Earths. 191 



taking away their transparency, and rendering 

 them brittle. Some are liable to be intersected 

 with innumerable flaws. The hardest kinds re- 

 sist the fire the most. The alkalies act very 

 forcibly upon silica, and the best known and the 

 most useful property of this earth is, that of 

 melting with a moderate quantity of fixed alkali 

 into a glass. Flints and alkaline salts are the 

 most indispensable ingredients in glass ; but 

 sometimes other substances are employed to im- 

 prove the colour. 



Silica will not combine with any of the acids;, 

 except the fluoric, the phosphoric, and the bo- 

 racic, but the action of the first of these upon 

 flints is alone worth remarking. In this acid 

 silica dissolves with the greatest facility ; and, 

 what is most extraordinary, it is retained by the 

 acid even in the form of gas, so that the silica 

 rises with it even into the atmosphere, and is 

 then called sllicated Jluoric acid. If the gas is, 

 however, brought into contact with water, the 

 silica is immediately precipitated in the form of a 

 white crust. It is evidently by its attraction for 

 silica that the fluoric acid dissolves glass, as de- 

 scribed in a former lecture, for common glass is 

 formed of silica and carbonate of potass. 



IV. MAGNESIA has not been long known to 

 chemists. About the beginning of the last cen- 

 tury a Roman canon exposed to sale a white 

 powder, as a cure for all diseases, which he called 

 magnesia alba. It was at first suspected to be a 



