ivhen accompanied by SI Ilea. 35 L 



ing fluorine, provided it be present in the form of a fluoride 

 which admits of decomposition by oil of vitriol at its boiling 

 point. The first stage of the process consists, in both cases, 

 in heating the silicated fluoride in a flask along with strong 

 sulphuric acid, so as to occasion the evolution of the fluoride 

 of silicon, Si F 3 . This gas is conducted by a bent tube into 

 water, where it deposits a portion of gelatinous silica ; and 

 the liquid, after filtration (which, however, is not essential), 

 is treated as follows : — 



In the first process, I adopted one of Berzelius' well-known 

 methods for the isolation of silicon. The filtered liquid was 

 neutralised with potass : and the resulting gelatinous preci- 

 pitate of fluoride of silicon and potassium (2 Si F 3 + 3 KF), 

 after being washed, was dried, and transferred to a small 

 metallic crucible, in which it was heated with potassium, so 

 as to separate and set free the silicon, and convert the whole 

 of the fluorine into fluoride of potassium. This fluoride was 

 then dissolved out by water, evaporated to dryness, and 

 treated in the ordinary way with oil of vitriol, so as to evolve 

 hydrofluoric acid, which could be made to record its evolu- 

 tion by the etching which its vapour occasioned on a plate of 

 waxed glass, with lines written on it through the wax. 



This process is necessarily tedious, and is liable to several 

 objections. The most serious of these is the impossibility 

 of effecting the complete decomposition of the fluoride of 

 silicon and potassium, by potassium, so as to liberate the 

 whole of the silicon ; and the risk of the latter undergoing 

 oxidation into silica during the washing of the ignited mass. 

 Accordingly, though this method gives good results, and has 

 enabled me to detect fluorine in coal, in which I could not 

 previously detect more than the faintest traces of it, yet it 

 almost unavoidably necessitates a loss of the element in 

 question, and is much inferior in simplicity and certainty to 

 the process which I am about to describe. 



In the second process, as in the first, the substance under 

 examination is heated with oil of vitriol, so as to yield fluo- 

 ride of silicon, which is conducted into water. The resulting 

 solution (with or without filtration) is neutralised with am- 

 monia instead of potass, and then evaporated to dryness , 



2 a2 



