1134 MR. G. F. RODWELL ON THE EFFECTS OF HEAT ON CERTAIN 
pale green, solid. The more rapid expansion on cooling ceases at 142° C., and as it 
continues to cool an extremely slight expansion continues, until, according to Fizeau’s 
calculation, it attains a minimum density and maximum volume at —60° C. 
It is well known that plastic sulphur in becoming yellow, opaque, and crystalline, 
gives out a good deal of heat. Transparent plastic iodide of silver was also found to 
give out heat in changing from the amorphous to the crystalline condition. 
The iodide of silver obviously possesses at least two allotropic modifications : the 
one plastic, amorphous, and transparent above 156°'5 C. ; the other brittle, crystalline, 
and opaque below 142° C. If it be melted and poured into cold water a very brittle, 
yellow, opaque mass is the result. 
Successive fusions appeared to make the iodide more brittle, but did not affect the 
specific gravity materially. Thus a specimen which had been fused many times had a 
specific gravity of 5’675, while another specimen after a single fusion gave 5*66. A 
specimen prepared by the solution of silver in hydriodic acid was found to have a 
specific gravity of 5'812. Several specimens of precipitated iodide, fused, gave 5*681. 
Deville found 5*687, and in the case of the unfused precipitated iodide 5 '807. 
D amour found the native crystals to have a specific gravity of 5*667. 
When a mass of iodide passes from the amorphous into the crystalline condition, 
the molecular commotion is so considerable that portions of the mass are sometimes 
jerked off from the ends of the bar, and large fissures a millimetre broad and several 
centimetres in length appear in the rod. Within these, crystals are sometimes seen 
projecting from the sides oi the fissure. Under the microscope they present the 
appearance shown in fig. 6, 
Fig. 6. 
