when accompanied by Silica. 355 



was reduced to powder, and mixed with powdered glass so as 

 to add to it excess of silica. It was then subjected to the 

 ammonia process, and has yielded an etching as deep as the 

 purest fluorspar could have given with oil of vitriol. 



The result is so marked, that I should recommend the de- 

 liberate addition of silica to bodies suspected to contain 

 fluorine, as a provision for permitting such substances to be 

 analysed in glass vessels, in which the largest quantities may 

 be subjected to examination without risk of missing the ele- 

 ment in search, or permitting it to escape. 



Five points call for further notice. 



1st, When a silicated fluoride, as I may, for the sake of 

 brevity, call it, is distilled with oil of vitriol, the whole of 

 the fluoride of silicon comes away as gas, as soon as the oil of 

 vitriol has reached its boiling-point. It is not necessary, ac- 

 cordingly, to subject a body supposed to contain fluorine to 

 any lengthened ebullition ; and, in the case of plant-ashes, 

 it is desirable to arrest the boiling as soon as all the fluorine 

 has been evolved, for protracted ebullition only occasions evo- 

 lution of sulphurous acid. Besides the ultimate glass-etching, 

 the escape of fluorine is rendered manifest by the appearance 

 of a white gelatinous body in the water, through which the 

 gas evolved (Si F 3 ) is passed ; and by the production of a 

 gelatinous, flocculent precipitate, when the solution of this 

 gas is neutralised with potass. The coal-ashes gave all those 

 results. 



2d, It appears exceedingly probable, that much of the silica 

 occurring in the forms of quartz, chalcedony, opal, sinter and 

 the like, which is generally supposed to have been deposited 

 from aqueous or alkaline solution, has owed its origin to the 

 decomposition of fluoride of silicon by water, or has otherwise 

 been related to fluorine as its solvent or transferring agent. 

 This, or rather the less precise notion of fluorine conveying 

 silica, has been suggested by my friend Mr A. Bryson, and 

 by Dr H. Buchanan, E.I.C.S. 



'3d, The occurrence of fluorspar in drusy cavities in green- 

 stone, along with silica, as in the specimens obtained from 

 Bishopton, on the Clyde ; the similar occurrence of apophyl- 

 lite in the cavities of trap ; the association of topaz, pycnite, 



