HALOID COMPOUNDS OP SILVER, MERCURY, LEAD, AND COPPER. 1163 
But the contraction did not begin when the expansion ceased; the mass simply absorbed 
heat without undergoing change of volume through a varying range of degrees, which 
removes the point of contraction still further from that of the iodide of silver. 
Thus, while in the case of the chlorobromiodide of silver containing only 26T692 
per cent, of iodide of silver, and of the lead-silver iodide alloy containing 33‘794 per 
cent., the contraction commenced at 124° C., the following temperatures were those at 
which the contraction commenced in the case of the copper-silver iodide alloys con¬ 
taining in all cases more iodide of silver than either of the alloys mentioned above:— 
Percentage of 
iodide of silver. 
38-2232 
55-3066 
64-9884 
71-2225 
88-1304 
Temperature at whieh contraction 
on heating commenced. 
284° C. 
233° 0. 
214° C. 
199° C. 
153° C. 
Thus the effect of 61*7767 per cent, of iodide of copper was to raise the point at 
which the iodide of silver commences to pass from the crystalline to the amorphous 
condition 142° C., viz.: from 142° C. to 284° C.; and the influence of 28*7775 of 
iodide of copper was sufficient to raise the temperature of the point of change to 
199° C., while the presence of 66*206 per cent, of iodide of lead, and of 74 per cent, of 
chloride and bromide of silver, not only did not raise the temperature at which 
contraction commences, but lowered it 18° C. When the percentage of iodide of 
silver in the copper-silver iodide alloys was very considerable the temperature at 
which contraction commenced fell to 153° C. 
17. This difference in the commencement of the period of contraction on heating 
would lead us to look for differences of structure in the case of those alloys which 
begin to contract at 124° C., and those in which the contraction is masked and 
annulled until a higher temperature is attained. Such differences we have no 
difficulty in finding. If we compare the lead-silver iodide alloy, PbI 3 .AgI, previously 
described (p. 1146), which contains 33*794 per cent, of iodide of silver, with the copper- 
silver iodide alloy, Cu 3 I 3 .AgI, which contains 38*2232 per cent, of iodide of silver, and 
which, hence, most nearly approaches it in composition, the differences are very con¬ 
spicuous. PbI 3 .AgI is crystalline in structure, and in cooling it expands considerably 
and breaks the tube in which it is cast. Harsh noises are emitted during cooling, and 
strong tremors are propagated through the mass. The expansion of the alloy ceases 
at 118° C., contraction commences at 124° C., and continues to 139° C. rapidly, almost 
equalling that of iodide of silver, itself; the mass decreasing in volume to *984120. 
Cu 3 I 3 .AgI is resinous in structure, and in cooling it contracts considerably, and comes 
out of the tube in which it is cast. No harsh sounds are emitted during cooling, and 
no tremors are propagated through the mass. The expansion of the alloy ceases at 
