Of late years, the question has been taken up again with more exactness, thanks to the views of a new science called "heat-chemistry," which occupies itself with chemical phenomena in their relations to heat. Heat-chemistry, by the aid of very delicate apparatus for measuring heat, ascertains the number of heat-units developed or absorbed by bodies entering into combination, beginning with the noted experiments of Favre and Silbermann. Berthelot, who had made profound researches into this subject, reduces the sources of animal heat to live varieties of transformation: first, the effects resulting from the fixation of oxygen with different organic principles; then the production of carbonic acid by oxidization; then the production of water; in the fourth place, the formation of carbonic acid by decomposition; and, last, hydrations and dehydrations. The learned chemist attempted to show how the numbers obtained in the study of the heat of combustion of the different organic acids, alcohols, etc., might be applied to the compounds burned in the animal organism; but, while admitting the theoretic verity of the analogies he establishes, we cannot refrain from re marking that their practical verification is exceedingly delicate and difficult. How can we measure, at any one point of the system, the heat produced by a fleeting reaction occurring in the inmost depths of a tissue that must be lacerated to be examined?
If thermo-chemistry seems not to throw much light on physiology on this side, it reveals to it on another sources of heat that had hitherto escaped notice. Berthelot shows that carbonic acid in the system is not always formed by oxidization of carbon, but sometimes proceeds from decomposition absorbing heat. We know that alimentary substances are reducible to three fundamental types—fats, hydrates of carbon (sugars, fecula, starch), and the albuminoids. Now, the fats, in decomposing and combining with water, as it occurs under the influence of the pancreatic juice, evolve heat; and so it is with the hydrates of carbon, independent of any oxidization. And albuminous substances, too, produce very clear calorific phenomena, when their combination with water takes place with its consequent various decompositions. These facts, noted by Berthelot, must have their place in the minute and exact calculation of animal heat, which it is perhaps as yet too early to undertake. At any rate, this heat originates in the totality of those chemical transformations which are going on unceasingly in the depths of the animal organs, and are bringing about the continual renovation of the whole organized substance; in other words, nutrition; but why that nutrition—why that perpetual production of heat in the living machine?
We have now the means of answering this question, which involves the secret of one of Nature's most beautiful arrangements. The heat produced by animals is the source of all their movements; in other words, the mechanical labor they perform is a mere simple transformation of the activity of heat they develop. They do not create mo-
