C 113 3 



ments in the greater number of the vegetable sub- 

 stances which can be used as food, have been already 

 ascertained by philosophical chemists, and have been 

 stated in the preceding pages ; the analysis by distil- 

 lation may, however, in some cases, be useful in esti- 

 mating the powers of manures in a manner that will 

 be explained in a future lecture. 



The statements of the composition of vegetable 

 substances, quoted from M. M. Gay Lussac and Then- 

 ard were obtained by these philosophers by exposing 

 the substances to the action of heated hyper-oxymu- 

 riate of potassa ; a body that consists of potassium, 

 chlorine, and oxygene ; and which afforded oxygene to 

 the carbon and the hydrogene. Their experiments 

 were made in a peculiar apparatus, and required great 

 caution, and were of a very delicate nature. It will 

 not' therefore be necessary to enter upon any details 

 of them. 



It is evident from the whole tenor of the state- 

 ments which have been made, that the most essential 

 vegetable substances consist of hydrogene, carbon, 

 and oxygene in different proportions generally alone, 

 but in some few cases combined with azote. The 

 acids, alkalies, earths, metallic oxides, and saline com- 

 pounds, though necessary in the vegetable ceconomy, 

 must be considered as of less importance, particularly 

 in their relation to agriculture, than the other princi- 

 ples : and as it appears from M. de Saussure's table, 

 and from other experiments, they differ in the same 

 species of vegetable when it is raised on different soils, 



