288 



Mr. G. F. Rodwell on the Effect of Heat on 



bent, but it cannot be made transparent without re-fusing. By rapidly- 

 casting a bar and introducing it into the trough of the expansion-ap- 

 paratus filled with paraffine above 200° C, the expansion of the trans- 

 parent amorphous iodide was determined, and this gave -00001206. 

 "Wernicke considers that at a high temperature " part of the iodine is 

 separated from its combination with the silver, and is absorbed by the 

 remaining substance in the liquid state, for the spectrum, like that of 

 solid and liquid iodine, contains no blue or violet light. In the normal 

 state, below 138° C, silver iodide gives a spectrum less bright, but twice 

 as long, and particularly developed in the blue-violet portion." 



It is well known that red, transparent, plastic sulphur in becoming 

 yellow, opaque, and crystalline gives out a good deal of heat. Trans- 

 parent plastic iodide of silver was examined at the moment of change, 

 and was found to give out heat. A mass of the plastic iodide at 200° C. 

 was placed in paraffine at 200° C, and the cooling was observed. When 

 the iodide began to change from the amorphous to the crystalline con- 

 dition the rate of cooling (previously determined) was far less rapid, 

 proving clearly that heat was given out. 



The iodide appeared to become more brittle with successive fusions ; 

 but this did not seem to materially affect its specific gravity. Thus a 

 specimen which had been many times fused gave a sp. gr. of 5*675, 

 while another specimen after a single fusion gave 5* 66. Taking the 

 former as the true sp. gr., we find that the sp. gr. at the maximum 

 density (142° C.) will be 5-771, and at the minimum density (- 60° C.) 

 5-673; while the sp. gr. of the molten iodide at 450° C. will be 5-522. 



It was noticed that the platinum cone was altered in capacity when 

 fused iodide of silver was allowed to cool in it ; in fact it became more 

 or less deformed. This was found to be due to the considerable expan- 

 sive force exercised by the iodide in expanding. We did not succeed in 

 breaking an iron bottle by this means, but a thick porcelain tube was 

 easily broken by the passage of the iodide from the amorphous to the 

 crystalline condition. 



An attempt to determine the relative conductivities of the iodide, 

 bromide, and chloride of silver was made in the following manner : — 

 Homogeneous rods of each of these substances were taken in weights 

 corresponding to their specific heats. Any one rod whose conductivity 

 was to be determined M^as fixed in the bottom of a copper trough, the 

 projecting end dipped into a small reservoir of mercury in contact with a 

 delicate thermopile and galvanometer. Great care was taken to sur- 

 round every thing with non-conductors of heat, and to insulate the rod 

 from contact with the copper trough. Paraffine at 300° C. was poured 

 into the trough, and thus came into contact with the exposed end of the 

 bar. The time which elapsed before the movement of the galvanometer 

 took place was noted in seconds. The results were not satisfactory ; but 

 they appeared to prove that the bromide of silver conducts heat better 



