[ 5'4 1 



of absorption in a very marked degree, absorptive power has 

 been noticed even in compact rocks, such as basalt, shale, 

 or marl zeolites. 



821. Soils with double silicates have higher absorptive 

 power. The hydrated double silicates in the soil resemble 

 zeolites, which contain Ca, Mg, K and Na and are de- 

 composed easily by HC1. Some of the natural double silicates 

 of the soil have been actually identified as zeolites, and those 

 containing such have the high absorptive power of zeolites. 

 An artificial preparation of silicate of alumina and soda pos- 

 sesses an absorptive power resembling that of clay loams. 

 The artificially prepared hydrated double silicate which shows 

 the highest absorptive power, contains 46 per cent, of SiO 2 , 

 26 per cent, of A1 2 O 3 , 16 percent, of Na 2 O and 12 percent, of 

 H 2 O. When this artificially prepared double silicate is treat- 

 ed with a lime salt most of the Na 2 is replaced by CaO, and 

 when it is afterwards treated with K 2 O, CaO is partly 

 replaced by K 2 O. In the same way, MgO and NH 3 can be 

 made to enter into the composition of this artificial mixture 

 which may now be called soil. That natural soils con- 

 tain similar double silicates to those of this artificially 

 prepared soil, is rendered certain by the following facts : 

 (i) Soils after treatment with HC1 yield to a solution of 

 Na 2 CO ;3 much larger quantity of soluble SiO 2 than before treat- 

 ment with HC1, and such soils generally have a high absorp- 

 tive power. (2) Soils treated with HC1 which lose their 

 absorptive power regain this power on addition of Na 2 CO 3 or 

 CaCO 3 which enables the precipitated Si0 2 to reform hydrated 

 silicates. (3) Hydrates of Fe and Al have the power of 

 absorbing small quantities of NHs, K 2 O &c., when presented as 

 hydrates, carbonates, or phosphates ; but they have very little 

 power of absorbing bases when presented in the form of chlo- 

 rides, sulphates, or nitrates. They have the power of fixing 

 P 2 O 5 and also absorbing small quantities of HC1 and H 2 SO 4 

 fixing them as highly basic compounds. (4) Hydrated Silicic 



