1824.] M. Berzelius on Fluoric Acid. 457 



silica, whose acid remains in solution. From the salts of the 

 earths proper, and of the metallic oxides, the base is precipi- 

 tated in the state of a bisilicate, while the whole of the fluoric 

 acid remains dissolved, in combination with the alkali. Even 

 when the metallic oxides are soluble in ammonia, a determinate 

 portion of them is carried down by the silica. When the solu- 

 tions of certain of these salts are mixed with a less quantity of 

 acid than is requisite to produce complete decomposition, pecu- 

 liar subsalts are precipitated, but I am uncertain whether these 

 are mixtures of silica with a subfluate, or actual fluosilicates, in 

 which the same base is shared between two acids. 



In most cases I prepared these salts by digesting the liauid 

 acid over the carbonate or hydrate of the base, until it was 

 nearly saturated : the solution was then concentrated to a cer- 

 tain point in a flat platinum capsule, and allowed to crystallize 

 of its own accord in a temperature between 64° and 68°. When 

 the gelatinous silica on the filter has been pretty thoroughly 

 washed, so large a portion of it is dissolved, that the solution of 

 the salts prepared from the liquid acid not unfrequently gelati- 

 nizes during concentration. This excess of silica may be redis- 

 solved by the addition of a few drops of pure fluoric acid, and 

 the compound thus formed is volatilized by the subsequent eva- 

 poration in the form of gas. Fluoric acid does not decompose 

 these double salts, and when the mixture is evaporated to dry- 

 ness, the whole excess of acid flies off, and leaves the salts 

 unaltered. 



Sulphuric acid instantly acts upon these double salts, and 

 disengages gaseous fluate of silica : the decomposition is ren- 

 dered complete by the application of heat, and there is expelled 

 at the same time a quantity of liquid acid, which rapidly attracts 

 moisture from the atmosphere. The salts of lime and barytes 

 are not decomposed by sulphuric acid, except in temperatures 

 above 212°. Nitric and muriatic acids decompose these double 

 salts only partially. On the contrary, the liquid silicated fluoric 

 acid, in the humid way, completely deprives these acids of the 

 bases with which it forms difficultly soluble con pounds ; in other 

 cases the decompositions which it also produces are only partial. 



(To be continual.) 



