FLUORIDE OF CALCIUM IN WATER. 151 



tained whether it is simply a fluoride of barium, as it is hliiely to he, when nitrate 

 of barjrta is employed ; or a double fluoride and chloride of barium, as it may be, 

 when the latter is the precipitant. Berzelius 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 difiiculty 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 lai'ger 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 Middleton 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 fluorine 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 Daubeny, for example, 

 mentions that phosphorite from Estremadvu-a, 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 Daubeny 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. 



I 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. 



