MR. Gr. GORE ON HYDROFLUORIC ACID. 
197 
of potassium, strong solution of fluoride of potassium, excess of sulphuretted hydrogen 
gas (either before or after neutralization of the acid by ammonia), dilute hydrochloric 
acid, solution of nitrate of silver, nor by adding a single drop of solution of nitrate of 
barium acidulated with nitric acid; it was therefore free from all impurities at all 
likely to be present. Additional tests were employed which need not be enumerated. 
This aqueous acid was employed in making the various fluorides described in this paper. 
I have found a plentiful supply of the pure aqueous acid of considerable service for 
removing (by digestion and evaporation) the silica in minerals, especially in searching 
for minute ingredients in those containing much silica. 
The amount of anhydrous hydrofluoric acid contained in the pure aqueous acid of 
various degrees of strength was ascertained both by the silica-process (see pages 176, 
177, 178, and 195) and also by the lime-process (see page 178). 
Aqueous hydrofluoric acid of a high percentage was also on two occasions prepared by 
heating dry fluoride of chromium in a suitable platinum apparatus, and condensing the 
vapour by immersing the condensing-tube in a mixture of snow and salt. The chromium 
salt was prepared by precipitating a solution of chrome-alum by aqueous ammonia, and 
boiling with excess of the ammonia, washing the precipitate with boiling water, dis- 
solving it whilst still wet in pure dilute hydrofluoric acid, and evaporating the solution 
to perfect dryness in a platinum dish. The residue was a pale green earthy powder, non- 
deliquescent, and very slowly soluble in water or dilute hydrofluoric acid ; it lost upwards 
of 40 per cent, in weight of acid vapour when heated to redness in platinum vessels, 
without contact of the substance with the products of combustion from the gas-burner. 
The following apparatus was employed: — A (fig. 15) is a 
retort-tube 7 inches (=17*8 centimetres) long and of an 
inch ( = T6 centim.) diameter; B is an angle-tube, and C is 
a curved tube about 5 inches ( = 12*8 centims.) high and an 
inch (=1*2 centim.) diameter. Each tube was formed of 
platinum without the use of solder. The tubes fitted tightly 
upon each other, and the junctions were made gas-tight by 
means of the melted mixture of sulphur and lampblack. 
The outer end of the tube C was closed by a paraffin stopper containing a small platinum 
escape-tube. 
50 grains weight of the dry chromium salt was placed in the tube A at its closed end, 
and the whole tube heated somewhat above the boiling-point of water ; no perceptible 
loss of weight occurred : the tubes were then joined together, the curved tube C 
immersed in snow, a wet rag laid upon A near its junction with B, and heat very gra- 
dually applied to the outer end of A during about twenty minutes until about 3 inches 
in length of the tube was red-hot ; the tube A was then taken off, the adhering cement 
removed, and the tube weighed; the loss of weight was 2210 grains=44'20 per cent, 
(this number, however, is a little too high because traces of the solid substance were 
carried forward mechanically into the angle-tube) ; only a trace of vapour escaped con- 
mdccclxix. 2 E 
Fie. 15. 
