TRANSACTIONS OF THE SECTIONS. 69 
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 affusion with warm distilled water, 
and in this state employed for the solutions to be evaporated. An aqueous solution 
of fluor-spar was prepared by boiling distilled water on the salt contained in a plati- 
num basin, and the liquid was then transferred to a pewter vessel, in which it was 
collected, and left for some days at the temperature 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 mea- 
sured in a pewter vessel, which had been carefully graduated, so as to contain, when 
nearly full, 7000 grains of the solution. The liquid thus obtained and measured, and 
which had never come in contaet with silica, was then evaporated to dryness in a 
platinum capsule, and the amount of residue ascertained. Six careful trials were 
made, and gave as a mean 0°25166 gr. as the amount of fluoride of calcium soluble 
in 7000 grains, or 16 fluid ounces, ὁ. 6. a pint apothecaries’ measure. This result 
approaches so closely to that previously obtained with glass vessels, that the number 
found must be considered as making a near approximation to the truth. 
A similar series of observations was made this summer ; but the fluor-spar, which 
was of great apparent purity, as furnished through the kindness of Mr. Tennant 
of London, was not subjected to any preliminary treatment with hydrochloric acid, 
but simply boiled with distilled water, and the solution collected and cooled as before 
in a pewter vessel. The liquid was allowed thus much contact with silica that it was 
passed through a paper filter placed within a tin funnel. Few however will suspect 
that it can have transferred to itself any silica from the saline constituents of the 
paper. Six trials were made in this way, the mean of which gave 0:26 gr. as the 
aes of fluoride of calcium soluble in 16 ounces of water. The numbers of which 
is is the mean, like those obtained in the previous determinations with metallic 
vessels, differ more from each other than the numbers did in the first series of expe- 
riments, where the solutions were made in glass flasks. This however was to be ex- 
pected, for the liquid employed in the first series was prepared at once to the extent 
of many pints, and the uniform composition of the whole secured before any of the 
solution was evaporated. In the case of the metallic vessels, on the other hand, 
owing to their smallness, each pint had to be prepared separately, and its evaporation 
completed before another was procured. The numbers therefore could not but differ 
more in the second and third determinations than in the first. The highest number 
was 0°28, the lowest 0°24. We may therefore consider 0°26 as sufficiently nearly 
representing the true solubility of fluor-spar, so that pure water may be considered as 
able to dissolve 5,3,51d part of its weight of this salt. The residue of 16 ounces of 
the solution etches glass rapidly and powerfully. 
The amount of solubility observed, though comparatively small, is large for a salt 
reputed quite insoluble, and is plainly sufficient to occasion an appreciable error in 
the quantitative determination of fluorine by the ordinary process, since as much as 
a pint of water, and that perhaps at the temperature of 212°, must often be employed 
in washing a precipitate of fluoride of calcium. 
A few unpublished particulars concerning the late Dr. Black. 
By Greorce Witson, M.D., F.R.S.E. 
The object of this communication was to lay before the Section a few characteristic 
incidents concerning Dr. Black, gathered from Mrs. Elizabeth Wordsworth, who was 
a servant in his household during the last five years of his life. 
The facts recorded do not admit of abridgement, but they completely confirmed the 
accounts contained in the published biographies of Black concerning his valetudina-~ 
rian and methodical habits, whilst they gave no countenance to the statement which 
had been credited in some quarters that the great chemist was an avaricious or penu- 
_ rious man. Some interesting particulars were adduced illustrative of the amiability 
and gentleness which characterized Dr. Black; and the author concluded by noticing 
that an error had been committed as to the date of the philosopher’s death, which was 
not the 26th of November 1799, as stated by Robison, but the 6th of December of 
that year, a fact which Mr, Muirhead first pointed out (Watt Correspondence, p. xxii.), 
and which is confirmed by the newspapers of the period. (Vide Edinburgh Mercury 
of the 14th of December 1799.) 
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