260 ANIMAL HEAT. 



II. In vegetables there is an internal production of heat, as well 

 as in animals ; a fact which has been fully demonstrated by the 

 experiments of Dutrochet and others, already described. In vege- 

 tables, however, the absorption of oxygen and exhalation of car- 

 bonic acid do not take place ; excepting, to some extent, during the 

 night. On the contrary, the diurnal process in vegetables, it is well 

 known, is exactly the reverse of this. Under the influence of the 

 solar light they absorb carbonic acid and exjiale oxygen. And it 

 is exceedingly remarkable that, in Dutrochet's experiments, he 

 found that the evolution of heat by plants was always accompanied 

 by the disappearance of carbonic acid and the exhalation of oxygen. 

 Plants which, in the daylight, exhale oxygen and evolve heat, if 

 placed in the dark, immediately begin to absorb oxygen and exhale 

 carbonic acid ; and, at the same time, the evolution of heat is sus- 

 pended. Datrochet even found that the evolution of heat by plants 

 presented a regular diurnal variation ; and that its maximum of 

 intensity was about the middle of the day, just at the time when the 

 absorption of carbonic acid and the exhalation of oxygen are going on 

 with the greatest* activity. The proper heat of plants, therefore, can- 

 not be the result of oxidation or combustion, but must be dependent 

 on an entirely different orocess. 



III. In animals, the quantities of oxygen absorbed and of carbonic 

 acid exhaled do not correspond with each other. Most frequently 

 a certain amount of oxygen disappears in the body, over and above 

 that which is returned in the breath under the form of carbonic 

 acid. This overplus of oxygen has been said to unite with the 

 hydrogen of the food, so as to form water which also passes out 

 by the lungs : but this is a pure assumption, resting on no direct 

 evidence whatever, for we have no experimental proof that any 

 more watery vapor is exhaled from the lungs than is supplied by 

 the fluids taken into the stomach. It is superfluous, therefore, to 

 assume that any of it is produced by the oxidation of hydrogen. 



Furthermore, the proportion of overplus oxygen which disap- 

 pears in the body, beside that which is exhaled in the carbonic acid 

 of the breath, varies greatly in the same animal according to the 

 quality of the food. Eegnault and Reiset 1 found that in dogs, fed 

 on meat, the oxygen which reappeared under the form of carbonic 

 ,acid was only 75 per cent, of the whole quantity absorbed ; while 



1 Annales de Chimie et de Physique, 3d series, xxvi. p. 428. 



