CHEMICAL, MANUJiES. 21 



depths of the soil, which the leaves decompose and assimilate. The 

 carbonic acid of the soil is provided by the decomposition of vege- 

 table matter which is wanting. Thus, the economy of the origin of 

 carbon in vegetation is summed up in three facts : 



It is always absorbed in the form of carbonic acid. 



The leaves digest it. . 



The sun's rays are necessary to determine it. 



Let us proceed to the origin of oxygen and hydrogen. I could 

 tell you the same of these two bodies as I have already said of car- 

 bon. Their functions in the economy of vegetation have but a 

 theoretic interest. 



Both come from water, and plants, as regards the source of oxygen 

 and hydrogen, receive more through the rain than they can make 

 ifce of. 



Is it certain, you will probably ask, that oxygen and hydrogen are 

 derived from water ? 



No question easier to determine than this. 



Cultivate burnt sand, and let the plant receive oxygen and hydro- 

 gen only through distilled water ; you will see how the water changes 

 its condition under your eyes, and enters into the composition of the 

 plants. 



We come now to azote. 



The question changes its character with azote. The origin of this 

 body in plants opens to us a problem of the first order. 



Now, this problem may be resolved in two different ways by 

 science and practice. 



I prefer to demonstrate by practice. 



I lay down as an axiom that plants can assimilate azote in three 

 different forms : 



In the form of ammoniac or salts of ammonia ; 



In the form of nitrate ; 



In the form of gaseous azote. 



And I add, that each of these three forms adapts itself by prefer- 

 ence to certain lists of plants the ammoniac to wheat, the nitrate to 

 beets, while the legumes absorb azote, especially under the form of 

 elementary gas. 



This point admitted, I ask if harvests in general contain more 

 azote than the manures which produce them ? 



Facts prove this unanimously ; there is always an excess of azote 

 in the harvest. 



We find, for example, that the excess (and this is the minimum 

 value) in sweet potatoes rises to 38 pounds, and in lucerne to 151 

 pounds the acre. 



Here a new question arises : From whence this excess of azote ? 

 From the soil ? Evidently not, for it is a permanent and continued 

 phenomenon. This excludes the idea of its coming from the soil, 

 since its resources are limited and it yields yearly, through its 

 harvests, more azote than it receives by manures. 



We cannot doubt, then, that the excess of azote comes from the 

 air. But here another difficulty : In what form has the azote been 



