ABSORPTION BY SOIL CONSTITUENTS. 15 



ble hydrolytic products of colloidal nature. This action of course 

 becomes more pronounced the finer the particles. Cushman (6) 

 shows that this action takes place to an appreciable extent in a 

 few hours and suggests that a colloidal film is thereby formed on 

 the surface of the particles. Michaelis (16) states that some finely 

 powdered quartz which he prepared absorbed water and swelled; 

 also that a silicic acid gel may be made by boiling a quartz suspension 

 for several days. We found that treating powdered orthoclase with 

 hot water for two days, then leaching free from soluble salts, increased 

 its absorptive capacity for water vapor from 0.0016 gram to 0.0046 

 gram. 



The fact that the most finely powdered minerals, even though 

 they must have suffered considerable alteration, had absorptive 

 capacities less than the ultra clays, affords some evidence that the 

 colloidal matter in soils is not made up of merely finely divided 

 minerals. More conclusive evidence of this, however, is afforded by 

 analytical data which are being prepared for publication. These data 

 show that no probable combination of soil minerals has the same 

 chemical composition as the typical colloidal matter in soils. 



ABSORPTIVE CAPACITIES OF SYNTHETIC GELS. 



It was thought that further evidence on the nature of the col- 

 loidal matter in soils might be obtained from the absorptive capaci- 

 ties of inorganic gels; since, as previously stated, other investiga- 

 tions in this bureau indicate that the colloidal material in soils is 

 largely of a gel-like natui'e. An examination of the ultra clays 

 extracted from many different soils has indicated that they consist 

 chiefly of aluminum silicate with some ferric silicate and one or more 

 of the following substances in the gel form; ferric hydroxide, alumi- 

 num hydroxide, organic matter, and possibly silicic acid (20). If 

 this is true, it would seem that synthetic preparations of these indi- 

 vidual gels should have absorptive capacities of the same order as 

 the ultra clays. Four of these gels were therefore prepared and their 

 absorptions tested. 



An aluminum silicate gel was made from sodium silicate and 

 aluminum chloride mixed in the proper proportions for forming 

 kaolinite (AloOg. 2Si02. 2H2O) ; the ferric hydroxide was prepared 

 from ferric chloride and sodium hydroxide ; the aluminum hydroxide 

 from aluminum chloride and sodium hydroxide; and the silicic acid 

 from sodium sUicate and sulphm'ic acid. The gels thus formed were 

 repeatedly washed with large amounts of distilled water until prac- 

 tically free from soluble salts. The wash waters were removed 

 through Pasteur-Chamberland filters, using suction, and the gels, 

 after being concentrated by air-drying, were ground to pass a 200- 

 mesh sieve. 



For comparison with these freshly prepared gels, a sample of 

 silica gel was procured from a commercial chemical company and 

 samples of alumina and iron gels from the Chemical Warfare Service. 

 The absorptive capacities of these products all ground to pass a 

 200-mesh sieve are shown in Table 5. Included in this table also 

 are the absorptions of a peat soil and of an extract from it con- 

 taining no particles above 1 micron. The peat soil contained very 

 little mineral matter, so these samples are probably fairly repre- 

 sentative of the organic colloids in soils. 



