A casual glance at the animal kingdom will show that diet is a wide subject. A pigeon will eat peas; a tiger would not know what to do with the peas if he got them; while a monkey will eat almost anything he can lay hands on. A plant takes us still further afield, for it can use the atoms of substances with an extremely simple molecule—carbonic acid gas, for instance. Our task, however, is simplified by our having only man to consider; and although most of the higher animals are so much alike that they might be considered in general and contrasted in detail, it is a great thing to get rid of the whole vegetable kingdom with bacteria and parasitic animals. One of the first requisites for the maintenance of life, as was mentioned above, is nitrogen. Now, nitrogen is one of the commonest elements in the world, but it is the hardest to supply to the body. Four-fifths of the air is pure nitrogen, but pure nitrogen is useless as a food. We draw it into our lungs at every breath, and are none the better for it, for we breathe it out again unchanged; and if it were completely absent from the air we should not be so very much the worse. The Ancient Mariner exclaimed, ‘Water, water everywhere, and not a drop to drink’; a starving man might exclaim, ‘Nitrogen, nitrogen everywhere, and not an atom to assimilate.’ Animals have to get their nitrogen in the form of proteid, a substance whose molecule is composed of nitrogen, oxygen, hydrogen, carbon, etc., and might roughly be described as dead protoplasm. Plants on which animals feed, when they do not get their proteid by the simpler, though less moral, method of eating one another, are able to get their nitrogen in a simpler form; but with that we are not concerned. The proteids are a group of substances which resemble protoplasm in the elements of which they are composed and in the complexity with which they are combined. The various proteids seem, however, to have a definite chemical composition, and therefore differ from protoplasm in being true compounds; moreover, if kept from bacteria they undergo no changes. One of the best forms of proteid for examination is white of egg; this, as is known, sets or coagulates when boiled, dissolves in water, from which it may be precipitated by boiling, and displays various other chemical properties common to all proteids. There is, however, a good deal of difference between the several varieties of proteids, and the more complex ones have to be converted into the simpler before they can be absorbed. Hence the necessity for digestion. Now, as proteid resembles dead protoplasm, it might be supposed that a diet of proteid alone would be the most economical; but this is not so. If it were possible to live without work, _i.e._, without movement of any kind, it might be; but to do work, more carbon must be oxidized than the proteid molecule contains.