228 
ME. GEORGE GORE ON ELUORIDE OE SILVER. 
placed in a platinum tube-retort about 150 millims. long, and the salt heated to fusion 
to expel all moisture ; the retort was then closed by a tube containing fragments of 
chloride of calcium and cooled. The cup was removed, instantly closed, and reweighed. 
The salt was then dissolved in hot water, the solution filtered in the dark into a two- 
ounce platinum bottle, the washed filter burned, and the residuary silver weighed. The 
well-washed cup was also heated to faint redness, and its gain of weight of silver ascer- 
tained. By deducting these weights from that of the fused fluoride, nearly the true 
weight of soluble fluoride was found. For every single grain of soluble fluoride, there 
was now added to the contents of the bottle '22047 grain of perfectly pure caustic lime 
in powder (prepared as described, Phil. Trans. Boy. Soc. 1869, p. 179), which had just 
been heated nearly to whiteness until it ceased to lose weight. The bottle was now 
placed in a large platinum dish, with a small inverted platinum funnel supported above 
its mouth, and the mixture continuously evaporated to perfect dryness, the products of 
combustion from the gas-flame being deflected on one side (see page 232). The bottle 
was then heated to incipient redness until it ceased to lose weight. This method is 
capable of yielding very accurate results when sufficient care is exercised. 
The reaction upon which this method is based is represented by the following equa- 
tion : — 
2AgF + Ca O = Ca F 2 + 2Ag + O. 
Two molecules, or 254 parts, of the silver salt, and one molecule, or 56 parts, of caustic 
lime lose 16 parts of oxygen, and leave a mixture of 78 parts of fluoride of calcium and 
216 parts of metallic silver. The 16 parts of oxygen lost by the above weights of ma- 
terials equal 38 parts of fluorine present ; a greater proportion of loss would occur if 
the silver salt contained less fluorine and an equal weight of oxygen or water was pre- 
sent to supply the deficiency, because the lime prevents the expulsion of the fluorine by 
heat, but not that of any oxygen or water. 
The following are the particulars of an analysis of this kind: — 100 '92 grains of the 
brown fluoride was fused in the cup and retort ; it effervesced, and a little silver was set 
free upon the surface of the cup; the loss of weight was T08 grain =1-070 per cent., 
consisting of acid watery vapour, and some oxygen produced by the action of that 
vapour upon the heated salt. On dissolving the salt and filtering, 2'80 grains of silver 
was found upon the filter, and T3 grain upon the citp. The quantity of salt dissolved 
therefore was 96'91 grains =21-366 grains of CaO ; 21-39 grains of the lime was then 
added, and the mixture evaporated and ignited ; the process occupied about nine hours. 
The residue was perfectly neutral to test-paper, and weighed 112-20 grains = a loss of 
6-10 grains, theory requiring 6-104 grains. 
The contents of the bottle was now dissolved by warm dilute nitric acid, the solution 
filtered and precipitated; 108-545 grains of argentic chloride, =81'692 grains of silver, 
was obtained, theory requiring 82-411 grains*. 
To determine the amount of silver more accurately 1 made-the following experiment : — 
* Platinum vessels heated to low redness in contact with finely divided silver from fused argentic fluoride show 
an increase of weight even after having been cleaned by nitric acid (see page 229). — Added April 1870, G. G. 
