38 SOILS. 



both, the action of potassic or sodic chlorid will be to partially replace 

 the lime, while calcic and magnesic chlorids pass into solution, result- 

 ing in the partial or complete replacement of the lime by one or the 

 other, or by both bases. It is important to note that, other things 

 being equal, potash is usually absorbed in greater amounts and is held 

 more tenaciously than soda. The process may frequently be partially 

 or wholly reversed again, by subsequent treatment with large amounts 

 of solutions of the displaced base or bases. Thus while a solution of 

 potassic chlorid may be made to expel almost completely the sodium 

 present in analcite, subsequent treatment with sodic chlorid solution 

 will again almost completely displace the potash before taken up. The 

 same happens when the natural mineral potash leucite, (see p. 32) of fre- 

 quent occurrence in certain lavas, is pulverized and treated with a sodic 

 solution ; resulting finally in the production of a mass corresponding to 

 natural analcite, the sodium mineral corresponding to leucite. 



In other words, in any zeolitic powder the alkaline or alkaline 

 earth bases present may be partially or wholly displaced by 

 digestion with an excess of solution of any of these, varying 

 according to the amount of solution employed, and the length 

 of time and temperature of action. 



This characteristic behavior of zeolites is exactly reproduced 

 in soils. Few soils permit any saline solution to pass through 

 them unchanged ; solutions of alkaline chlorids filtered through 

 soils almost invariably cause the passing through of calcium 

 and magnesium chlorids, while a part of the alkaline base is re- 

 tained ; and as a matter of fact, we find that this absorbing 

 power of soils for alkaline bases is more or less directly pro- 

 portional to the amount of matter which may be dissolved or 

 decomposed with elimination of silica, by means of acids. 



This absorption of bases from solutions by chemical fixation will be 

 farther discussed later on ; but it should be mentioned here that both 

 naturally and artificially, rock-masses are very commonly cemented, 

 wholly or in part, by zeolitic material. Hydraulic concretes may be 

 considered as sandstones or conglomerates whose grains are cemented by 

 a zeolitic cement consisting of silica, lime and alumina, with usually some 

 potash or soda, and of course containing the basic water ; hence unaf- 

 fected by the farther action of the latter substance after the time of setting 

 has expired, which varies somewhat according to the nature of the material 

 used. That similar cements should occur in natural sandstones is to be 



