On the Application of Manures. 



247 



wheat will not flourish in a sandy or oven in a calcareous soil, 

 unless a considerable quantity of clay be also present, the latter 

 being required to furnish a sufficient proportion of alkali, which 

 the other earths do not contain. 



Accordingly, fir-trees and other evergreens grow well in sand- 

 stone and in limestone, whilst those which shed their leaves thrive 

 better on rocks of a granitic character ; for the latter require more 

 alkali, and can procure a larger supply of that constituent from 

 felspathic materials. 



But the inorganic matters already alluded to constitute, after 

 all, an inconsiderable part of the vegetable structure, when com- 

 pared with those volatilisable principles which accompany them 

 in plants. 



It becomes therefore a still more important inquiry, from 

 w^hence do the latter derive the carbon, the hydrogen, the oxygen, 

 the nitrogen^ which they all appear to contain ? Do these prin- 

 ciples proceed from the soil, or from the atmosphere, or must we 

 conclude that both the one and the other contribute to supply 

 them ? 



Until within the last century it would have been taken for 

 granted that the soil was the source from whence proceeded all 

 the solid matter at least which entered into the constitution of a 

 plant ; and there were several circumstances which tended to 

 countenance such an opinion. No plants, it was observed, would 

 continue long to thrive in earth unmixed with some proportion of 

 vegetable mould ; and the fertility of the latter is greatly en- 

 hanced by the addition of animal or vegetable matter in that state 

 of decay in which it becomes soluble in water, and therefore fitted 

 to obtain admission into the vessels of plants. 



Hence, when Priestley had demonstrated that leaves decompose 

 the carbonic acid of the atmosphere, giving out its oxygen and 

 assimilating its carbon, the doctrine alluded to still to a certain 

 extent maintained its ground ; and it was even questioned by 

 Ellis and others, whether, in fact, if we were to strike the balance 

 between the opposite influence of a plant during the day and the 

 night, as much carbonic acid might not be exhaled by it at one 

 period as had been decomposed at another. 



I was therefore induced myself to undertake some experiments,* 

 the results of which appear to establish that plants, even in a 

 confined atmosphere, do in reality add a great deal more oxygen 

 to the air than they abstract from it, whilst the amount of car- 

 bonic acid which may be introduced undergoes at the same time 

 a corresponding diminution. 



This effect I even found to take place in diffused light, as well 



* See Philosophical Transactions for 1836. 



VOL. II. S 



