Dr Davy's Agricultural Discourse. 389" 



ing to its quality, — that is, the proportion of finely-divided alunii- 

 nons matter it contains, and the proportion of sand. As it appears 

 that in some instances this process of subsoil-ploughing has been of 

 little advantage, not repaying the cost, it may be prudent to try the 

 effect of it on a small portion of the drained land, and to be guided 

 by the result as to its extension, — for example, the quality and 

 quantity of produce on the portion subsoiled, compared with the 

 quality and quantity of the crop on an equal portion merely drained. 



Allow me now to turn your attention to the atmospheric air, and 

 to the rain-water, for the admission and penetrating of which into 

 the soil, without stagnation of the latter, thorough-draining, as re- 

 gards its function, may be considered in the first place as instituted. 



Atmospheric air, we know, is composed of oxygen and azote and 

 carbonic acid in almost constantly the same proportion, viz., 21 

 parts in volume of oxygen, 79 of azote, and about the one-half of a 

 thousandth part of carbonic acid, with a very variable proportion of 

 water diffused through it in the elastic state in the form of vapour, 

 and when in the vesicular state in the form of clouds, and also an 

 extremely minute portion, there is reason to believe of carbonate of 

 ammonia, and of some other matters, chiefly saline, either held in 

 solution in it or in suspension. 



Kain, it is to be remembered, is never absolutely pure water : it 

 is variously impregnated ; and this in consequence of two offices 

 which it seems to have to perform, (not to mention others) ; namely, 

 the purifying of the atmosphere, and the fertilizing of the earth. 

 Carbonic acid, oxygen, and azote, are always contained in it, and 

 the former in considerably larger proportion than in the atmo- 

 sphere, oxygen being more soluble in water than azote. And be- 

 sides these, there are other matters, such as carbonate of ammonia, 

 and various substances which it brings down with it, exercising its 

 purifying function, from the atmosphere, in which they were sus- 

 pended or dissolved. 



The rain entering the soil thus impregnated, not only immediately 

 promotes active vegetation, but also has an ameliorating effect on 

 the soil and the subsoil, fitting it for the purposes of vegetation. 

 The water, impregnated with oxygen, promotes the decomposition 

 of animal and vegetable matter, thus forming food for plants; and 

 acting on compound minerals in the soil and subsoil, it produces the 

 separation of their elements ; and thus forms new mould. Thorough- 

 draining, by preventing the stagnation of water, and promoting its 

 descent, administers in a remarkable manner to these ameliorating 

 effects. And preserving the soil and subsoil in a porous condition, 

 it administers also to another effect, not insignificant in the economy 

 of vegetation, namely, the formation of ammonia by the union of 

 the azote of the atmospheric air penetrating into the earth, with 

 hydrogen, as it is disengaged from decomposing animal and vege- 

 table matter; tlius supplying an alkali, which appears to bo the 



