passes in and out ever so freely, is always moist. Here the venous blood readily gives up to the air its surplus of carbonic acid, here from the air it takes in oxygen and then goes on its way as arterial blood; while by the heaving of the chest the air itself is also being continually renewed.

It may be asked why does this interchange take place? why are the blood and the air thus obliged to give and take? why could not the carbonic acid remain in the blood and the oxygen in the air? To this question there are more answers than one.

In the first place, it is a general law of nature that, when two such things as blood and water come together they should each try to supply each other's deficiencies. If you expose pure distilled water, or what answers the same purpose, recently boiled water to the air, the water will always take up some of the air, will always dissolve some of it, the exact amount taken up or dissolved being dependent on the warmth of the water and air, and many other circumstances. By boiling, or by other means, this air may be driven out, and the water is then ready to take up a fresh supply. In the same way blood takes up air (or oxygen) in the lungs, gets rids of some of it in the inner recesses of the body, and is ready for a fresh supply when it gets back to the lungs. On the other hand, if you force into water more gas than it will naturally dissolve, as in the manufacture of sodawater, an escape will always take place when opportunity offers, as when the soda-water bottle is uncorked. During its journeyings through the tissues, while it is bathing those parts which are being consumed in the vital fire, blood gets overcharged with carbonic acid. and when it reaches the lungs the surplus is set free, not in bubbles or with effervescence, for the strain is not great enough for that, but silently and quietly, though none the less really.

There is also another process at work in this business of breathing. Venous blood, the blood that is on its way to the lungs and ready for the changes about to take place there, is, as every one knows, of a dark colour. If, however, venous blood is exposed to the air it becomes of a much brighter red. When a butcher kills a sheep the blood, as it flows from the neck, is, for the most part, dark, but when collected in a vessel becomes of a bright red at the top where it is exposed to the air. The same change takes place when the blood is exposed to the air in the lungs. Whether in the body or out of the body dark venous blood becomes bright arterial blood whenever it can get a chance of coming into contact with air. And if the proper means are used, a great deal more oxygen can be got out of the bright than out of the dark blood, for it contains much more. Hereupon hangs a tale. We say that blood is red, either dark or bright red; but its redness is not spread all over it equally; it is not red in the same way that port wine or the red liquids in a chemist's shop are red. Blood is made up of minute discs, the blood-corpuscles, swimming in a fluid; the discs are red, but the fluid is nearly colourless, at least, shows only a pale yellowish colour. Blood, in the mass, looks red because the discs are so small and so closely crowded together, but the redness is really due to the discs and the discs alone. If we allow blood to clot and wait till the clot has gathered all the blood-discs to itself, we shall be able to pour off what is called the serum, the fluid of blood free from discs. This serum is nearly colourless, and therefore we cannot expect to see it change in colour on being exposed to the air; and if we use the proper means we shall find that after ever so long an exposure to the air, it contains only a very little oxygen, nothing at at all to be compared with that which can be got from the bright red blood which has not been robbed of the discs. From which we learn this notable fact, that it is the blood-discs which are so fond of oxygen, not the blood-fluid or serum. We may go even a step further. The redness of the blood-discs is due to their being made up of a red substance, which we can by proper means separate and even crystallize. It is a substance as distinct as any other chemical substance, as tartaric acid for instance, or magenta. This red substance, too, is greedy of oxygen, and, moreover, differs somewhat in colour according as it is in possession of or in lack of oxygen. If you prepare a solution of it in such a way that no oxygen gets to it, you will have a dull purplish liquid; if you then shake the solution in an open bottle so that the air can get readily to it, its colour changes to a much brighter red, to something of a scarlet hue, and on examination you will find that it now contains a good deal of oxygen. You may even play

another trick with it. If you take some of the bright red oxygenholding solution, and add to it some substance, which the chemists can readily provide for you, still more greedy than it for oxygen, you will be able to see that the solution has given up its oxygen to the added substance, for it has become dark and purple instead of bright and scarlet. But the experiment need not end here. Shake up the mixture; the solution will take fresh oxygen from the air and become once more bright and scarlet. Add again some more of the oxygen-loving substance, the colour is again changed to purple; and so the experiment may be carried on for a long while, the red bloodsubstance, the essence so to speak of the red blood-discs, acting as a go-between, taking up oxygen from the air and handing it over to the oxygen-loving substance, itself shifting in colour between scarlet and purple; or as we might more truly say between arterial and venous, for the change thus brought about in a bottle or a test tube is probably identical with that which takes place in the lungs, not only in the matter of colour, but in other respects as well. When the purple venous blood reaches the lungs and comes very near to the air, the thinnest possible membrane alone intervening, there by virtue of its blood-discs, (and they by virtue of their red substance) it takes up oxygen and goes on its way as bright arterial blood. As it travels onwards through the body it meets with many oxygen-loving substances to which it'renders up its oxygen, helping them to burn themselves away, while it goes on its way as oxygen-robbed purple venous blood. to get oxygen once more from the air of the lungs. Retail dealers of oxygen are the blooddiscs, and this traffic of breath is probably the chief business of their lives. The air of the lungs is a great warehouse; from thence each tiny disc carries its load of oxygen. Driven by the heart's beat it wanders over the body, and brings the breath of life with it wherever it goes, so that the remotest and inmost spots breathe as freely, have as ready a supply of oxygen as those that are situate in the lungs themselves, or those on the very surface of the skin.

The blood is thus the life of the body, inasmuch as it bears in itself the essence of the two things on which we live, food and air. The food enters the blood from the digestive organs, the air from the lungs. And just as the food before it enters the blood is in some measure prepared for use, so, there is reason to believe that the oxygen, in becoming saddled on to red blooddisc is also to a certain extent "prepared." Oxygen, though we speak of it as of one thing, has somewhat different qualities under various circumstances. Thus under certain conditions it is said to be more "active" than under others, more prone to unite