340 M. Berzelius on Fluoric Acid. [Nov. 



saturate their excess of acid ; nevertheless these combinations 

 cannot be effected in the humid way with all the saline bases, 

 for the mixture, in some instances, separates either by precipi- 

 tation, or by crystallization, into two distinct filiates. The 

 fluates of potash and soda do not combine, and if either of their 

 acid salts be saturated with ammonia, the whole of that alkali is 

 gradually dissipated, even when the evaporation is performed in 

 the ordinary atmospheric temperatures, and we again obtain 

 the acid salts unaltered. 



One of the most interesting of these double salts is unques 

 tionably the combination of Jiuate of soda and filiate of alumina, 

 which occurs in the mineral kingdom, and is known to mineral- 

 ogists by the name of cryolite. It may also be prepared artifi- 

 cially. Thus if a solution of acid fluate of soda be cautiously 

 mixed with hydrate of alumina until it loses its acid reaction, 

 the liquid becomes little more than pure water, and when eva- 

 porated leaves a mere film of the double salt, which had remained 

 in solution. As both the fluate of soda and fluate of alumina 

 are readily soluble in water, this experiment of itself demon- 

 strates that in the double fluate which precipitates the propor- 

 tion of its constituent salts is such that both contain the same 

 quantity of acid ; otherwise an excess of one or other of the two 

 salts would have remained in solution. This compound may 

 also be formed by digesting hydrate of alumina in a solution of 

 neutral fluate of soda. If the experiment be made in a close 

 vessel, the liquid, when the decomposition is effected, possesses 

 an alkaline and caustic taste ; if under free exposure to the 

 atmosphere, the liquid attracts carbonic acid, and is found to be 

 a solution of carbonate of soda. During the digestion, the 

 alumina rapidly assumes the appearance of a semitransparent 

 mass ; but when dried, it loses the whole of its gelatinous 

 aspect, and becomes white and pulverulent. 



To satisfy myself of the identity of these double salts with the 

 one formed by nature, I subjected a quantity of cryolite to 

 analysis. When ignited, it gives off neither chemically com- 

 bined water, nor silicated fluoric acid. 100 parts were digested 

 with sulphuric acid so long as fluoric acid continued to escape ; 

 the mixture was then evaporated, until the greater portion of the 

 excess of sulphuric acid was dissipated. The saline mass being 

 redissolved in water, and the solution decomposed by ammonia, 

 gave 24-4 parts of alumina. The filtered liquid was now evapo- 

 rated to dryness, and the residue was cautiously ignited, in order 

 to free the sulphate of soda from sulphate of ammonia and excess 

 of acid. The salt after fusion weighed 101 parts, equivalent to 

 44-25 parts of soda. Consequently 100 parts of the mineral are 

 composed of 



