It is idle to diminish the power of these principles by proceeding to enumerate the whole of the exceptions to their validity. We know perfectly well that there are no absolute principles in nature. Let us say, then, that the energy which temporarily animates the living being is furnished to it by the external world under the exclusive form of potential chemical energy; but that, if there is only one door of entry, there are two exits. It may return to the external world in the principal form of thermal energy and in the accessory form of mechanical energy. § 2. MEASUREMENT OF THE SUPPLY OF ALIMENTARY ENERGY. _Calorimetric Method._—From what has preceded it is clear that if the energetic _flux_ which circulates through the animal emerges, _in toto_, in the state of heat, the measurement of this heat becomes the measurement of the vital energy itself, for the origin of which we must go back to the food. If the flux is divided into two currents, mechanical and thermal, they must both be measured and the sum of their values taken. If the animal does not produce mechanical work, and all ends in heat, we have only to capture, by means of a calorimeter, this energetic flux as it emerges, and thus measure in magnitude and numerically the energy in motion in the living being. Physiologists use for this purpose various types of apparatus. Lavoisier and Laplace used an ice calorimeter—that is to say, a block of ice in which they shut up a small animal, such as a guinea-pig; they then measured its thermal production by the quantity of ice it caused to melt. In one of their experiments, for instance, they found that a guinea-pig had melted 341 grammes of ice in the space of ten hours, and had therefore set free 27 Calories. But since those days more perfect instruments have been invented. M. d’Arsonval employed an air calorimeter, which is nothing but a differential thermometer very ingeniously arranged, and giving an automatic record. Messrs. Rosenthal, Richet, Hirn and Kaufmann, and Lefèvre have used more or less simplified or complicated air calorimeters. Others, following the example of Dulong and Despretz, have used calorimeters of air and mercury, or with Liebermester, Winternitz, and J. Lefèvre (of Havre), have had recourse to baths. Here, then, there is a considerable movement of research which has led to the discovery of very interesting facts. _Measurement of the Supply of Alimentary Energy by the Chemical Method._—We may again reach our result in another way. Instead of surprising the current of energy as it emerges and in the form of heat, we may try and capture it at its entry in the form of potential chemical energy.