THE COLLOID AZ MATTEL 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 ^ 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/ 

 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.^ He found that the 

 adsorption of certain dyes by soils afforded a very good 

 index to colloidal content. The difficulty in this method, 

 how^ever, 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- 



^A comparison of these methods is found as follows: 

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

 anorganiseher KoUoide in Zersetzten Gesteinen und deren 

 tonigen Unlagerungsprodukten. Zeitseh. 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, 



