100 CHYMISTRY APPLIED TO AGRICULTURE. 



the soil the oxygen and carbonic acid contained in it in a 

 free state, or dissolved in water, and also the juices and 

 salts which are mixed with the earth. 



Water appears to be the necessary vehicle of nearly all 

 the nutritive portions of the soil ; so that it not only serves 

 to nourish plants, by yielding to them the elements of 

 which it is itself composed, but it conveys into their inter- 

 nal organs all the substances which can serve them as 

 food. 



The substances which chiefly afford nourishment to 

 plants, present in their composition only carbon, hydrogen, 

 and oxygen ; the numerous products formed in the course 

 of vegetation, do not upon analysis furnish any other prin- 

 ciples ; the salts, the earths, and the metals are generally 

 found in them in very small quantities, and under a very 

 different form from that in which they exist in the soil. 



Strictly speaking, the three principles necessary to vege- 

 tation are oxygen, carbon, and hydrogen, combined in va- 

 rious proportions ; and it is this difference in the propor- 

 tions which causes the immense variety in the vegetable 

 kingdom : some hundredths more or less of carbon, oxy- 

 gen, or hydrogen change the character of the body. 



The chymist in experimenting upon dead plants pro- 

 duces at pleasure a part of these effects : fermentation and 

 spontaneous decompositions give rise to a great number. 

 But the constant uniformity of the products in the same 

 species of plants, and the analogy existing between those 

 derived from different species of the same genus; their 

 variety in the different organs, and the peculiar com- 

 pounds, apparently so complicated, of each one of them, 

 form altogether so many phenomena beyond the power of 

 art to explain. 



We know the substances received by plants, and those 

 which they reject; we determine by analysis the nature 

 and the composition of the products which they form ; but 

 this is the utmost extent of our knowledge. All that passes 

 within the plant is still a mystery, and belongs to the laws 

 of vitality, which modify by their action those physical 

 laws that are known to us. 



However, as the laws of vitality governing vegetables 

 are in their application less independent of the physical 

 laws, than those that reign in the animal kingdom, we can 

 even now raise a portion of the veil, and follow at least 

 the progress of the changes, though we can as yet neither 

 produce them nor discover their mode of action. 



