1881.] Effects of Heat on the Chloride, fyc., of Silver. 



293 



2. The Chloro-brom-iodides of Silver. 



Mr. Carnelley lias been so good as to determine for me, by bis new 

 metbod, tbe melting points of tbe five cbloro-brom-iodides of silver 

 described in my former paper (" Proc. Roy. Soc," vol. 25, p. 292), 

 and tbe resnlts bave been recalculated in accordance tberewitb. By 

 reference to tbe accompanying table, it will be seen that tbe fusing 

 point of No. 1 is 44° C. lower than tbat of its most easily fusible 

 constituent, and 144° C. lower than tbat of tbe least easily fusible 

 constituent. Tbe lowering is most conspicuous in No. 3, tbe fusing 

 point being 101° 0. lower tban tbat of bromide of silver, wbicb con- 

 stitutes nearly one-quarter of its weigbt, and 201° C. lower tban tbat 

 of iodide of silver, wbicb constitutes more tban balf its weigbt. 

 Again, in No. 4 it will be seen tbat tbe melting point is 147° C. lower 

 tban of iodide of silver, wbicb constitutes nearly three-fourths of its 

 weigbt. 



We may notice, moreover, the following points : — 



(1.) No. 1, containing the smallest quantity of iodide of silver, is 

 almost unaffected by it. It closely resembles bromide of silver, save 

 that a very slight contraction takes place between 125 0, 5 C. and 

 131°'5 C. ; and the orange- coloured powder furnished by pulverisation 

 turns green on exposure to light. In No. 3, which contains a little 

 more than half its weight of iodide of silver, we find the lowest melt- 

 ing point ; the highest specific gravity ; the greatest divergence in 

 every respect from the properties of its constituents, although the 

 influence of the iodide is very marked, both in the plasticity of the 

 substance above 250° C, and in the considerable contraction which 

 takes place between 124° C. and 133° C. In No. 5 the influence of 

 the iodide is most marked, and in some respects the substance 

 resembles its principal constituent. 



(2.) Let us note, moreover, tbat the fusing points diminish from 

 1 to 3, and increase from 3 to 5 ; while the specific gravities diminish 

 on both sides of No. 3. 



(3.) In No. 1, in spite of the presence of the iodide, the coefficients 

 of expansion above 131 "5 and below 125'5 are higher than those of 

 any of its constituents. Again, in spite of the presence of such 

 expansible bodies as the chloride and bromide of silver, tbe amount 

 of contraction undergone by some of the chloro-brom-iodides, on 

 heating between 124° C. and 133° C, exceeds tbat of the iodide itself. 

 The case becomes one of great complexity. Take the most distinctive 

 of the chloro-brom-iodides, No. 3. Between 0° C. and 124° C, any 

 mass of a hundred molecules consists of 58 which are undergoing 

 slight contraction, and 42 which are undergoing rapid expansion. 

 Rapid contraction takes place with the iodide alone at 142° C. ; here 

 it commences 18° C. lower, and while it takes place in the case of 58 



