EARTHS. 275 



2. PREPARATION. 



(a.)Flint or rock crystal, ignited, thrown into water, and pulver- 

 ized, affords silica sufficiently pure for every common purpose. 



(b.) But the more correct process is, to mix these powders with 3 

 or 4 parts of carbonate of potash or soda,* and to melt the mixture 

 in a crucible, giving a higher heat, for half an hour or an hour, to- 

 wards the last, and stirring it to prevent overflowing. f 



(c.) Caustic potash or soda is, of course, more energetic in its ac- 

 tion, but is more expensive ; there is however an advantage in using 

 caustic alkali, as it does not intumesce ; if a silver crucible is used, 

 it should be thick, that there may be the less danger of melting it. 



(d.) Dissolve the melted alkalino-siliceous mass in water, filter, 

 and add diluted muriatic or sulphuric acid as long as precipitation 

 continues ; the acid must be added in excess. J 



(e.) The solution was formerly called liquor silicum, liquor of 

 flints; the vitreous mass from which it is obtained is deliquescent, 

 and if the solution formed from it is dilute, and the acid is added 

 gradually, the alkali may be saturated without precipitating any of the 

 silica, but by evaporation to dryness the silica is rendered insoluble ; 

 the salt formed by the alkali may be dissolved out, and the earth thus 

 obtained pure after ignition. 



(/.) If the proportions of alkali and earth are reversed, then the 

 compound produced is glass ; of which mention will be made again. 



3. PROPERTIES. 



White, insipid, harsh. 



7V0 effect on test colors, no causticity, or any alkaline proper- 

 ty, except its union with a single acid, the fluoric. 



(c.) Water does not directly dissolve silica, nor is it absorbed by 

 that earth, but when it is newly precipitated, it retains 26 per cent, 

 of water, at 70 Fahr. 



(d.) When dry it is insoluble in water, but when just precipita- 

 ted, it is dissolved by that fluid, in the proportion of about ToVT$ an( ^ 

 and if taken in its nascent state, || it is even largely dissolved, and a 



* Dry pearl ashes will do. 



t It is recommended to dissolve the alkali first, in as little water as may be, to mix 

 it with the silica, evaporate to dryness, and then fuse it, which may be done in a 

 silver crucible. From my own experience, I should however recommend caution 

 in the use of silver vessels, as they melt at about the degree of heat which produ- 

 ces the combination between the silica and the fixed alkali. 



t Dr. Henry remarks, " the alkaline liquor must be added to the acid, and not the 

 reverse ; for, in the latter case, the precipitate will be glass and not silica." Vol. I. 

 p. 642, Mh ed. 



Found naturally dissolved, as in the Geysers in Iceland, in which the solution 

 is aided by soda, contained in the water: in the similar hot fountains of the Azores, 

 silica is found in solution, &c. there are natural hydrates, and the immense num- 

 ber of crystals of quartz, evince that silex has been in solution on a great scale. 



|| Particularly when the sulphuret of silicium is dissolved in water, and the silica 

 is regenerated by the oxygen of that fluid, while its hydrogen is evolved, combined 

 with sulphur. 



