54 BULLETIN OF THE 



small matter to the heat generated withiu them ; of # course, too, 

 mechanical power is continually transformed into heat within the 

 body of animals, but this neither increases nor diminishes the total 

 amount of energy liberated. 



I yield my hearty assent to that modern scientific induction * 

 which sees in the potential energy of the complex chemical com- 

 pounds supplied to animals by their food, the essential source of all 

 the actual energy of the body, whether manifested in the form of 

 heat or work. In a general way the reduction of these complex 

 chemical compounds by oxidation into the much simpler ones, urea, 

 carbon dioxide, and water, is the means by which potential is con- 

 verted into actual energy. In the case of plants, too, the source of 

 any little heat that may be developed under special conditions, and 

 of such sluggish motions as actually occur, is doubtless to be found 

 in the reduction to simpler combinations by oxidation of a part of 

 the organic matter already formed. The chief function of the veg- 

 etable world, however, is to build up, by means of the solar energy, 

 those complex and unstable organic compounds that supply the 

 animal world with food. Nevertheless, while I yield my hearty 

 assent to this generalization, and freely admit that it is more than 

 a mere deduction from the general doctrine of the conservation of 

 energy — that in fact it affords the most satisfactory explanation yet 

 suggested for a large number of observed phenomena — it is my 

 duty to caution you against the erroneous supposition that any one 

 has ever yet succeeded in affording a rigorous demonstration of the 

 truth of the generalization by an adequate series of actual experi- 

 ments. 



Various attempts have, indeed, been made of late years to de- 

 termine experimentally both for animals and for man, the potential 

 energy contained in the food of a given period, and the actual 

 energy liberated during the same time in the form of heat and work. 

 I think, however, that all practical physiologists who have looked 

 into the question will agree with me that the numerical results 

 hitherto obtained must be received with the utmost caution. 5 

 Difficulties exist on both sides of the problem. It is comparatively 

 easy, no doubt, to obtain a close approximation to the quantity 

 and composition of the food ; but to represent numerically what 

 becomes of it in the body, to deduct correctly what passes through 

 unchanged, and ascertain with reasonable accuracy the amount of 

 carbon dioxide, water, and urea, into which the rest is transformed ; 



