No. 10. Last week I described to you the process by which the food is prepared to nourish the system and poured into the blood. Now let us track it along and see how it is burned up to warm our bodies. The vein, into which the fuod is poured in the neck, leads dIrectly to the right side of the hcart; and as the heart is essentially a thick bag which is constantly expanding and contracting, as it expands, the blood from the great vein which has just received the food runs into the heart, and when the heart contracts, this blood is forced into the Inngs. The lungs arc what butchers call the lights, in sheep, hogs, &c., in which animals they are of substantially the same stmcture as iu man. They are a grey, spongy mass, and, on close examinatIOn, are fOllnd to consist of an immense number of very fine blood vessels and air passages, which are separated from each other by an exceedingly thin membrane. As the blood is forced into all these blood vessels, the air vessels are filled at the same time by drawing in the breath, and thus the blood and the air lie side by side, separateu only from actual contact by the very thiu membranc of which I spoke. When the blood anu the air are in this situation, there takes place a very curious operation, which has attracted a g"cat ueal of the attention of physiologists within the last few years. It is called osmose from the Greek, aSIilOS. Do you know what that means, J ? “ “ No, Sil'.'l .”Do you know Ivhnt otlteO means ? “ “Yes, sir, to push forward." .”Osmos is the noun, and means a pushing forward. Tile opemtion is this. When two fluids of opposite characters rest against each side of a thin animal membrane, they are found to mix together through the membrane, though the membrane might be entirely imper-violls to each when the other was away. It is by osmose that the blood is purified in the lungs. The waste carbonic acid and watery vapor pass through the membrane. and are thrown out into the atmosphere with the brcath. as it leaves the lungs, and at the same time, oxygen from the ait passes th"ough the membrane and mixes with the blood. Now, let us get out the microscope anu look at some blood." .”Where shull we get any blood ?.”“Oh, we only want a very small drop, and I will prick my wrist and get a little." .”Here, father, take it out of my wrist. Give me a piu, Charles, and I will get some." "There is a plenty, John ; there is a plenty. Here, squeeze up the skin and let me touch tIllS glass to it..”.Let me look, father." .”As soon as I get the foous right. Ah! hcre we have it, boys. Now, look. Don't shake the table..”“ Why ! What arc all those little flat things ? “ “ Those are the blood corpuscles, and those that look like little square sticks are the same things seen edgeways, for they are shaped like coin, thickest at the euge, as you see. They are so small that the little drop which I took from John's wrist would contaIn about a million of them. These corpuscles absorb the oxygen which passes through the lung mcmbrane, and they are borne by the blood as it retums from the lungs into the left side of the heart. As the heart contracts, the blood is forced through the arteries, which branch off, like the limbs of a tree, throughout every part of the system. 'rhe arteries, dividing down smaller and smaller finally termiuate in exceedingly small pipes, called capillary vessels, which do not taper like the arteries, but are of uniform size, and mesh together in a manner somewhat similar to the webs of a net. These capillaries pervade every portion of the system, so that the point of a cambric needle cannot. be thrust into any part of the body without puncturing some one of them nnd allowing the blood to escape. 'fhrough the thin membrane which furms the walls of the capillary vessels another opemtion of osmose takes place ; a part of the oxygen which has been brought by the blood corpuscles from the lung, passes outward into the surrounding tissue, and carbonic acid and water pass inward into the capillary. Here, too, is the place where the burning takes plnce, by which the heat of the body is maintained. The oxygen, which has been brought by the blood corpuscles, combines with the waste carbon of the system, und with the car bon which has been brought by the food for that purpose, producing carbonic acid and generating heat, precisely as we saw it do in the burning gas which was the subject of our first conversation. Hydrogen is also burnt, generating heat and produeing water, as in the bUl'Iling of gus." .”If we should cut into the body could we see the fire burning?" "No; it is such a slow fire that it does not prod.uce light. It takes a heat of abont 1,000 to give light, and Ihis slow fire only heats the body to a temperature of 98. The capillary veisels Icau from the termination of the artcries to the beginning of the veins, nnd the blood passes on through them from the arteries into the veins, by which latter vesscls it is returned to t,he heart. The blood in the arteries is of a bright scarlet color, bllt when it gives up, in the capillaries, the oxygen which it nbsorbed in the lungs, it changes to a deep purple color, which color it maintains in its passage through the veins to the heart, and from the heart to the lungs. In the lungs it again gives up its carbonic acid, IThich passes through the membrane of the lungs and is exhaled with the hreath. And now, having again sent forth the atom of carbonic acid into the air, we will, for the present, bid it good bye. You remember how it is composed, one atom of oxygen united to two of carbon, like a bean sandwiched between two girls ; away they float through the air, destined to a long and close union, nnless they come in contact with a treacherous leaf, which, like the tongue of a scandalous gossip, will sow discord between them, and divide their firm nnion asunder. I propose that we begin next week, an inquiry into what the several artirles on the breakfast table are composed of." . —> m