CEMENTING MATERIALS IN SOIL 27 



shown by a chemical analysis. The physical properties of 

 a soil cannot safely be inferred from its chemical composition. 

 We shall presently have to refer to a variety of other 

 conditions which affect the character of clay soils. 



Besides clay, soil contains other colloid bodies which help 

 to bind the particles together. Humic acid is well known 

 as a colloid substance ; when combined with lime it possesses 

 considerable cementing power. Schloesing prepares calcium 

 humate by first extracting a soil with dilute hydrochloric 

 acid -to remove the bases with which the humic acid is 

 combined, and then, after washing on a filter, extracting with 

 ammonia. To the dark-coloured solution of ammonium 

 humate thus obtained, hydrochloric acid is cautiously added 

 with constant stirring till a precipitate begins to form. A 

 solution of calcium chloride is then added, and the precipitate 

 of calcium humate collected. Schloesing determined the 

 amount of freshly prepared calcium humate required to give 

 a suitable coherence to a siliceous sand, a comparative experi- 

 ment being made at the same time with a good plastic clay. 

 He found that i per cent, of humic acid in the form of 

 calcium humate had as great a cementing power as n per 

 cent, of plastic clay. If however the humate is thoroughly 

 dried, and then remoistened, it will be found to have lost 

 its cementing power, while the cementing power of clay 

 remains unaltered by this treatment. Humus is well known 

 as one of the most effective materials for improving the 

 physical condition of sandy soils ; it acts partly by giving 

 cohesion to the particles, but still more by increasing the 

 power of the soil to retain water. 



We have already mentioned that humus diminishes the 

 tenacity of clay, an action which is plainly the reverse of 



