232 WALL'S MANUAL 



Silicic Acid (Silica, Sand'). However strange it 

 may seem, that such substances as sand and flint 

 should be classed among the acids, yet such is the fact. 

 At high temperatures, silica exhibits powerful acid 

 properties, and neutralizes bases, forming a class of 

 salts the silicates. Most rocks and minerals are 

 silicates. 



Although common quartz and sand are totally 

 insoluble in water, yet they are rendered soluble by 

 the action of the alkalies ; hence the reason for apply- 

 ing potash and lime to soils to dissolve their silica. 

 AYhen liberated from its combinations by the agency 

 of the air, it is soluble in water; hence it is always 

 present in springs. Silica is necessary to the growth 

 of vegetation, and exists abundantly in many plants ; 

 particularly in the stalks of grains and grapes. 

 It is this which gives stiffness to their stems,, as 

 the skeleton does to the bodies of animals. If there 

 is a deficiency of soluble silica in the soil, the grain 

 stalks will be weak, and liable to break down or 

 lodge. How necessary, then, to apply substances 

 which will dissolve silica or sand. 



CHAPTEE VIII. 



ANIMAL AND VEGETABLE MANURES. 



Animal maures are compounds of vegetable matter 

 and salts. They, of course, contain all the elements 

 of fertility. The immense variety of substances used 

 and commended for manures, w^ould seem to render 

 the subject both extensive and complicated. It is 

 capable .of simplification. 



