320 SALTS. 



.812. CLAY. Clay is a silicate of alu- 

 composition mina, commonly containing silicate of po- 

 tassa and other materials in small pro- 

 portion. The best kaolin or porcelain clay is perfectly 

 white, and is nearly pure silicate of alumina. 

 How is soluble 813. SOLUBLE GLASS. Soluble glass is 

 glass made? made by fusing sand with potassa or soda. 

 Its production may be illustrated in a soda bead, by 

 subsequently re-fusing it, with addition of sand. As 

 the silicic acid combines with the soda, carbonic 

 acid is expelled, as will be evident from an efferves- 

 cence on the surface of the bead. Soluble glass is 

 sometimes used as a sort of varnish for rendering wood 

 fire proof. 



814. WINDOW GLASS. Common window 

 manufacture glass is a silicate of lime and soda. To 



f la W ss nd W f rm il > Chalk > S da > q uartz salld and ld 



glass are fused together until the mass be- 

 comes fluid. The molten glass is then blown, by 

 means of an iron tube, as soap bubbles are blown 

 with a pipe. The first form of the bubble is 

 that represented in the figure. The glass blower 

 next contrives to lengthen out the bubble, as he 

 blows it, to a larger size, and finally to blow out 

 the end by a strong blast from his lungs. It 

 is then trimmed with a pair of shears, and the 

 other end cracked off by winding round it a thread 

 of red hot glass. Such a thread is readily produced 

 by cfipping an iron rod into the pot of molten glass, and 

 then withdrawing it. The bubble of glass is thus 



