456 M. Berzelius on Fluoric Acid. [Dec. 



a quantity of water of crystallization, whose oxygen is double 

 that of the lime. 



In other respects, the composition of all these salts is strictly 

 analogous. 



The composition of these double salts affords a simple expla- 

 nation of a phenomenon which at first appears very paradoxical. 

 If a solution of superflnate of potash or soda be digested with as 

 much silica as is sufficient to saturate the excess of acid, it loses 

 altogether its acid reaction, and becomes alkaline. This reac- 

 tion, however, is not occasioned directly by the silica. The 

 excess of acid in the salt is exactly sufficient to form a silicated 

 fluate of potash or soda with one-half of the neutral fluate : in 

 proportion, therefore, as this excess combines with silica, the 

 double salt precipitates, while the other half of the neutral fluate, 

 which remains in solution, exhibits its characteristic alkaline 

 reaction. Zeise had already shown, that a similar change is 

 produced upon the acid fluates by boracic acid. 



The double fluates of silica with the other bases have all an 

 acid bitter taste, which in most instances cannot be distin- 

 guished from that of cream of tartar. They all redden litmus 

 paper, and the greater number of them are soluble in water. The 

 only difficultly soluble salts which I found, were those of potash, 

 soda, lithia, barytes, lime, and yttria. Many contain water of 

 crystallization, and a few of them fatiscerate.* In a high tem- 

 perature they all undergo decomposition, gaseous fluate of silica 

 being expelled, while a neutral fluate of the stronger base 

 remains. If the salts contain water of crystallization, it passes 

 off along with the fluate of silica, and there is obtained a con- 

 centrated liquid, silicated fluoric acid, which deposits silica 

 when it comes in contact with water. The quantity of wafer of 

 crystallization may be determined by heating the salts along 

 with oxide of lead, in which experiment there is formed an 

 actual fluosilicate of the oxide. This compound is so remarka- 

 bly fusible, that it becomes liquid in a temperature below a 

 visible red heat, particularly when the ingredients are propor- 

 tioned with some exactness. 



The aqueous solutions of these double salts are all of them 

 decomposed by alkalies. The alkaline salts are decomposed in 

 such a manner, that the silica is precipitated, while the acid 

 with which it had been combined remains in solution, in the 

 state of a neutral fluate. From the earthy salts the earth is 

 precipitated in the state of fluate, mixed or combined with the 



• This term is applied by Berzelius to certain compounds containing water of crystal- 

 lization, which lose a determinate quantity of it, but not the whole, when exposed to a 

 temperature considerably under 212°. Thus citric acid, sulphate of ammonia, oxalate 

 of ammonia fatiscerate, when exposed to a low heat, losing by this treatment exactly 

 one-half of their combined water. 



