FLUORIDE OF CALCIUM IN WATER. 14g 



animals. The frequent association of phospliate of lime and fluoride of calcium 

 in minerals, natm-ally suggested that whatever substance enabled water to become 

 charged with the one salt, would cause it to dissolve the other. Carbonic acid is 

 known to be one agent which coufers upon water the power of taking up phos- 

 phate of lime ; it seemed worth while, therefore, to try whether it Avould cause it to 

 dissolve fluoride of calcium as it does so many other lime-salts. I was not aware 

 that Dana the American mineralogist,* and Professor Graham of London,f had 

 anticipated me in this idea, or I should probably not have performed any experi- 

 ments on the subject. In ignorance of their views, the following trials were made. 

 A portion of pale green crystallised fluor-spar was reduced to fine powder and 

 digested for some hours in warm nitromuriatic acid, so as to remove any car- 

 bonate of lime, metallic oxides, or other foreign matters, which might be present. 

 It was then washed on a filter, dried, and suspended in pure distilled water, 

 through which a current of carbonic acid was passed for two hours. At the end 

 of this period the liquid was filtered through paper, and tested for lime by oxalate 

 of ammonia. A cloudiness was soon occasioned, and speedily a white precipitate. 

 On evaporating the liquid to dryness, a greyish- white residue was left which gave 

 off sharp acid fumes when moistened with oil of vitriol. When this residue was 

 warmed with Nordhausen sulphuric acid in a platina crucible covered by glass, 

 the latter was deeply corroded in a few minutes. The process was repeated many 

 times, and always with the same result. I shew the Society squares of glass 

 which were etched in this way ; the engraved words having been traced through 

 wax, as in the ordinary method of testing for hydrofluoric acid. The experiments 

 referred to were made in January last, and were supposed to justify the idea 

 which led to their trial, namely, that carbonic acid was the agent which enabled 

 water to dissolve fluor-spar. 



If carbonic acid, however, had been essential to the retention of fluor in solu- 

 tion, the expulsion of that gas, by warming the liquid, should have been followed 

 by the deposition of the fluoride. I was struck, however, by observing that the 

 solution could be raised to the boiling-point, without any troubling or opalescence 

 appearing, and that no precipitate shewed itself after protracted ebullition. It 

 was manifest that water was able of itself to retain in solution the fluoride if once 

 dissolved in it ; and highly probable that it would prove equally suSicient to com- 

 mence the solution of the lime-salt. The experiment was accordingly tried of 

 suspending fluoride of calcium in cold distilled water, and shaking it occasionally 

 in a stoppered bottle for two hours. The liquid, after filtration, shewed lime with 

 oxalate of ammonia as readily as the carbonic acid solution had done, and left, 

 after evaporation, a residue which gave, with oil of vitriol, acid vapours etching 



* Edin. Phil. Jour., vol. xxxix., p. 255. 



t Note to Mr Middleton's paper. Quart. Jour. Geol. Soc, vol. i., p. 216. 



VOL. XVI., PART II. 2 P 



