Carbon, the next item on our list, is familiar to everyone in the comparatively pure form of coal, charcoal, and the ‘lead’ of pencils. It is commonly used to burn—_i.e._, oxidize—that heat may be obtained to boil water and to work machinery. This is precisely what it is required to do in the body, where it is burnt by oxygen taken in by the lungs, that heat and energy may result. It is a commonplace that severe exercise causes laboured breathing, and the reason of this is that the carbon in the body is being oxidized, and the product, carbonic acid gas, has to be got rid of. The more work is being done, the more oxygen is required to burn carbon in the muscles. The more carbon is burnt, the more heat is evolved, and the more necessary it is that the blood should be cooled by drawing cool air into the lungs. Hence the more rapid breathing. The air normally breathed out is always warmer than that taken in, and always contains extra carbonic acid gas. After exercise the quantity is increased, and its increase on the normal amount given off can readily be demonstrated by analyzing samples of the air taken in and given out. But carbon, like nitrogen, cannot be taken in in the crude form. No one would try to make a meal of charcoal. A certain amount is contained in the proteid molecule, enough, no doubt, to secure the basis of the protoplasmic structure; but unless one is prepared to eat an excessive quantity of proteid, a proceeding entailing waste and exhaustion of the digestive apparatus, the balance must be made up by eating carbohydrate. The forms in which people are most familiar with carbohydrate are starch and sugar. Sugar is the better food, as it is so much more soluble than starch; and, in fact, starch is always turned into a kind of sugar before it is used by the body. The common cane-sugar, which everyone knows so well, is about the most useful food we have, owing to its purity, and therefore concentration, and its simplicity. A very small amount of digestion is necessary to convert it into the simplest of all carbohydrates, a substance easily stored, as glycogen, till wanted, which is present in muscle after a meal, and is used up when the muscle is active, being oxidized to carbonic acid gas, sarcolactic acid, and alcohol. The importance of carbon in the diet is therefore obvious; and people who intend doing extra muscular work should take extra sugary food rather than extra proteid. A locomotive which is about to make a record run takes in more coal, not more engine-drivers, and our athletes now follow the same principle. We shall, however, have a good deal more to say about athletes presently.