136 HOW CEOPS GROW. 



solution. It is formed when the solution of an alkali- 

 silicate is decomposed by means of a large excess of some 

 strong acid, like the hydrochloric or sulphuric. 



EXP. 57. Dilute half the solution of potassium silicate obtained in 

 Exp. 56 with ten times its volume of water, and add diluted hydrochloric 

 acid gradually until the liquid tastes sour. In this Exp. the hydrochlo- 

 ric acid decomposes and destroys the potassium silicate, uniting itself 

 to the base with production of chloride of potassium, which dis- 

 solves in the water present. The silica thus liberated unites chemi- 

 cally with, water, and remains also in solution. 



By appropriate methods Doveri and Graham have 

 obtained solutions of silica in pure water. Graham pre- 

 pared a liquid that gave, when evaporated and heated, 

 14 per cent of anhydrous silica. This solution was clear, 

 colorless, and not viscid. It reddened litmus-paper like 

 an acid. Though not sour to the taste, it produced a 

 peculiar feeling on the tongue. Evaporated to dryness at 

 a low temperature, it left a transparent, glassy mass, 

 which had the composition H 2 Si0 3 . This dry residue 

 was insoluble in water. These solutions of silica in pure 

 water are incapable of existing for a long time without 

 suffering a remarkable change. Even when protected 

 as much as possible from all external agencies, they 

 sooner or later, usually in a few days or weeks, lose their 

 fluidity and transparency, and coagulate to a stiff jelly, 

 from the separation of a nearly insoluble hydrate of silica, 

 which we shall designate as gelatinous silica. 



The addition of 7^^ of an alkali or earthy carbon- 

 ate, or of a few bubbles of carbon dioxide gas to the strong 

 solutions, occasions their immediate gelatinization. A 

 minute quantity of potash or soda, or excess of hydro- 

 chloric acid, prevents their coagulation. 



Gelatinous Silica. This substance, which results 

 from the coagulation of the soluble silica just described, 

 usually appears also when the strong solution of a silicate 

 has strong hydrochloric acid added to it, or when a sili- 

 cate is decomposed by direct treatment with a concen* 

 trated acid. 



