276 EARTHS. 



bulky gelatinous hydrate is obtained, by a gentle evaporation : it is 

 decomposed at a common temperature, but entirely at ignition. Dr. 

 Thomson,* has shown that there are several hydrates of silica. 



(e.) Insoluble in acids, except the fluoric, which attacks it with 

 great energy. 



(f.) When newly precipitated, soluble to some extent, in several 

 acids, and readily forms triple salts. Dr. Marcet recommends to 

 precipitate it with muriate of ammonia. 



(g.) Specific gravity 2.66, 

 (h.} "" 



Infusible in any furnace, but readily melted by the compound 

 blowpipe ; this was done originally by Lavoisier, with oxygen gas 

 directed upon burning charcoal ; afterwards, and often, by Dr. Hare, 

 and in the laboratory of Yale College :f it forms a perfect glass. 



(h.) Silica, minutely divided, is dissolved at a boiling heat, by caus- 

 tic fixed alkali; the alkali should be twice the weight of the silica; 

 after evaporation, the white puffy mass forms a clear solution with 

 warm water, as already mentioned under (e.) 



(i.) Silica is hard, and when rubbed between two plates of glass 

 wears them so as to spoil their polish. 



4. POLARITY. I believe it is not distinctly determined. Several 

 chemists of eminence regard silica as being an acid rather than 

 an earth. This opinion is founded upon the fact that it satu- 

 rates the fixed alkalies, and that in its natural combinations, it sat- 

 urates the other earths. It has therefore been called the silicic 

 acid, and its compounds, silicates. This however, appears to be 

 a forced arrangement. In every other particular, silica is quite 

 foreign from the nature of acids, and as regards its combinations with 

 earthy and alkaline bases, it is not uncommon for one oxide to unite 

 with another ; the alkalies dissolve many metallic oxides, and potassa 

 and soda readily dissolve alumina, and should therefore, upon this prin- 

 ciple be called acids. The student will, however, do well to remem- 

 ber that the silicates mentioned in modern books, and frequently in 

 the analyses of minerals, are compounds of silica with bases. Wheth- 

 er we regard silica as an earth or an acid, there appears no reason 

 why these combinations should not take place in definite proportions, 

 such as are actually found to exist. 



5. COMBINING WEIGHT. According to Dr. Thomson, it is 16, 

 of which one proportion is oxygen, 8, and one silicium, 8. Accord- 

 ing to Berzelius, it is 1 proportion of silieinm, and 3 of oxygen. 



* First Principles, Vol. I, p. 191. 



t Not first by Dr. Clarke, as stated by Dr, Henry. Vol. I, p. 643. 10th London cd. 



