165 



undergo spontaneous decompositions, wliich, independently 

 of it, would not take place. 



Soils are surpiisini^ly benefited by aeration, and the free 

 admission of the weather into their interior parts. This is 

 generally considered as the principal use of fallowing ; and 

 its importance in gardening is proved by compost heaps, and 

 both winter and summer ridging up. The precise advanta- 

 ges, however, of exposure to the aii-, independently of the 

 concurrent influence of water, heat, and the other eflects 

 above mentioned, as resulting from pulverization, do not 

 seem at present to be fully ascertained. It is admitted on all 

 hands, that carbonic acid gas is absorbed by calcareous 

 earths ; and Dr. Thomson, who is among the ablest of our 

 chemical writers, is of opinion, that the earths alone may 

 thus, in all likelihood, administer food to plants. Sir H. 

 Davy seems to consider mere exposure to the atmosphere as 

 of no benefit whatever to soils: he, therefore, condemns 

 fallowing as useless and unprofitable. But the reasons given 

 for such an opinion are merely speculative, and founded on 

 nothing experimental or conclusive ; accordingly, they will 

 convince no skilful agriculturist, nor no practical planter. 



To these facts and observations we may add a suggestion 

 of Darwin's, respecting temperature, which though fanciful, 

 is ingenious, namely, that a certain portion of atmospheric 

 air being always taken down into the soil, at the time of 

 pulverization, its internal heat is thereby promoted, and its 

 fertility increased. The confinement of the air occasions 

 decomposition, by means of the moisture in the earthy por- 

 tions. Ammonia is formed, by the union of the hydrogen 

 of the water, with the nitrogen of the atmosphere, and nitre, 

 by the union of oxygen and nitrogen. The oxygen hke- 

 wise probably unites with the carbon contained in the soil, 

 and forms carbonic acid gas, and carburetted hydrogen. 

 Hence the heat given out, during these processes. 



Such is a short, and very imperfect view of the manifold 



