448 BIOLOGY. [Book Tir. 



combustion of living substances, and say ■with Lavoisier : — 

 "The animal machine is governed by three regulating principles, 

 respiration, which consumes hydrogen and carbon and furnishes 

 caloric ; transpiration, which augments or diminishes, according 

 as it is necessary to get rid of more or less caloric j finally, 

 digestion, which restores to the blood what it loses by respiration 

 and transpiration." 1 



But, thanks to the progress of modern physics, chemistry, and 

 physiology, we can follow, step by step, minutely, these impor- 

 tant phenomena, indicate what processes organisms employ to 

 produce, according to need, heat and cold, state what organisms 

 are the most active producers of heat ; finally, specify the office 

 of the aliments, and apply to the working of living machines 

 the great law of the correlation of physical forces. 



' Lavoisier, Mimoires de I'Academie des Sciences, 1789. 



