352 Dr George Wilson on the Detection of Fluorine 



which has the effect of rendering the silica produced insoluble. 

 On digesting water on the residue, fluoride of ammonium is 

 dissolved, and the solution requires only to be evaporated to 

 dryness and moistened with sulphuric acid to give off hydro- 

 fluoric acid, which readily etches glass. The stages in the 

 ammonia process are thus : — 



1st, Distillation of the substance with oil of vitriol, so as 

 to produce fluoride of silicon, Si F 3 . 



2d, Neutralisation of the aqueous solution of the distillate 

 with ammonia in excess, so as to produce fluoride of silicon 

 and ammonium, 2 Si F 3 -f 3NH 4 F. 



3d, Evaporation of the neutralised liquid to dryness, so as 

 to separate silica and render it insoluble. 



Ath, Exhaustion of the residue with water, and evaporation 

 to dryness, so as to leave fluoride of ammonium. 



5th, Moistening of the ammonio-fluoride with oil of vitriol, 

 so as to liberate hydro-fluoric acid, which will act upon glass. 



I have tried this process with Aberdeen and Peterhead 

 granite ; with three trap rocks from the neighbourhood of 

 Edinburgh, namely, basalt from Arthur Seat, greenstone 

 from Corstorphine Hill, and clinkstone from Blackford Hill ; 

 with a deposit from the boiler of the Atlantic steamer, Ca- 

 nada ; with a fossil bone ; with the ashes of charcoal, of 

 barley-straw, and of hay ; and in all with such success, that 

 the applicability of the process to the end proposed is certain. 

 The pieces of glass, etched by hydrofluoric acid evolved from 

 the substances referred to, which I lay upon the table, are 

 not selected successful specimens, but represent the whole of 

 the trials made by the ammonia process. The etchings on 

 the majority of them are as deep as could be obtained from 

 pure fluorspar and oil of vitriol ; and, with the experience 

 which I have now acquired, I have no doubt that I shall be 

 more successful in succeeding trials with vegetable ashes, 

 which, for reasons to be presently mentioned, require more 

 precautions than fragments of rock do. 



The examination of a hard crystalline mineral, such as 

 granite or an unweathered trap, presents no difficulties. It 

 must be reduced to a tolerably fine powder, and employed in 

 considerable quantity. A little sulphurous acid is always 



