HALOID COMPOUNDS OP SILVER, MERCURY, LEAD, AND COPPER. 1161 
8. The general colour of the alloys was in the mass from brown-yellow to green- 
yellow, as the percentage of iodide of silver was increased. In thin laminse bright 
yellow, and in a powder a brilliant dark yellow, becoming slightly lighter as the 
percentage of iodide of silver was increased. 
9. The coefficient of expansion of the alloys below the point at which contraction on 
heating commences, was found to decrease as the percentage of iodide of silver was 
augmented:— 
Percentage of Agl. 
Coefficient of expansion. 
38*2232 
•00004998 
55-3066 
•00003750 
64-9884 
•00002307 
71-2225 
•00001998 
88-1304 
•00000636 
It was in all cases considerably less than that of iodide of copper. 
The same fact was observed in the case of the chlorobromiodides of silver :— 
Percentage of Agl. 
26-1692 
41-4840 
58-6404 
68-0171 
73-9285 
10. On the other hand, the expansion 
heating finishes, was greater than that of 
copper. 
Coefficient of expansion. 
■00012216 
•00009529 
•00008307 
•00006000 
•00005400 
above the point at which contraction on 
either the iodide of silver or the iodide of 
11. Matthiessen asserts that the coefficient of expansion of a metallic alloy is the 
mean of the coefficients of expansion of the several volumes of the metals composing 
it, and in the case of ordinary alloys this law undoubtedly holds good. But in the 
iodide of silver alloys the conditions are so complex that we can scarcely hope for the 
same result. Nor do we find it. We are dealing, on the one hand, with a substance 
which possesses several allotropic modifications; several distinct coefficients of con¬ 
traction on heating, and one coefficient of expansion; and, on the other hand, with 
bodies, which like the iodides of lead and mercury also possess allotropic modifica¬ 
tions, each with its own coefficient of expansion, or with a single coefficient of expansion 
and no allotropic modification like the iodide of copper. 
12. Matthiessen considers that in nearly all cases two-metal alloys may be regarded 
as solidified solutions of the one metal in the other, as glass is a solidified solution of 
different silicates, or gold and silver in the gold-silver alloys. 
13. The lowering of the fusing point of alloys undoubtedly means that the cohesion 
of the particles in the alloy is less than that of the particles in its constituents ; and 
7 i 2 
