3 % 
on different Combinations of Fluoric Acid. 
of ammonia is formed and sublimed. Its action on glass is so 
powerful, that I have successfully employed it instead of fluoric 
acid itself, for etching on this substance. It has one advan- 
tage, that it is more manageable. The solution may be ap- 
plied by means of a hair pencil or a common pen to the glass, 
and the erosion will be produced by exposure to a moderate 
temperature. 
The fixed alkalies, and all the earths that I have tried, de- 
compose this salt ; they expel the ammonia, and form true 
fluats with the acid itself. I have examined all the fluats thus 
formed, and have endeavoured to ascertain the proportions of 
their constituent parts ; but I am not sufficiently satisfied of 
the accuracy of the results, to venture to give an account of 
them. 
Sect. III. On Fluoboracic Acid Gas. 
MM. Gay Lussac and Thenard, who first discovered this 
gas, obtained it by heating strongly, in an iron tube, a mix- 
ture of fiuor spar and fused boracic acid. I have found that 
it may be more easily procured, in greater abundance, and at 
less expence, by gently heating, in a common glass retort, a 
mixture of finely pounded boracic acid * and fluor spar with 
concentrated sulphuric acid. 1 part by weight of fused boracic 
acid, 2 parts of fluor spar, and about 12 of sulphuric acid ap- 
pear to be the proportions best adapted for the purpose. This 
method will require no explanation when it is considered that 
boracic acid, as has already been observed, precipitates silex 
from liquid subsilicated fluoric acid. If the heat is gentle, not 
* Common calcined borax answers the same end, but no so well. Its only recom- 
mendation to preference is cheapness. 
MDCCCXII. 3 B 
