358 J. Davy’s Account of some Experiments 
deposition of silex is considered as an obstacle to the free ex- 
posure 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 re- 
tort 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; but could obtain no sulphureous acid gas, as Dr. 
Priestley 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 
