244 DISINTEGRATION 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, 

 O'OIS per cent, of ammonia in their castings. 

 It may be here added that I have recently 

 been informed by Dr. Grilbert "that several 

 " square yards on his lawn were swept clean, 

 " and after two or three weeks all the worin- 

 " castings on the space were collected and 

 '* dried. These were found to contain 0'35 

 *' of nitrogen. This is from two to three times 

 " as much as we find in our ordinary arable 

 " surface-soil ; more than in our ordinary 

 " pasture surface-soil ; but less than in rich 

 " kitchen-garden mould. Supposing a quantity 

 "of castings equal to 10 tons in the dry 



* A. A. Julieu " 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. 



