We now know--thanks to the labors of Joule, Lyon Playfair, Clausius, Tyndall, Helmholtz, etc.--that heat itself is a mode of motion, a form of convertible energy, which can be made to do useful or productive work, and be expressed in terms of actual work done. Modern experiment shows that all our energy is derived from that of food, and, in particular from the non-nitrogenous part of it, that is, the fat, starch, and sugar. The nutrition of man is best maintained when he is provided with a due admixture of all the four classes of aliment which we have mentioned, and not only that, but he is also better off if he has a variety of each class. Thus he may and ought to have albumen, fibrine, gluten, and casein among the albuminates, or at least two of them; butter and lard, or suet, or oil among the fats; starch of wheat, potato, rice, peas, etc., and cane-sugar, and milk-sugar among the carbo-hydrates. The salts cannot be replaced, so far as we know. Life may be maintained in fair vigor for some time on albuminates only, but this is done at the expense of the tissues, especially the fat of the body, and the end must soon come; with fat and carbo hydrates alone vigor may also be maintained for some time, at the expense of the tissues also, but the limit is a near one, In either of these cases we suppose sufficient water and salts to be provided. We must now inquire into the quantities of food necessary; and this necessitates a little consideration of the way in which the work of the body is carried on. We must look upon the human body exactly as a machine; like an engine with which we are all so familiar. A certain amount of work requires to be done, say, a certain number of miles of distance to be traversed; we know that to do this a certain number of pounds, or hundredweights, or tons of coal must be put into the fire of the boiler in order to furnish the requisite amount of energy through the medium of steam. This amount of fuel must bear a certain proportion to the work, and also to the velocity with which it is done, so both quantity and time have to be accounted for.