OF AGRICULTURE. 225 



silica or sand, with various bases. Silicate of potash 

 and soda forms a colorless glass which is soluble in 

 water. This soluble glass is applied to wood and 

 cloth, etc., to render them incombustible. Silicates 

 of soda and lime forms window glass, the soda gives 

 it a slight greenish tinge. The lime hardens the 

 glass and adds to its lustre. 



Silicates of potash and lime forms plate glass and 

 crown glass (the finest window glass), 



Silicates of potash and lead' yield flint glass and 

 crystal glass. The oxyde of lead gives it great trans- 

 parency, brilliancy and refractive power; it is hence 

 used for chandiliers and optical lenses. 



Silicates of alumina, of oxyde of iron, and potash 

 or soda produce green bottle glass, the color being 

 due to the impurities of the materials. 



Earthenware. Silicate of alumina or clay is the 

 basis of all the varieties of pottery. Its adaptation 

 for this puroose depends upon its plasticity when 

 mixed with water, the readiness with which it may 

 be molded and shaped, and on its capability of 

 being hardened when exposed to a high heat in 

 furnaces or kilns. 



ACIDS. 



Some more extended explanation of the acids may 

 be necessary, we therefore lay them before the 

 reader : 



Carbonic Acid. This acid being one of the most 

 important, we will describe it first. Carbon unites 

 with oxygen in the proportion of one combining 

 weight to two, forming carbonic acid (c. o. 2). Car- 

 bonic acid is a colorless gas, with a slightly sour 

 10* 



