HALOID COMPOUNDS OF SILVER, MERCURY, LEAD, AND COPPER. 1151 
The Tables (A, p. 1145, B, p. 1147) show the curves of contraction and expansion 
of the lead-silver iodide alloy, and of its constituents. In Table B the scale has been 
enlarged in order to show the details of the contraction of the alloy on being heated 
from 124° C. to 139° C. 
Other alloys of iodide of lead with iodide of silver were made, having the following 
composition:— 
( 1 ) 
( 2 ) 
(3) 
(4) 
(5) 
2AgI.PbI 2 , containing 50’51 7 per cent, of iodide of silver. 
3AgI.2PbI 2 „ 43*360 „ 
4AgI.3PbI 2 „ 40-497 
5AgI.4PbI 2 „ 38-950 
10AgI.9PbI 2 „ 36-190 
They all possessed the same general appearance as the alloy AgI.PbI 2 described 
above, which contains 33’794 per cent, of iodide of silver. But with the exception 
of No. 5 they were so brittle that they could not be cast into rods suitable for use 
with the expansion apparatus. During cooling large rifts appeared in the rod at 
right angles to its length, at the time when the iodide of silver commenced to expand. 
In the case of No. 1 the rod was violently broken during its cooling by the expanding 
iodide of silver; even when slowly annealed in hot paraffine. It may be noted that 
no such effect was produced in the case of the chlorobromiodides of silver, having the 
composition respectively : Ag 2 I 2 . AgBr.AgCl; Ag 3 I 3 .AgBr. AgCl; Ag 4 I 4 . AgBr.AgCl; 
and containing in each case a larger percentage of iodide of silver (viz.: 58‘6404; 
68'0171; and 73*9285) than the silver-lead iodide alloy No. 1. The chlorobrom¬ 
iodides, although, of course, their brittleness increased with the percentage of iodide 
of silver, formed less brittle rods than the iodide of silver, and than the first of the 
silver-lead iodide alloys, although the latter contains 23 per cent, less iodide of silver 
than the chlorobromiodide Ag 4 I 4 . AgBr.AgCl. 
10. Iodide of Copper. 
The cuprous iodide was prepared by the method of Soubeiran, in which an aqueous 
solution of two molecules of cupric sulphate and two molecules of ferrous sulphate is 
precipitated by potassic iodide, according to the equation 
2 (CuS0 4 )+2(FeS0 4 ) + 2KI=K 2 S0 4 + Fe 2 3S0 4 +Cu 2 I 2 . 
The precipitated iodide was washed with dilute sulphuric acid and with water, and 
was completely dried at 200° C. It then presented the appearance of a nearly white 
powder, not of a dead white colour like oxide of zinc, but white with a tinge of grey. 
On heating to a temperature considerably exceeding 200° C. it becomes pale canary- 
yellow, which just before its fusion changes to orange. When fused it forms a 
bromide-red liquid, which solidifies to a steel-grey crystalline mass. If the fused mass 
MDCCCLXXXIII. 7 H 
