64 ANIMAL CHEMISTRY LECTURE III. 



without some oxygen, in the form of vegetable tissue or secretion. 

 When the tissue or secretion is subjected to a full red heat, it 

 yields, among other products, free carbon, free hydrogen, and 

 various compounds of carbon with hydrogen. The piece of 

 wood-charcoal now in my hand, for instance, has resulted indi- 

 rectly from a gradual deoxidation of carbonic anhydride by the 

 living plant, just as this other piece of charcoal in the flask has 

 resulted directly from a violent deoxidation of carbonic anhydride 

 by the metallic sodium. 



(67.) Thus, then, we have presented to our notice the most 

 important terrene, or rather cosmical function of plant life. The 

 living plant effects a decomposition of carbonic anhydride and 

 water, evolves the liberated oxygen, and retains within its organ- 

 ism the united carbon and hydrogen, which becoming the food of 

 animals, are simultaneously disunited and re-oxidised once more 

 into carbonic anhydride and water. Now, I wish to consider 

 with you I was going to say more minutely, but I should rather 

 say more broadly what is the essence of these complementary 

 actions in their relation to the first principles of that dynamical 

 philosophy which is often spoken of as the science of energetics. 

 It formed part of my original plan to give merely a passing glance 

 at this subject, and I certainly should not have ventured to dis- 

 cuss it in the elementary form in which I now propose to bring it 

 under your notice, had it not recently come to my knowledge 

 that certain principles of mechanical philosophy admitted by that 

 class of naturalists who are called physicists, to be as fundamental 

 even as the laws of gravitation, are not generally acknowledged 

 by that other class of naturalists who are called physicians. Now, 

 in order to contrast with one another the great antagonistic func- 

 tion of plants and animals the decomposition of carbonic anhy- 

 dride and water by the one class, and recomposition of carbonic 

 anhydride and water by the other it would not conduce to my 

 object, even if it were within my competency, to discuss with 

 you the simplest functions of organic life, as manifested by the 

 most minute and simple organisms, in some of which it is scarcely 

 possible for us to distinguish between the animal or vegetable 



