459 



Colourless crystalline quartz was ignited, finely pulverized, and 

 then boiled for an hour with a solution of its weight of perfectly 

 pure carbonate of soda ; the amount of silica thus dissolved was 1-5 

 per cent, of the quartz, but on repeating the treatment of the same 

 quartz with a second portion of the carbonate, only '35 percent, was 

 dissolved. The object of this process was to remove any soluble 

 silica, and the quartz thus purified was employed for the following 

 experiments, which were performed in a vessel of platinum. 



I. 1000 parts of quartz and 200 of carbonate of soda were boiled 

 with water for ten hours, and the mixture was several times evapo- 

 rated to dryness, and exposed for a few minutes to a temperature of 

 about 300 F. The amount of silica taken into solution was 12 

 parts. 



II. A hydro-carbonate of magnesia was prepared by mingling 

 boiling solutions of sulphate of magnesia and carbonate of potash, 

 the latter in excess ; the precipitate was washed by boiling with 

 successive portions of water. 1000 parts of quartz were mixed with 

 about as much of this magnesian carbonate and boiled as above for 

 ten hours. An excess of hydrochloric acid was then added, the 

 whole evaporated to dryness, and the magnesian salt washed out with 

 dilute acid. The residue was then boiled for a few minutes with car- 

 bonate of soda, and gave 33 parts of soluble silica. 



III. A mixture of 1000 parts of quartz, 200 of carbonate of soda 

 with water, and an excess of carbonate of magnesia was boiled for 

 ten hours, and the residue, treated as in the last experiment, gave 148 

 parts of soluble silica. The alkaline liquid contained a little mag- 

 nesia but no silica in solution. That the soluble silica was really 

 combined with magnesia was shown by boiling the insoluble mixture 

 with sal-ammoniac, which, dissolving the carbonate, left a large 

 amount of magnesia with the silica. This silicate was readily decom- 

 posed by hydrochloric acid, the greater part of the silica separating 

 in a pulverulent form. 



The third experiment was suggested by some observations on the 

 reactions of silicate of soda with earthy carbonates. Kuhlmann has 

 remarked the power of carbonate of lime to abstract the silica from 

 a boiling solution of soluble glass*, and it is known that alumina 



* Comptes Rendus de 1'Acad. des Sciences, Dec. 3rd and Dec. 10th, 1855, 

 where will be found many important observations on the alkaline silicates. 

 VOL. VIII. 2 M 



