SECT. VI. MANURES. 85 



action of the atmospheric air upon the soil, whether 

 in rendering it more friable, or in hastening the 

 putrefaction of noxious plants ; or, it is owing to the 

 abstraction and accumulation of oxygene. In the 

 case of trenching, it is owing to the increased facility 

 with which the roots can now penetrate to the pro- 

 per depth ; and in the case of deep ploughing, it is 

 owing, as it would appear, to the same cause. 



But it often happens that the soil can no longer By the a p- 

 be ameliorated by any of the foregoing means, and manures. 

 in this case there must be a direct and actual appli- 

 tion made to it of such substances as are fitted to 

 restore its fertility. And hence the indispensable 

 necessity of manures, which consist chiefly of ani- 

 mal and vegetable remains that are buried and 

 finally decomposed in the soil, from which they 

 are afterwards absorbed by the root of the plant, in 

 a state of solution. 



But as carbon is the principal ingredient furnished 

 by manures as contributing to the nourishment of 

 the plant, and is not itself soluble in water, nor even 

 disengaged by fermentation in a state of purity ; 

 under what state of chemical combination is its 

 solution effected ? Is it effected in the state of char- 

 coal ? It has been thought, indeed, that carbon in 

 the state of charcoal is soluble in water; because 

 water from a dunghill, when evaporated, constantly 

 leaves a residuum of charcoal, as was first ascertained 

 by the experiments of Hassenfratz. But there seem 

 to be reasons for doubting the legitimacy of the 



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