Agricultural and Horticultural Chemistry. 365 



plants, in decaying, give rise to the formation of those very 

 substances requisite for the support and growth of fresh 

 plants. 



We know that plants, in decaying, evolve ammonia, 

 carbonic acid, and other substances which constitute the 

 principal food of plants; but we know also that, in decay- 

 ing, they evolve sulphuretted hydrogen ; it is reasonable, 

 therefore, to expect that that substance, like the other pro- 

 ducts of vegetable decay, should have some influence on 

 the growth of plants. Its presence in manures is well 

 known and readily proved ; but its presence in the air, in 

 which it exists in exceedingly minute quantity, is less 

 readily shown. We know, however, that it is constantly 

 being formed on the surface of the earth, and we have evi- 

 dence of its presence in the air by several effects, — such as 

 the tarnishing of some metals and the blacking of white 

 paint; these effects, which take place gradually and slowly, 

 are principally occasioned by the presence of sulphuretted 

 hydrogen and its compounds, particularly the hydrosul- 

 phuret of ammonia in the air. 



Lastly, if the vegetable kingdom is the great means of 

 purifying the air and retaining it in a fit stale for the res- 

 piration of men and animals, the absorption and decompo- 

 sition of sulphuretted hydrogen by plants nuist constitute 

 not the least important of their functions. It must be ob- 

 vious that, if the inferences to be drawn from such experi- 

 ments as those just described are correct, sulphuretted 

 hydrogen is not the poisonous substance to plants which it 

 is commonly supposed to be; that is to say. that even 

 though in large quantity it may be, and no doubt is, hurt- 

 ful to vegetation, yet in small quantity it may produce 

 most beneficial effects. However advantageous the prepa- 

 ration of manures may be, and however unquestionably 

 important in practical horticulture and agriculture the 

 application of chemistry in preparing and ascertaining the 

 probable value of manure, there will nevertheless be found 

 some who assert that its advantages are doubtful, and that 

 the preparation of manure and the application of chemistry 

 in the way spoken of, occasions unnecessary labor and too 

 much trouble, without producing any essential benefit. 

 This is one great reason why cultivators of the soil are 

 unwilling to enter upon speculations of this kind, until they 

 are assured of their results by ocular demonstration; es- 



