THE EE- ACTIONS OF ORGANIC MATTER ON FORCES. 43 



in them other actions which are much more important. In- 

 deed, what we may call the indirect re-actions thus caused, 

 are so great in their amounts compared with the direct re- 

 actions, that they quite obscure them. 



In strictness, these two kinds of re-action should not be 

 dealt with together. The impossibility of separating them, 

 however, compels us to disregard the distinction between 

 them. Under the above general title, we must include botli 

 the immediate re-actions and those re-actions mediately 

 produced, which are among the most conspicuous of vital 

 phenomena. 



§ 18. From organic matter, as from all other matter, 

 incident forces call forth that re-action which we know as 

 heat. More or less of molecular vibration almost necessarily 

 results, when, to the forces at work among the molecules 

 of any aggregate, other forces are added. Experiment 

 abundantly demonstrates this in the case of inorganic 

 masses ; and it must equally hold in the case' of organic 

 masses. In both cases the force which, more mark- 



edly than any other, produces this thermal re-action, is that 

 which causes the union of different substances with each 

 other. Though inanimate bodies admit of being greatly 

 heated by pressure and by the electric current, yet the 

 evolutions of heat thus induced, are neither so common, nor 

 in most cases so conspicuous, as those resulting from chemical 

 combination. And though in animate bodies, there are 

 doubtless certain amounts of heat generated by other actions ; 

 yet these are all secondary to the heat generated by the 

 action of oxygen on the substances composing the tissues and 

 the substances contained in them. Here, however, 



we see one of the characteristic distinctions between inani- 

 mate and animate bodies. Among the first, there are but 

 few which ordinarily exist in a condition to evolve the heat 

 caused by chemical combination ; and such as are in this 

 condition soon cease to be so, when chemical combination 



