HALOID COMPOUNDS OF SILVER. MERCURY, LEAD, AND COPPER. 1141 
Several facts require to be noted in regard to these compounds. 
(1) No. 1, containing 26 per cent, of Agl is almost unaffected by it, and closely 
resembles bromide of silver, save that a very slight contraction occurs on heating 
between 125°*5 and 131°‘5 C., and the orange coloured powder produced by pulverisa¬ 
tion turns green on exposure to light. In No. 3, which contains a little more than 
half its weight of iodide of silver, we find the greatest divergence in every respect 
from the properties of its constituents, although the influence of the Agl is very 
marked both in the plasticity of the substance above 250° C., and in the considerable 
contraction which takes place between 124° C. and 133° C. It possesses the lowest 
melting point (viz. : 326° C., which is 101° C. lower than that of AgBr, which con¬ 
stitutes nearly one-fourth of its weight, and 201° C. lower than that of Agl, which 
constitutes more than half its weight), and the highest specific gravity. In No. 5, 
the influence of the Agl is most marked, but the melting point is still 147° C. lower 
than that of the Agl, which constitutes nearly three-fourths of its weight. 
(2) We may note that the fusing points diminish from 1 to 3, and increase from 3 
to 5 ; while the specific gravities diminish on both sides of No. 3. 
(3) While iodide of silver commences its considerable contraction consequent upon 
change of state at 142° C., and finishes it at 156°*5 C. (mainly, however, between 148° 
and 151°*3 C.), the chlorobromiodides all commence their contraction at 124° C., viz. : 
18° C. lower, and finish at 133° C., or 23°*5 C. lower. The lead-silver iodide (p. 1149) 
practically does the same thing. This matter is alluded to in greater detail in the 
general discussion of results (pp. 1162, 1163). 
(4) It is a curious fact that until the percentage of iodide of silver in the compounds 
becomes considerable, its chief influence is exerted between a narrow range of tempe¬ 
rature ; and more than this, so soon as the contraction is over the mass undergoes far 
more rapid expansion than any of its constituents when heated through the same 
range of temperature. The condition is obviously one of great complexity. Take 
the case, for example, of the chlorobromiodide No. 3. Between 0° C. and 124° C. any 
mass of 100 molecules consists of 58 which are undergoing slight contraction on 
heating, while 42 are undergoing rapid expansion. Then between 124° C. and 133° C. 
58 molecules are rapidly contracting; heat is disappearing in internal work and is 
changing the crystalline into the amorphous iodide, ie., converting an opaque, brittle, 
highly crystalline body into a transparent, plastic, denser body; while the 42 molecules 
of chloride and bromide are still expanding. Finally, from 133° C. to the fusing point, 
all the molecules are expanding, 42 of them quickly, and 58 of them slowly. 
7. Di-iodide of Mercury, Hgl 3 . 
This substance is dimorphous. In the amorphous condition it presents the appear¬ 
ance of a brilliant scarlet powder, which, if heated, fuses at 200° C. and volatilises 
just above the fusing point to a vapour more than twice as dense as that of mercury. 
