Now, while the cells which make up the body of man differ very greatly owing to the different tasks which they have to perform in obtaining food and getting rid of refuse, they all require very much the same fuel to enable them to live, and having got it, they all treat it in very much the same way; therefore our first business is to consider what the body wants, and what it does with it. Afterwards we can try to find out how it gets it, and where. The first and most indispensable requirement of protoplasm is water. The next is probably nitrogen, compounds of which seem to form the framework of the protoplasmic structure. The next is probably carbon, and the next free oxygen. The two last-mentioned combine with a release of energy. This happens in the grate when coal burns, and the result is heat. In the tissues of a body the result may be heat, growth, or movement, all three being present in the phenomenon of muscular activity. Finally there are mineral salts, the most important being sodium chloride, which is placed on the table at every civilized meal. But though these elements are given here in order, their importance is really equal, for all are necessary. That is about as much as it is wise to say here. The chemistry of the living cells—their anabolism, or how fresh material is built into their structure; their katabolism, or how the same structure is broken down that work may be done; in fact, the general metabolism—is so complicated, and so little understood as yet, and requires so extensive a knowledge of chemistry to follow, that it is best left alone by people who do not want to go into it deeply. At best, such a discussion resolves itself into an exposition of different observers’ theories, with the reasons why they hold them—reasons based on laborious and technical studies. Pages might be written on the various theories, backed by pages more of chemical formulæ, to show why this view deserves deep consideration, while that, in spite of the obstinacy with which it is upheld, is absurd; but though such discussions take one nearest the secret of life, the general public is not unnaturally apt to stigmatize this side of physiology as dry. It is a matter which interests experts, not the casual reader. Quite a different affair is the question of diet. That is everybody’s business, as the number of faddist societies and blatantly advertised ‘foods’ attest. And though the preparation of the food in the body up to the point where it merges into living matter and is lost sight of—in a word, ‘digestion’—is again a question of chemistry, it is one which may be approached without such an exhaustive knowledge of that science as the previous considerations would have required. It is, moreover, to judge from the way it is discussed, a topic of universal interest.