1881.] On the Coefficient* of Expansion, Sfc. 14& 



IX. " On the Coefficients of Contraction and Expansion by Heat 

 of the Iodide of Silver, Agl, the Iodide of Copper, Cu 2 I 2 , 

 and of Five Alloys of these Iodides." By G. F. Rodwell, 

 F.R.A.S., F.C.S., Science Master in Marlborough College. 

 Communicated by Professor A. W. WILLIAMSON, For. Sec. 

 R.S. Received November 11, 1881. 



(Abstract.) 



The experiments described in this paper are a continuation of those 

 published at intervals during the last five years in the " Proceedings ,r 

 in connexion with the anomalous expansion by heat of certain 

 iodides. 



Fresh and more accurate determinations of the coefficients of con- 

 traction and expansion of iodide of silver are given. 



Certain physical and chemical properties of cuprous iodide are- 

 detailed, and determinations of its coefficient of expansion by heat. 



^Eive alloys of iodide of silver with cuprous iodide were prepared,, 

 having the following composition and percentage of iodide of silver : — 



Composition. Percentage of iodide of silver. 



Cu 3 I 3 .AgI 38-2233 



CuJ^Agl 55-3066 



Cu 3 I 3 .3AgI 64-9884 



Cu.I3.4AgI 71-2225 



Cu 2 I 2 .12AgI 88-1304 



The physical properties of these bodies are described, and their 

 coefficients of contraction and expansion are determined, and the- 

 volumes between 0° C. and the melting point are deduced therefrom. 



A general discussion of the results is afterwards given, in which 

 these alloys are compared with the five chlorobromiodides of silver 

 previously described (" Proc. Roy. Soc," vol. 25, p. 303), and with 

 the lead-silver iodide ahoy, the properties of which were described in 

 the last communication of the author on this subject ("Proc. Roy. 

 Soc," vol. 32, p. 540). 



The following are some of the facts noticed in connexion with the 

 alloys : — 



1. The specific gravity varies but slightly, viz., from 5*7302 to 

 5-6950, and is little above the mean specific gravity of the con- 

 stituents. 



2. The melting points are in all cases much lower, for while the 

 melting point of iodide of silver is 527° C, and of iodide of copper 



