336 



PROCEEDINOS OF THK 



the priru ip'.il food of plants, but wo know also that in decayintj 

 tliey cvolvo sulphurrttcd li\droy;en ; it is reasonable therefore to 

 expect that that siibstanee, like the other products of vegetable 

 decay, should have some influence on the growth of plants. Its 

 presence in manures is well known and readily jiroved, but its 

 presence in the air in which it exists in excec(lini;ly uiinute (pian- 

 tity is less readily shewn. \\'e know however that it is constantly 

 beinj; formed on the surface of the earth, and we have evidence 

 of its presence in the air by several elTects, sucli as the tarnishini^ 

 of some metals, and the blackintz; of white paint j these elfects 

 which take place j^radually and slowly, are j)rincipally occasioned 

 by the presence of sulphuretted hydrogen and its compounds, 

 more particularly the hydrosulphuret of ammonia, in the air. 

 Lastly, if the vegetable kingdom is the great means of purifying 

 the air, and retaining it in a fit state for the respiration of men 

 and animals, the absorption and decomposition of sulphuretted 

 hydrogen by plants must constitute not the least important of 

 their functions. It must be obvious, that if the inferences to be 

 drawn from such experiments 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, hurtful to 

 vegetation, yet in small quantity it may produce most beneficial 

 effects. One object gained by exposing dung and similar kinds 

 of manure to the air for some time previous to applying it to the 

 surface of the land is to get rid of much of the sulphuretted hy- 

 drogen e\ olved during its decay, and therefore in preparing 

 artificial manures many chemists have been very careful to ex- 

 clude that substance, regarding it as a poison ; the above experi- 

 ments would lead to the conclusion that its presence is desirable 

 rather than otherwise, but it must certainly be supplied only in 

 hraited quantity, and with cautioL, I may add one fact which 

 has considerable interest in connexion with this matter. It is the 

 custom in most parts of England to form the manure intended to 

 be used for the land into a heap and leave it exposed to the air for 

 some time j in certain counties, and more particularly in the 

 South-west, it is a common practice to plant such manure heaps 

 with cabbages. These plants then, grow in a soil rich in sul- 

 phuietted hydrogen and its compounds, and they are generally 

 very large and of a fine deep green colour. They have it is said 

 a bad taste and are only used for feeding cattle ; this bad taste has 

 however nothing to do with the health and vigour of the plant, 

 and involves another question. The economy of such a system 

 may be questionable, but it is certain that the cabbages must 

 absorb a large quantity of ammonia and carbonic acid, and pro- 



