HALOID COMPOUNDS OF SILVER, MERCURY, LEAD, AND COPPER. HG5 
21. It would therefore appear that in the case of the lead-silver iodide alloy, 
PbI 2 .AgI, the ground mass of PbT 2 , with which the iodide of silver is surrounded, 
offers no opposition to the passage of the iodide from the plastic to the crystalline 
condition, but rather, by separating the crystals from the influence of their own 
attraction, promotes it. While, on the other hand, that in the case of the copper- 
silver iodide alloy the ground mass of Cu 2 I 2 , by its attraction or by other influence, 
prevents the passage of the iodide of silver from the plastic to the crystalline 
condition, or at least considerably hinders it, and allows it only to take place at a 
temperature far above the usual point, or when the percentage of iodide of 
silver is so considerable that the adverse influence of the Cu 3 I 2 is overcome, as in 
the last alloy containing 88 per cent, of Agl. 
22. If a small thin layer of sulphur be melted on a piece of mica and suddenly 
cooled in cold water, and placed under the microscope, it is seen to be perfectly 
transparent; gradually, as we watch it from day to day, crystals are seen forming 
themselves out of the ground mass, and these increase rather in number than in size, 
until the whole is converted into a crystalline opaque brittle mass. In this case 
molecular motion being, as we know, slowly lost, the crystalline forces become more 
and more able to build up crystals out of the ground mass. On the other hand, if a 
thin layer of iodide of silver be melted on a slip of mica, and placed, while in the 
plastic condition, under the microscope, it is seen to be transparent, but at the moment 
when the temperature sinks to 142° C. the mass suddenly crystallises all over. 
Between these two extremes we, of course, have instances in which the passage from 
the amorphous to the crystalline state takes place with greater or lesser readiness. 
If iodide of lead be fused on a piece of mica, and placed under the microscope, it is 
seen to crystallise rapidly, and it presents the appearance of crystalline plates with 
numerous veined markings (fig. 7). When alloyed with iodide of silver the mass is 
seen to be made up of a multitude of small tabular crystals (fig. 8), altogether 
different from either the iodide of lead, or the iodide of silver when similarly and 
separately treated. The copper-silver iodide alloys (except when the percentage of 
iodide of silver is very large) present the network appearance under the microscope 
shown in fig. 9. They are transparent moreover in thin layers, and translucent in 
thicker layers, while the chlorobromiodides, and the iodide of lead, and the lead-silver 
iodide alloy are opaque from the presence of multitudes of crystals. 
23. It is conceivable that the iodide of lead possesses so strong a tendency to 
crystallise, and so little attraction for the molecules of iodide of silver, that it in no 
way interferes with the crystallisation of the latter; while the iodide of copper may 
possess but little tendency to crystallise itself and considerable attraction for the iodide 
of silver homogeneously diffused throughout. Or, in the one case we may have an 
actual chemical attraction of the molecule of the one iodide for that of the other; 
