168 SOILS: PROPERTIES AND MANAGEMENT 



able, mode of colloidal estimation is that of Mitscherlich, 1 

 in which the adsorptive capacity of the soil is again made 

 the comparative index. Water instead of dye is used as 

 the adsorbed material. In this method the air-dry soil 

 in a thin layer is brought to absolute dryness over phos- 

 phorus pentoxide. It is then placed in a desiccator over 

 a 10 per cent solution of sulfuric acid and the condensa- 

 tion is hastened by a partial vacuum. The sulfuric acid 

 is used in order to prevent the deposition of dew on the 

 soil. After exposure for at least twenty-four hours the 

 soils are found to have taken up their maximum moisture 

 of condensation, which is called the hygroscopic water. 

 The soil is then weighed, and the increase, figured to a 

 percentage basis, is taken as a measure of colloidal con- 

 tent. The reverse process may also be followed, by 

 exposing air dry soil in a saturated atmosphere and 

 afterwards drying over phosphorus pentoxide. The 

 hygroscopicity of the soil, or its hygroscopic coefficient, 

 is thus the basis for colloidal comparison. It is now 

 clear why the term colloidal estimation is employed in 

 this discussion, rather than colloidal determination. 



An objection to the Mitscherlich method is advanced 

 by Ehrenberg and Pick, 2 who claim that the drying over 



1 Rodewald, II., und Mitscherlich, A. E. Die Bestimmung 

 der Hygroskopizitat. Landw. Ver. Stat., Band 59, Seite 

 433-441. 1903. Also, Mitscherlich, E. A., und Floess, R. 

 Ein Beitrage zur Bestimmung der Hygroskopizitat und zur Bewer- 

 tung der physikolischen Bodenanalyse. Internat. Mitt. f. Boden- 

 kunde, Band I, Heft 5, Seite 463-480. 1912. 



2 Ehrenberg, P., und Pick, II. Beitrage zur physikalischen 

 Bodenuntersuchung. Zeit f. Forst- und Jagdwesen, Band 

 43, Seite 35-47. 1911. Also, Vageler, P. Die Rodewald- 

 Mitscherlichsche Theorie der Hygroskopizitat vom Standpunkte 

 der Colloidchemie und ihr Wert zur Beurteilung der Boden. 

 Fuhling's Landw. Zeit., Band 61, Heft 3, Seite 73-83. 1912. 



