FLUORIDE OF CALCIUM IN WATER. 151 
tained whether it is simply a fluoride of barium, as it is likely to be, when nitrate 
of baryta is employed ; or a double fluoride and chloride of barium, as it may be, 
when the latter is the precipitant. Brrzerius has described such a salt. But I 
have frequently availed myself of the fact that barium forms a sparingly soluble 
compound with fluorine, in seeking for the latter in liquids. They are often most 
conveniently tested for that substance by precipitating them by a salt of baryta, 
and testing the precipitate for hydrofluoric acid. This reaction, moreover, has an 
important relation to qualitative chemical analysis, inasmuch as it throws an 
unsuspected difficulty in the way of distinguishing dissolved sulphates from 
fluorides. The barytic precipitate, with solution of fluoride of calcium, is soluble 
in excess of nitric and hydrochloric acids, but it requires a much larger addition 
of these to redissolve it, than the carbonate, borate, or phosphate of baryta does. 
A fluoride, therefore, may readily be mistaken for a sulphate, or a mixture of 
both for only the latter. This mistake must have been frequently made in 
analysing mineral waters, where fluorine is certainly more abundant than has 
hitherto been suspected. When fresh analyses of these bodies shall be made, I 
have little doubt that where fluorine is met with, as Mr Mrmp.erton has already 
discovered it in the pipe-water of London, and I have detected it in one of the 
wells of Edinburgh, it is the sulphates that will be found to have been over- 
estimated, at the expense of whatever proportion of fluorine was also present. 
The fact of the solubility of the fluoride of calcium in water introduces an 
insurmountable objection to the present method of estimating fluorine quanti- 
tatively. It accounts in part for the discrepance between the result obtained, 
when fiuorine has been estimated by the loss which a substance containing it 
sustained when heated with sulphuric acid, as contrasted with that which has 
been procured when the hydrofluoric acid evolved was condensed in ammonia, 
and precipitated by solution of chloride of calcium. Dr Davuseny, for example, 
mentions that phosphorite from Estremadura, yielded, according to the first 
method, fifteen per cent. of fluoride of calcium, according to the second, not nine 
per cent.* Part of the difference was doubtless owing to the difficulty with 
which fluor-spar is made to abandon all its fluorine when distilled with oil of 
vitriol, in consequence of the pasty condition of the sulphate of lime which is 
formed. But when we find Dr Dauseny mentioning, that he subjected the pre- 
cipitated fluoride of calcium to “repeated washings with water,” in order to 
remove any accompanying sulphate of lime, we may well suspect that fluoride of 
calcium was also washed away. 
T regret that I cannot yet announce the proportion of fluor-spar which water 
dissolves. Owing to the corroding action which the solution occasionally exerts 
on glass, I thought it unadvisable to employ vessels of that material, or of porce- 
* Chemical Society’s Memoirs, vol. ii., p. 98. 
