The calculation may be carried out to a high degree of accuracy if, instead of confining ourselves to the broad features of the problem, we enter into rigorous detail. It is only, in fact, approximately that we have reduced all foods to albumen, sugar, and fat, and all excreta to water, carbonic acid, and urea. The reality is a little more complicated. There are varieties of albumen, carbo-hydrates, and fatty bodies, the heats of combustion of which in the organism oscillate in the neighbourhood of the numbers 4.8, 4.2, and 9.4. Each of these bodies has been individually examined, and numerical tables have been drawn up by Berthelot, Rubner, Stohmann, Van Noorden, etc. The tables exhibit the thermal value or energetic value of very different kinds of foods. In our climate, the adult average man, doing no laborious work, daily consumes a maintenance ration composed, as a rule, of 100 grammes of albuminoids, 49 grammes of fats, and 403 grammes of carbo-hydrates. This ration has an energetic value of 2,600 Calories. It is therefore, thanks to the victories won in the field of thermo-chemistry, and to the principles laid down since 1864 by M. Berthelot, that this second method of attack on nutritive dynamism has been rendered possible. Physiologists, by the aid of these methods, have drawn up _balance-sheets of energy_ for living beings just as they had previously established _balance-sheets of matter_. Now, it is precisely researches of this kind that we have indicated here as a consequence of biological energetics, which in reality have helped to build up that principle. These researches have shown us that, in conformity with the _principles of thermodynamics_, there was not, in fact, in the organism, any transformation of heat into mechanical work, as the physiologists for a short time supposed, on the authority of Berthelot. With the help of our theory this mistake is no longer possible. The doctrine of energetics shows us in fact the current of energy dividing itself, as it issues from the living being, into two divergent branches, the one thermal and the other mechanical, external the one to the other although both issuing from the same common trunk, and having between them no relation but this, that the sum of their discharges represents the total of the energy in motion. Let us now translate these very simple notions into the more or less barbarous jargon in use in physiology. We shall be convinced as we go on of the truth of the saying of Buffon, that “the language of science is more difficult to learn than the science itself.” We shall say, then, that chemical energy, that the unit of weight of the food which may be placed in the organism, constitutes the alimentary _potential_, the _energetic value_ of this substance, its _dynamogenic power_. It is measured in units of heat, in Calories, which the substance may leave in the organism. The evaluation is made according to the principles of thermo-chemistry, by means of the