230 
ME. GEOEGE GOEE ON FLUOEIDE OF SILVEE. 
All these numbers, especially those representing the amounts of free silver and of water, 
are slightly variable, owing to the extreme deliquescence of the salt and its easy decom- 
posability by heated aqueous vapour *. 
According to M. Prat (Comptes Rendus, No. 9, August 26th, 1867), ordinary fluoride 
of silver in the anhydrous state is composed of oxygen, fluorine, and silver, its composi- 
tion being represented by the formula AgF, AgO ; and true fluoride of silver, prepared 
by him in a different way, and possessing very different properties from the ordinary 
fluoride, has the following composition, — 
Silver, 1 equiv. or . 
. 108-0 
0-785 
Fluorine, 1 equiv. or . 
. 29-6 
0-215 
AgF 
. 137-6 
1-000 
He considers the equivalent or atomic weight of fluorine to be 29*6 ; and he represents 
the composition of fluor-spar as follows : — 
Calcium, 2 equivs. or ... . 4(h0 
Fluorine, 1 equiv. or . . . . 29*6 
Oxygen, 1 equiv. or ... 8-0 
77-6 
The results obtained by me in the foregoing analytical experiments, as well as those 
obtained in determining the molecular volume of anhydrous hydrofluoric acid in the 
gaseous state (Phil. Trans. Roy. Soc. 1869, pages 179-183), do not agree with the view 
that ordinary fluoride of silver in the anhydrous state, prepared in the way I have 
described, contains oxygen. 
Physical Properties. 
The salt is a troublesome one to prepare, vessels of silver or platinum are essential 
for the evaporation of its solution. The solution must be heated and filtered before 
evaporating. When its solution was nearly concentrated by evaporation skin-like films 
formed upon its surface, and the liquid beneath was filled with very minute silky fibres, 
probably crystals of the salt. When heated nearly to dryness it became a tenacious 
sticky mass, requiring a rigid and sharp-edged spatula to remove it. The dried salt was 
in brown earthy fragments, which rapidly attracted moisture and deliquesced. In a fused 
state it formed a highly lustrous, mobile, and jet-black liquid, which on cooling became 
hard and tough f. On dissolving the cooled salt in hot water, filtering, and evapo- 
rating the clear solution by heat, some jet-black crystalline powder separated. On 
several oocasions a clear solution of the pure salt, containing free hydrofluoric acid, 
became blackish tvith separation of a little black powder on heating. 
The salt is very soluble, and evolves a small amount of heat whilst dissolving in the 
minimum amount of water. 103-58 grains of its aqueous solution (saturated at 15 0, 5 C.), 
* (Added April 1870.) — I have since made numerous analyses and quantitative experiments relating to 
fluoride of silver, and they have all confirmed the above composition of the salt. — G. G. 
t In a state of fusion it corrodes silver. See page 233. 
