76 On the Injiaence of ike 



some of which, however, rendered it doubtful. The prac- 

 tical agricuhurist has made no uise of this doctrinv, as he 

 observed in many cases the inipossibilitv of subjcctinii; his 

 fields to so expensive a revolution, and knew, from long 

 experience, that ground without manure prt)duces verv 

 little ; and therefore he has neglected those works which 

 recommended it. 



But the case has been diflcreut with the modern chemis- 

 try, which perhaps will make us better aco,uainted with the 

 nourishment and growth of vegetables. Bv the help of the 

 pneumatic apparatus other prmciples have been exliibited 

 to us in the vegetable kingdom than those supposed to exist 

 in them bv our forefathers- Modern chemistry gives as 

 the component parts of vegetables, carbon, hvdrogen, azote, 

 and oxygen, and shows that the existence of earths and 

 metals m the residuum is merely accidental ; or at least 

 considers them as doubtful component parts of vciietables. 

 Whether alkalies exist in a substantial form in plants during 

 their vegetation cannot with certainty be determined, as it 

 is Iiighiy probable that they may be formed from hydrogen 

 and azote during the decomposition of organic bodies by 

 the help of tire. The native saltpetre found m the Glcanna 

 hederacea, Hc/iaJithi/s cuir.iiHS, Ti/ssihigo farj'ura, and some 

 species of gourd, aiid which is, obtained from extracts of 

 Xhe above plants, siiows at ativ rate the existence of the 

 principles of these salts, which can onlv unite when the 

 vital power ceases to act. It is proved, by the experiments 

 and discoveries of modern chemistry, that the separation and 

 union of manv principles take place in the course of vegeta- 

 tion. For it is shown by experience, that when plants are irri- 

 tated in the light or by the light, the water is decomposed 

 oxygen set free, and that thty take into them, as a coiu- 

 ponent part, the hvdrogen ; also that a similar decomposi- 

 tion with the carbonic acid must take place when the plants 

 are irritated by light; for it is only in the dark that carbonic 

 acid is exhaled. 



The presence of the fourth principle, azotic gas, in vege- 

 tables, is proved by the component parts of the gluten in 

 maize, which by distillation in the dry way gives ammonia; 

 and this, as is well known, can be asrain decomposed into 

 azote and hydrogen. These principles the plants receive 

 from the atmosphere ; for in the atmospheric air with w hicU 

 the plants are continually surrounded azote exists ^s a com- 

 ponent part ; and Priestley luis proved, by a series of experi- 

 ments, 

 3 



