550 



Mr. G. F. Rodwell. 



silver is passing from the amorphous plastic condition to the brittle 

 crystalline condition, within the mass of the iodide of lead. 



6. The fusing point of the alloy is 177° C. lower than that of the 

 iodide of silver, which constitutes one- third of its weight, and 33° C. 

 lower than that of the iodide of lead, which constitutes two-thirds of 

 its weight. 



7. If the lowering of the fusing point (also markedly apparent in 

 the case of the chlorobromiodides of silver) is due to the fact that 

 similar particles of matter attract each other more powerfully than 

 dissimilar, and hence, when the particles of two bodies are mutually 

 diffused, the attraction becomes, less, and the molecular motion is 

 consequently more readily assimilated ; the same cause may serve to 

 explain the commencement of the phase of contraction on heating the 

 alloy at a temperature 18° 0. lower than the substance to which it 

 owes this property. 



8. It is interesting to compare one of the chlorobromiodides of 

 silver with the lead- silver iodide alloy. For this purpose we will take 

 the chlorobromiodide which contains the nearest approach to the same 

 quantity of iodide of silver as the alloy. The second of the chloro- 

 bromiodides before described (" Proc. Roy. Soc," vol. 25, p. 295) 

 contains 41*484 per cent, of iodide of silver, and 58*5160 per cent, of 

 the chloride and bromide of silver, which latter, from the heat point 

 of view, may be regarded as the same substance, because their co- 

 efficients of expansion are practically the same. It may be noted 

 (vide below) that while the expansion of the bromide (which is 

 slightly greater than that of the chloride) scarcely exceeds that of 

 the iodide of lead, and while, moreover, the chlorobromiodide contains 

 8 per cent, more iodide of silver than the lead-silver iodide alloy, the 

 amount of contraction by heat of the latter is more than twenty times 

 greater than that of the former, although we must believe this effect 

 to be solely due to the iodide of silver in each case. 



Comparison of the Coefficients of the Iodide of Lead and the Bromide 

 of Silver, used in conjunction with Iodide of Silver in the forma- 

 tion of the two Alloys given below. 



Iodide of lead. Bromide of silver. 



Melting point 383° C 427° C. 



Volume at 0° C. ..... . 1*000000 1*000000 



205 1*015608 1*021945 



253 1*019594 1*027369 



265 1*027248 1*028725 



383 (solid) . . 1*048488 1*042531 



383 (liquid) . . 1*078080 



427 (solid) 1*047855 



427 (liquid) 1*107225 



