16 CHEMICAL MANURES. 



In our next we will broach the subject under its new view, which 

 will bring us into the domain of practice. 



,'NIVKK'S.I rv 



lALIFOI.'N! A. LECTURE SECOND. 



f^\ ENTLEMEN : In our first meeting I endeavored to show you 

 \JT the nature of the elements composing vegetation. You remem- 

 ber that these elements are very unequally distributed in the different 

 organs, or rather between those forming ephemeral combinations 

 before passing into the state of tissues and organs. 



To complete this almost preliminary study, we must now ask in 

 what state we find these elements of nature, the source and cause of 

 fertility of soil, under what form plants absorb them, and to what 

 degree we can, by their aid, act upon the products of vegetation. 



I begin with carbon. 



The quantity of carbon which enters into the composition of plants 

 is, in round numbers, from 40 to 45 per cent. Carbon, then, plays a 

 prominent part in vegetation. If, however, I add that in agriculture 

 it is not necessary that it may be entirely excluded from manures 

 without affecting the fertility of the soil I will appear to contradict 

 myself. 



The contradiction is but apparent, and to prove it, permit me to 

 remind you that the carbon of plants has its origin in the carbonic 

 acid of the air, and the atmosphere is an inexhaustible source of it. 



I need not, therefore, treat of the assimilation of carbon ; in many 

 respects this omission will not be inconvenient ; nevertheless, I have 

 determined to stop here and make this the object of a deep study. 

 Why ? For two reasons because the explanation of this phenom- 

 enon marks an era in the history of science, but particularly because 

 its study will help us to show clearly the essential characteristics of 

 vegetable productions. 



The act which determines the assimilation of carbon is a simple 

 phenomenon. Carbonic acid, formed from carbon and oxygen, is ab- 

 sorbed by the leaves, where it is decomposed. The carbon remains in 

 the plant, while the oxygen, being freed, returns to the atmosphere. 

 Here is produced a truly extraordinary phenomenon, and one which 

 we cannot imitate in our laboratories without calling to our aid the 

 most powerful means of analysis at the disposal of chemistry ; this 

 phenomenon the delicate tissue of the leaf performs without affecting 

 its organization. 



You will see, farther, that vegetable respiration produces effects 

 opposite to animal respiration. Plants borrow carbonic acid from 

 the air and return oxygen to it, while animals, who borrow oxygen, 

 return carbonic acid. This explains the reason why the composition 

 of the atmosphere B not changed by the incessant drain made on it 

 by plants and animals. 



