Fluoride of Calcium is soluble in Water. 231 



fluor spar is soluble in water at 60° F. The result of my re- 

 searches at that period was, that 7000 grains of distilled 

 water dissolve gr. 0*26 of the salt in question, at the tem- 

 perature mentioned. Some objections were afterwards made 

 by Mr Nisbet, to the method of investigation followed in 

 some of my experiments, which seemed to imply that a doubt 

 was entertained, whether the substance dissolved by water 

 when it is digested on fluor spar, is fluoride of calcium, or the 

 fluoride of silicon and calcium. I did not see the force of 

 those objections, but I felt, nevertheless, that as all the so- 

 lutions had been procured by boiling water on native fluor 

 spar in glass vessels, which became slowly corroded if long 

 exposed to the action of the solution, it was not impossible 

 that silicon might have been present, in some of the solutions 

 which were employed to determine the amount of solubility 

 of the fluoride. I thought it well, accordingly, to repeat the 

 results with solutions made in metallic vessels, and never 

 allowed to come in contact with silica in any condition. 



One set of trials was made last summer (1849) in the fol- 

 lowing way : — Well crystallised, transparent fluor spar, was 

 boiled for some hours in a platinum basin, with fine hydro- 

 chloric acid, so as to secure the conversion of any silica pos- 

 sibly present, into fluo-silicic acid, and remove any metallic 

 oxide, sulphate of lime, carbonate of lime, or other foreign 

 matter present in the spar, and soluble in the acid. The 

 purified fluor was then washed in the same vessel, by copious 

 afl*usion with warm distilled water, and in this state em- 

 ployed for the preparation of the solutions to be evaporated. 

 An aqueous solution was prepared by boiling distilled 

 water on this purified salt contained in a platinum basin, 

 and the liquid was then transferred to a pewter vessel, 

 in which it was collected and left for some days at the tem- 

 perature of 60°, that it might deposit the excess of fluor it 

 had dissolved at 212°. The clear liquid was then filtered 

 through a tin funnel, with the neck partially choked by zinc 

 filings, and the filtrate was measured in a pewter vessel, 

 which had been carefully graduated, so as to contain, when 

 nearly full, 7000 grains of the solution at 60°. The liquid 

 thus obtained and measured, and which had never come in 



