THE COLLOIDAL MATTER OF SOILS 167 



certain qualities, supposed to be developed largely by 

 colloid content. This indicates that the methods are 

 largely comparative, rather than exact or strictly analyti- 

 cal in nature. These important methods l are three in 

 number: Van Bemmelen's, Ashley's, and Mitscherlich's. 



Van Bemmelen. — The first investigator to advance 

 a method for colloid estimation was Van Bemmelen, 2 

 who considered that the amount of silica dissolved from 

 a soil by digestion with hydrochloric or sulfuric acids 

 was a measure of its colloidal content. It is now known 

 that some materials, such as crushed rock, may yield 

 as much silica with this treatment as a highly colloidal 

 clay. This method is not of great importance at the 

 present time, except as to the information that it gives 

 regarding the evolution of colloidal soil study. 



Ashley. — A second method, and one of much more 

 value, has been evolved by Ashley. 3 He found that the 

 adsorption of certain dyes by soils afforded a very good 

 index to colloidal content. The difficulty in this method, 

 however, lies in choosing the most effective dye and regu- 

 lating its concentration. Moreover, different colloids 

 vary so much in adsorptive capacity for the same dye, 

 that only roughly comparative results have thus far been 

 possible. 



Mitscherlich. — The third, and as yet the most valu- 



1 A comparison of these methods is found as follows : 

 Stremme, H., and Aarnio, B. Die Bestimmung des Gehaltes 

 anorganischer Kolloide in Zersetzten Gesteinen und deren 

 tonigen Unlagerungsprodukten. Zeitsch. f. Prak. Geol., 

 Band 19, Seite 329-349. 1911. 



2 Van Bemmelen, J. M. Die Adsorptionsverbindungen 

 und das Absorptionsvermogen der Ackererde. Landw. Ver. 

 Stat., Band 35, Seite 69-136. 1888. 



3 Ashley, H. E. The Colloid Matter of Clay and Its 

 Measurement. U. S. Geol. Sur., Bui. 388. 1909. 



