numerical tables of Berthelot, Rubner, and Stohmann. The same number also expresses the _thermogenic power_, virtual or theoretical, of the alimentary substance. This energy being destined to be transformed into _vital energies_ (Chauveau’s _physiological work_, _physiological energy_), the dynamogenic or thermogenic value of the food is at the same time its biogenetic value. Two weights of different foods which supply the organism with the same number of Calories,—_i.e._ for which these numerical values are the same,—will be called _isodynamic_ or _isodynamogenic_, _isobiogenetic_, _isoenergetic_ weights. They will be equivalent from the point of view of their alimentary value. And finally, if, as is usually the case, the cycle of energy ends in the production of heat, the food which has been utilized for this purpose has a real _thermogenic value_, identical with its theoretical thermogenic value. In this case it might be determined experimentally by direct calorimetry, measuring the heat produced by the animal supposed absolutely unchanged and identical before and after the consumption of the food. § 3. DIFFERENT TYPES OF FOODS. THE REGULAR, BIOTHERMOGENIC TYPE AND THE IRREGULAR, THERMOGENIC TYPE. Food is a source of thermal energy for the organism because it is decomposed within it, and undergoes within it a chemical degradation. Physiological chemistry tells us that whatever be the manner in which it is broken up, it always results in the same body and always sets free the same quantity of heat. But if the point of departure and the point of arrival are the same, it is possible that the path pursued is not constantly identical. For example, one gramme of fat will always give the same quantity of heat, 9.4 Calories, and will always come to its final state of carbonic acid and water; but from the fat to the mixture of carbonic acid gas and water there are many different intermediaries. In a word we get the conception of varied cycles of alimentary evolutions. From the point of view of the heat produced it has just been said that these cycles are equivalent. But are they equivalent from the vital point of view? This is an essential question. Let us imagine the most ordinary alternative. Food passes from the natural to the final state after being incorporated with the elements of the tissues, and after having taken part in the vital operations. The chemical potential only passes into thermal energy after having passed through a certain intermediary phase of vital energy. This is the normal case, _the regular type of alimentary evolution_. It may be said in this case that the food has fulfilled the whole of its function, it has served for the vital functional activity before producing heat. It has been _biothermogenic_.