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 
scoriz. 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 
