86 



FLUORIC ACID. 



Subsilicatedfiu- 

 oric acid|is de- 

 composed by 

 alkalies, and by 

 earths and 

 acids. 



Habitudes of 

 subsilicated flu 

 oric acid with 

 heat. It came 

 over in distil- 

 lation; &c. 



This differs 

 from Doctor 

 Priestley's re- 

 suit. 



Fluoric acid 

 gas cannot be 

 had by di filia- 

 tion free from 

 silex. 



tion of silex is considered as an obstacle to the free exposure 

 of the surface of the water to the gas. 



Subsilicated fluoric acid is decomposed by ammonia and the 

 fixed alkalies, and by all the earths that I have made trial of. 

 It is also decomposed by the sulphuric acid and the boracic, as 

 well as by the muriatic acid gas. 



Of the particular changes which occur when it is acted upon 

 by the alkalies, I defer giving any account at present, as it is 

 my intention to do it in the next section. 



To learn the effect of heat on it, a small quantity of strong 

 acid, pure and transparent, was introduced into a retort con- 

 nected with mercury. A spirit lamp being applied, about three 

 cubic inches of silicated fluoric acid gas were produced. The 

 neck of the retort was lined with silex in a gelatinous state, 

 and much liquid subsilicated fluoric acid, that had distilled 

 over, was condensed in the colder part of the neck, and was 

 absorbed by bibulous paper previously introduced, to prevent 

 the distilled fluid from entering the jar for the reception of the 

 gas. When the whole of the acid in the bulb of the retort had 

 been evaporated, little or no silex remained. 



The general result of this experiment is very different from 

 that which Dr. Priestley, who first made it, obtained. Instead 

 of silicated fluoric acid gas, he procured " vitriolic acid air," 

 sulphureous acid gas. 



I have tried also the effect of heat on the silicious crust, 

 formed by the decomposition of silicated fluoric acid gas, by 

 water j but could obtain no sulphureous acid gas, as Dr. Priest- 

 ley did only a small quantity of silicated fluoric. 



The correctness of Dr. Priestley's observations cannot be 

 doubted. I can only account for his results, by supposing that 

 some sulphuric acid in consequence of the high temperature 

 employed in making the gas was volatilized, and mixed with 

 the subsilicated fluoric acid, and that mercury also was present, 

 from the acid being prepared over this metal. 



These experiments too oppose another statement relative to 

 a method prescribed for making fluoric acid gas free from 

 silex, by merely heating strong subsilicated fluoric acid in a 

 retort, and collecting the gas over mercury. It is asserted, in 

 chemical works of some reputation, that this process is suc- 

 cessful. I have never found it so, having always obtained 



results 



