178 
ME. a. GORE ON HYDROELUOEIC ACID. 
and distilled water added to it in minute drops at long intervals at first, until about a 
quarter or half an ounce had been added, taking great care not to cause an escape of 
vapour. A quantity of hydrate of potash, more than sufficient to convert all the acid into 
fluoride of potassium, was now dissolved in a separate vessel, and the solution chilled ; 
some of this liquid, more highly diluted, was added in drops to the dilute acid, at inter- 
vals at first, and gradually the whole, taking care to cause no escape of vapour. The 
mixture (of strong alkaline reaction) was washed into a platinum beaker, and gradually 
evaporated until a film formed upon it. Precipitated silica, quite dry and perfectly pure, 
and especially free from matters soluble in hot concentrated sulphuric acid, was weighed 
in a stoppered bottle in more than sufficient quantity to convert the whole of the fluorine 
of the acid into tetrafluoride, the contents of the bottle added to the alkaline pasty liquid, 
and the bottle at once closed and reweighed. The mixture of silica and alkaline fluoride 
was incorporated by stirring, and an excess of concentrated sulphuric acid added gra- 
dually to it, and the mixture then heated until it was a thin liquid, and the sulphuric 
acid freely volatilized. The residuary mixture was then dissolved, transferred to a filter, 
and the silica washed, dried, ignited, transferred to a stoppered bottle and weighed. 
This method gave nearly accurate results. 
The best method of analysis, however, consists in diluting the chilled acid with extreme 
care, and adding to it very gradually a known weight of pure caustic lime, and weighing 
the dried and ignited product ; the following are the particulars of the process. A six- 
ounce platinum bottle provided with a neck about 2-| inches (=6 A centimetres) high 
and \ an inch (=l - 8 centimetre) in diameter, with a flanged mouth, plate with paraffin 
layer (see page 175) and clamp screws, and previously weighed, was placed in a closely 
fitting tin case immersed in the freezing-mixture of ice and chloride of calcium (see 
page 173), and about 30 to 50 grains of the acid from a chilled and similar platinum 
bottle transferred to it. The bottle was then securely closed, removed from its case, and 
allowed to return to the atmospheric temperature and reweighed; it was now reim- 
mersed in the freezing-mixture, and when quite cold the plate and screws were removed 
and minute portions of distilled water were allowed to drip down the inner side of the 
tube until the^acid was sufficiently diluted to be able to add gradually in the whole about 
If an ounce of water. Pure caustic lime (which had been repeatedly heated to whiteness 
until it ceased to lose weight), more than sufficient in quantity to neutralize all the 
hydrofluoric acid present, was now intensely heated in a small platinum box, and cooled 
with the lid (a closely fitting one) upon it, and weighed ; the whole of the lime was 
then very gradually added to the dilute acid with shaking, and the mixture digested 
with shaking and heating until the liquid became alkaline to test-paper. On first 
heating, much heat spontaneously developed itself in the mixture, and required to be 
moderated by immersing the bottle in cold water, and after this development of heat 
the liquid was alkaline. The mixture was now evaporated and dried in the platinum 
bottle, then repeatedly ignited to redness and cooled until it ceased to lose weight, and 
finally weighed, every 22 grains increase of weight being estimated to represent 40 
