=53.= But if the blood is thus continually being made rich by things, it must also as continually be getting rid of things. The things with which it parts are not, however, the same as those which it takes. The blood, as we have said, is fuel for the muscles, for the brain, and for other parts of the body. These burn the blood, burn it with heat but without light. But, as you have learnt from your Chemistry Primer, Art. 4, burning is only change, not destruction; in burning nothing is lost. If the muscle burns blood, it burns it into something; that something, being already burnt, cannot be burnt again, and must be got rid of. Into what things does the body burn itself while it is alive? I have already said that if you were to take a piece of meat or some blood, and dry it and burn it, you would find that it was turned into four things--water, carbonic acid, ammonia, and ashes. The body is made up of nitrogen, carbon, hydrogen, and oxygen, with sulphur, phosphorus, and some other elements. The nitrogen and hydrogen go to form ammonia; the hydrogen, with the oxygen of combustion, forms water; the carbon, carbonic acid; the phosphorus, sulphur, and other elements go to form phosphates, sulphates, and other salts. =In whatever way the body be oxidized, whether it be rapidly burnt in a furnace, whether it be slowly oxidized after death, as when it moulders away either above ground or in the soil, whether it be quickly oxidized by living arterial blood while still alive--in all these several ways the things into which it is burnt, into which it is oxidized, are the same. Whatever be the steps, the end is always water, carbonic acid, ammonia, and salts.= These are the things which are always being formed in the blood through the oxidation of the body, these are the things of which the body has always to be getting rid. In addition to the water which comes from the oxidation of the solids of the body, we are always taking in an immense quantity of water; partly because it is absolutely necessary that our bodies within should be kept continually moist, partly because food cannot pass into the blood except when dissolved in water, and partly because we need washing inside quite as much as outside; if we had not, so to speak, a stream of water continually passing through our bodies to wash away all impurities, we should soon be choked, just as an engine is choked with soot and ashes if it be not properly cleaned. We have, then, to get rid daily of a large quantity of washing water over and above that which comes from the burning of the hydrogen of our food. We have already seen that a great deal of the carbonic acid goes out by the lungs at the same time that the oxygen comes in. A large quantity of water escapes by the same channel. You very well know that however dry the air you breathe, it comes out of your body quite wet with water.