J. A. Prescott 



119 



solution were made up to 100 c.c. with dilute nitric acid (making the 

 acidity N/20) and varying amounts of sodium phosphate solution; 

 after three hours' shaking, the precipitates were filtered and PgOg deter- 

 mined in 50 c.c. of the filtrate. The following results were obtained : 



^ma. per 25 c.c. humus solution 



Much work remains to be done on the quantitative relationships 

 in the absorption by the organic matter of soils. 



The introduction of the conception of colloids. 



Van Bemmelen was the first to introduce the conception of colloids 

 into soil problems. The work of Thomas Graham (1861-1864) on 

 colloidal solutions had led to the more precise definition of these 

 bodies. He showed that solutions of gelatine, glue, gum arable would 

 not diffuse freely through organic membranes like parchment, whilst 

 solutions of the ordinary crystalline substances would. He therefore 

 divided substances into "crystalloids" and "colloids." This division 

 is no longer sharply defined, as many substances can be obtained both 

 in the crystalloid condition and in colloidal solution. Graham also 

 found that certain solutions such as silicic acid, tungstic acid, aluminium 

 and iron hydroxides would not diffuse through parchment ; he therefore 

 called these: "colloidal solutions" or "sols." These solutions however 

 did not behave like true solutions in that they were radically changed 

 by the addition of small quantities of electrolytes. The "sol" of 

 silicic acid sets to a jelly in the presence of a trace of carbon dioxide, 

 while a trace of sodium sulphate would precipitate a ferric hydroxide 

 or aluminium hydroxide "sol." The precipitates or the jellies were 

 termed "gels." Van Bemmelen investigated their properties, especially 

 the rate at which they lost water by evaporation and their absorptive 

 power for water and solutions. He further showed that these phenomena 

 in gels resembled some of the known reactions of soils. It is interesting 



