CHAP. V. AND DENUDATION. 245 



" state were annually deposited on an acre, 

 " this would represent a manuring of 78 Ibs. 

 " of nitrogen per acre per annum ; and this 

 "is very much more than the amount of 

 " nitrogen in the annual yield of hay per 

 "acre, if raised without any nitrogenous 

 " manure. Obviously, so far as the nitrogen 

 "in the castings is derived from surface- 

 " growth or from surface-soil, it is not a gain 

 "to the latter; but so far as it is derived from 

 " below, it is a gain." 



The several humus-acids, which appear, as 

 we have just seen, to be generated within the 

 bodies of worms during the digestive process, 

 and their acid salts, play a highly important 

 part, according to the recent observations of 

 Mr. Julien, in the disintegration of various 

 kinds of rocks. It has long been known that 

 the carbonic acid, and no doubt nitric and 

 nitrous acids, which are present in rain-water, 

 act in like manner. There is, also, a great 

 excess of carbonic acid in all soils, especially 

 in rich soils, and this is dissolved by the water 

 in the ground. The living roots of plants, 

 moreover, as Sachs and others have shown, 

 quickly corrode and leave their impressions 



s 2 



