136 NATURE AND LIFE. 



remain, then, nearly 2,600 heat-units to account for. Other 

 analogous estimates have been made, and physiologists 

 have deduced from them the conclusion that a man of aver- 

 age weight produces in our climate 3,250 heat-units every 

 day; that is to say, a sufficient amount of heat to raise 

 seven gallons of water to the boiling-point. These figures, 

 though approximations, give a sufficiently clear notion of 

 the power of the animal economy to generate heat. 



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 chemi- 

 cal 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 has made profound researches into this subject, re- 

 duces the sources of animal heat to five varieties of trans- 

 formation : first, the effects resulting from the fixation of 

 oxygen with different organic principles ; then, the produc- 

 tion 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 dif- 

 ferent organic acids, alcohols, etc., might be applied to the 

 compounds burned in the animal organism ; but, while admit- 

 ting the theoretic verity of the analogies he establishes, we 

 cannot refrain from remarking 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 



