FLUORIDE OF CALCIUM IN WATER. 149 
animals. The frequent association of phosphate of lime and fluoride of calcium 
in minerals, naturally 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 confers upon water the power of taking up phos- 
phate of lime ; it seemed worth while, therefore, to try whether it would 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 Granam of London,+ 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 Jeft 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 sufficient 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 MipprzTon’s paper, Quart. Jour. Geol. Soc., vol. i., p. 216. 
VOL. XVI., PART II. 2P 
