332 M. Berzelius on Fluoric Acid. [Nov. 



as the excess of potash is in a situation to combine either with 

 carbonic acid or with water. Fluate of potash in a red heat dis- 

 solves silica, and forms with it a transparent mass ; and no sili- 

 cated fluoric acid is disengaged, in temperatures below that 

 necessary to melt glass. The mass on cooling has a white 

 porcelaneous aspect, and water extracts from it a deliquescent 



salt. 



Filiate of Soda. — a. The acid fluate may be crystallized in 

 transparent rhomboids. It possesses a sharp and purely acid 

 taste, and is but sparingly soluble in cold water. It is composed 

 of an atom of fluate of soda, and an atom of hydrous fluoric 

 acid. 



b. The neutral finale of soda is most economically prepared by 

 mixing 100 parts of dry silicated fluate of soda and 112 parts of 

 anhydrous subcarbonate of soda with as much water as will form 

 with them a thin pap, and boiling the whole until it ceases to 

 effervesce. After about an hour, the mixture concretes to a solid 

 mass : this must be reduced to powder, and again boiled in 

 water, so long as it effervesces. By this means we obtain a 

 mixture of fluate of soda and silica: the former is to be sepa- 

 rated by repeatedly washing the insoluble portion with water.* 

 The solution, when slowly evaporated, deposits the salt in crys- 

 tals. But with whatever precautions the preparation of this salt 

 may be conducted, a small quantity of the silica invariably 

 passes into solution : hence after the greater portion of the fluate 

 of soda has crystallized, the liquid becomes opalescent, being 

 unable to retain the whole of the uncombined silica in solution. 

 It must be evaporated to dryness, and the residue ignited, in 

 order to render this silica insoluble. If any of the double fluate 

 had escaped decomposition at the commencement of the process, 

 it forms a part of this dry mass, and its excess of acid is expelled 

 during the ignition : the dissipation of the last portions may be 

 greatly facilitated by introducing into the crucible, while red- 

 hot, a bit. of carbonate of ammonia, and immediately after 

 covering it up with its lid. 



Fluate of soda crystallizes in cubes and regular octahedrons : 

 the crystals are transparent, and have sometimes the lustre of 

 mother of pearl when viewed with reflected light. It is always 

 obtained in octahedrons, when the solution contains carbonate 

 of soda. It is remarkable that the fluates of potash and soda are 

 isomorphous Avith the muriates of the same bases (chloride of 

 potassium and chloride of sodium), and also, so far at least as 

 can be concluded from what has been already ascertained, with 

 the analogous compounds formed by iodine. This salt is less 

 fusible than glass. Water dissolves it very slowly ; and its solu- 



* The object of this process is to prevent the silicate from gelatinizing ; the gelatinous 

 silica resulting from the decomposition of fluosilicates being sensibly soluble in water. 



