CHEMICAL SCIENCE. 249 



silicate, by means of acetic or other weak acid, and, after -well wash- 

 ing, heating it for some time under pressure, with a small quantity 

 of water in a closed vessel. A liquid is thus obtained which gelatin- 

 izes on addition of a saline solution. 



3. By passing gaseous fluoride of silicium over crystallized boracic 

 acid, and separating the hydrofluoric and boracic acids by digestion 

 with a large excess of ammonia, a hydrate of silica remains, which, 

 when well washed from the above acids, is very soluble in water. 

 This solution gives no precipitate when boiled, but leaves silica as an 

 insoluble powder on evaporation. 



4. By the beautiful method recently pointed out by Professor Gra- 

 ham, in which advantage is taken of the new means of separating 

 bodies by dialysis. A solution of silicate of soda, supersaturated with 

 hydrochloric acid, is placed on one side of a parchment paper sep- 

 tum, pure water being on the other side ; in a few days the hydro- 

 chloric acid and chloride of sodium will be found to have completely 

 passed through the diaphragm, leaving the silica in aqueous solution, 

 and so pure that acid nitrate of silver fails to detect chloride in the 

 liquid. This solution remains liquid for some days, but it ultimately 

 gelatinizes. We have generally adopted this last plan of preparing 

 the aqueous solution of silica, although a stronger solution is obtained 

 by the method first given. 



When a pure aqueous solution of silicic acid prepared as above is 

 allowed to soak into the pores of chalk or dolomite, a process of hard- 

 ening rapidly occurs, which goes on increasing for several days, 

 whilst, owing to its considerable depth of penetration, and to there 

 being no soluble or efflorescent compounds to be removed, there is 

 every probability that this hard silicious impregnation will afford 

 permanent protection to the stone. London Chem. News. 





