THE ABSORPTIVE PROPERTIES OF SOILS 361 



in certain definite proportion as do crystalloids in the 

 process of crystallization. The more water adsorbed by 

 colloids, the less firmly is it held in combination. There- 

 fore it is easier to evaporate the water when a large quan- 

 tity has been taken up, and as the amount decreases it 

 becomes more difficult to drive it off. 



Another property of colloidal matter is that when it 

 is separated from solution it carries down with it other 

 substances in the solution from which it is precipitated. 

 If, on the other hand, the colloidal matter has been pre- 

 cipitated in a pure state, it absorbs substances from 

 solutions with which it remains in contact for some time. 

 The substances taken up in this way are not chemically 

 combined, but substances that unite chemically may be 

 absorbed. 



The combinations produced by absorption are weak 

 and it is possible to leach out the combined substances, 

 which are generally held in the water of the gels. The 

 following example of one kind of absorption is given 

 by Van Bemmelen : 1 ten grams of a hydrogel having the 

 composition SiO 2 . 4.2 H 2 O, shaken with 100 cubic cen- 

 timeter solution of 20 molecular equivalent KC1, will 

 absorb 0.8 to 1.1 molecular equivalent of the dissolved 

 substance. The absorption in this case was as if the 

 solution had been diluted with 4.2 to 5.8 cubic centimeters 

 of water. As the amount of gel water in 10 grams of 

 hydrogel of SiO 2 is about 5 cubic centimeters, the as- 

 sumption may be made that the dissolved substance is 

 taken up in equal concentration by the gel water. Ten 

 grams of hydrogel of SiO 2 shaken with 100 cubic centi- 



1 Van Bemmelen, J. M. Die Absorptionsverbindungen und 

 das Absorptions vermogen der A.ckererde. Landw. Vers. Stat., 

 Band 35, Seite 75. 1888. 



