on some Chloro-brom-iodides of Silver. 



301 



G-EJ^EEAL CoN'CLiisioi^s. — There are several questions connected with 

 the chloro-bromo-iodicles of silver which require to be discussed, and it 

 may be well to take them under separate headings. 



Comjoarison of the alloys ivitli their constituents. — For all purposes of 

 these comparisons we may take the bromide and chloride o£ silver to- 

 gether, since their coefficients of expansion and certain other relations 

 to heat are very much the same. It will be noticed that the first alloy 

 contains only 26 per cent, iodide of silver, while the four succeeding alloys 

 contain respectively 41, 58, 68, and 74 per cent. If we compare the 

 percentage of silver we find : — No. 1, 60 per cent. ; No. 2, 57 ; No. 3, 54 ; 

 No. 4, 52 ; and No. 5, 51 : or, again, in No. 1 we have 14 per cent, of 

 iodine to 25 of Br and CI ; in No. 2, 21 of I to 20 of Br and CI ; in No. 

 3, 31 of I to 14 of Br and CI : in No. 4, 36 of I to 11 of Br and CI ; and 

 in No. 5, 40 of I to 8 of Br and CI. The first alloy is scarcely affected 

 at all as regards its coefficients of expansion by the presence of the iodide, 

 and, in fact, resembles bromide of silver in all its properties ; on the 

 other hand, the alloys Nos. 4 and 5 are very much affected by the pre- 

 sence of the large amount of iodide of silver they contain, and in many 

 respects resemble the iodide. The greatest divergence from the proper- 

 ties of the constituents is to be found in the alloys Nos. 2 and 3, in which 

 the iodide varies betw^een 40 and 60 per cent. Perhaps this is due to 

 the fact that the iodide only dissol^'es to a certain extent in the fused 

 bromide and chloride ; for we notice that certain properties of the iodide 

 are masked so long as the iodide does not exceed a certain percentage, 

 while they become very apparent as the amount of iodide is increased. 



Of the point of maximum density of the alloys. — While the bromide and 

 chloride of silver expand regularly like any ordinary solid, it has been 

 shown that the iodide contracts slightly up to 142° C, considerably be- 

 tween 142° C. and 145°'5 C, and that it possesses its point of maximum 

 density at the latter temperature. Now nothing could possibly be more 

 definite or decided than the behaviour of the alloys at the critical tem- 

 perature at which contraction commences during the heating of the mass. 

 In the case of the alloys Nos. 2, 3, 4, and 5, this contraction invariably 

 commenced at 124° C, and invariably finished at 133° C. In the case of 

 No. 1 alloy, in which the percentage of iodide of silver was smallest, the 

 contraction began at 125°*5 C. (1°"5 C. higher than the others). The action 

 took place with great precision in every instance. Here, then, we have the 

 curious fact that while the iodide of silver commences its considerable con- 

 traction (which occurs simultaneously wdth its passage from the brittle 

 crystalline state into the plastic amorphous state) at 142° C. and finishes it 

 at 145°'5 C, the chloro-brom-iodide alloys commence their contraction 18° 

 C. lower, and end it 12°-5 C. lower. Thus in the iodide it is effected in 

 the heating through 3°-5 C, w^hile in the alloy it requires 9° C. We 

 must remember that in the alloy the iodide passes into the amorphous 

 condition while it is disseminated through the mass of the bromide and 



YOL. XXV. T 



