104 



AGRICULTURAL CHEMISTRY 



water, and are commonly called water-glass. Some of 

 the silicates are soluble in acids, but most of them are 

 insoluble complex compounds which are difficult to de- 

 compose. 



When K 4 SiO 4 is treated with HC1, a gelatinous mass 

 containing silicic acid is obtained : K 4 SiO 4 -f 4-HC1 = 

 H 4 SiO 4 + 4KC1. 



H 4 SiO 4 is normal silicic acid. Upon exposure to the 

 air it loses a molecule of water and forms ordinary silicic 

 acid, H 2 SiO 3 , which is decomposed by heat and in the 

 presence of acids forms H 2 O and SiO 2 . 



In addition to the two silicic acids, H 2 SiO 3 and H 4 SiO 4 , 

 there are other forms known as polysilicic acids as : 



H 2 Si 3 O 7 , H 4 Si 3 O 8 , and 



H 2 Si 2 5 , 



obtained by removing 



water from the normal and ordinary silicic acid. 

 2 H 2 Si0 3 = H 2 Si 2 O 5 + H 2 O. 

 3 H 4 Si0 4 = H 4 Si 3 8 + 4 H 2 0. 

 124. Dialysis. In the preparation of silicic acid, the 



process known as dialysis is employed for dissolving and 

 removing the impurities. Some sub- 

 stances, as NaCl and HC1, dissolve 

 and readily pass through animal mem- 

 brane ; such substances are called 

 crystalloids, while bodies like silicic 

 acid, which do not penetrate animal 

 membrane, or do so very slowly, are 

 called colloids. The removal of the 

 HC1 from the solution containing the 



gelatinous silicic acid is accomplished by means of the 



dialyzer, Fig. 46. This property of materials, readily or 



Fig. 46. Dialyzer. 



