THE COLLOIDAL MATTER OF SOILS 169 



phosphorus pentoxide will coagulate certain colloids and 

 lower their adsorptive power, thus causing the hygro- 

 scopic coefficient to become an unreliable comparative 

 figure. They suggest first the exposure of the field 

 soil over the water and sulfuric acid, and then the ex- 

 traction of the hygroscopic water over phosphorus pent- 

 oxide. Since this modification is very slow, the original 

 Mitscherlich method, in spite of its faults, remains the 

 most valuable up to the present time. 



