HEAT AND LIFE. 133 



then of hydrogen and respiration regarded as throwing 

 off out of the animal carbonic acid and vapor of water. 



Lavoisier's experiments have been repeated and varied, 

 and his conclusions discussed in many ways for nearly a 

 hundred years. Several experimenters have corrected or 

 perfected some points, but the general doctrine has not 

 been shaken by the recognition of its secondary and very 

 subtile difficulties, several of which still puzzle physiologists. 

 It is, indeed, undeniable that the greater part of the reac- 

 tions which occur in the system, with the production of 

 heat, do bring out, as a result, the exhalation of watery va- 

 por and carbonic acid from the lungs ; but these two gases 

 cannot arise from a direct combustion of hydrogen and car- 

 bon, because the system does not contain such substances 

 in a free state. They represent really only the close of a 

 succession of transformations, often distinct from combus- 

 tions, properly so called. On the other hand, these are not 

 the only residue of the chemical operations performed in 

 the vital furnace. Besides the water and carbonic acid 

 thrown off by animals in breathing, which are like the 

 smoke of this elaboration of nutrition, they excrete by 

 other channels certain principles which are, as it were, the 

 scorias. Now, these principles of disassimilation, among 

 which should be noted urea, uric acid, creatine, cholesterine, 

 etc., could not be results of pure combustion, and they de- 

 note that the circulating current is the seat of extremely 

 manifold reactions, the laws of which we are only begin- 

 ning to gain a glimpse of. 



The latest advances of chemistry allow us, indeed, to 

 follow the linked sequence of the gradual transformations 

 of nutritive substances into the cycle of vital operations. 

 It is well, at the outset, to fix exactly the seat of these 

 phenomena. They take place in all the points of the sys- 

 tem traversed by the capillary vessels. The glands, the 

 muscles, the viscera, in brief, all the organs, are in a state of 



