242 DISINTEGEATION Chap. V. 



carbonate of lime and on the oxides of iron. 

 It is, also, known that some of these acids, 

 which were called long ago by Thenard azo- 

 humic, are enabled to dissolve colloid silica in 

 proportion to the nitrogen which they contain.* 

 In the formation of these latter acids worms 

 probably afford some aid, for Dr. H. Johnson 

 informs me that by Nessler's test he found 

 0*018 per cent, of ammonia in their castings. 

 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 



* A. A. Julien " On the Geological action of the Humus-acids," 

 ' Proc. American Assoc. Science,' vol. xxviii., 1879, p. 311. 

 Also on "Chemical erosion on Mountain Summits;" 'New York 

 Academy of Sciences,' Oct. 14, 1878, as quoted in the ' American 

 Naturalist.' See also, on this subject, S. W. Johnson, " How 

 Crops Feed," 1870, p. 138. 



