40 Assistant-Prof. J. W. Clark on certain Cases 



soft. A rise of temperature which is insufficient to render a 

 solid liquid, or so soft as to change its shape, is also sufficient 

 for the equalization of strain as is shown in the annealing of 

 glass. Both electrolytic conduction and the annealing of glass 

 take place more readily at a high than at a low temperature. 



(ii.) Substances of apparently similar constitution may exhibit 

 opposite forms of electrical conduction (Cu 2 Se, Ag 2 Se, and Ag 2 S, 

 Cu 2 S). 



The bodies to which my investigation refers are mercuric 

 chloride and mercuric iodide, which Faraday believed to 

 conduct metallically in the fused condition. 



Beetz, as I have recently found, states in a paper, " Ueber 

 die Leitungsfahigkeit fur Elektricitat welch e Isolatoren durch 

 Temperaturerhohung annehmen " (I. c), that he has obtained 

 evidence of the electrolytic decomposition of fused mercuric 

 iodide; and he attributes its apparent conduction without 

 decomposition to recombination of the products. So far as I 

 am aware, no attempt has been made to examine the nature 

 of the conduction of fused mercuric chloride since Faraday 

 concluded that it probably conducted metallically. 



Mercuric Iodide. 



Mercuric iodide is dimorphic, and at the ordinary tempera- 

 ture forms a scarlet powder which at 110° C. becomes yellow, 

 and at that temperature acquires a very slight electrical con- 

 ductivity (Beetz). Mercuric iodide melts at 247° C. and 

 boils at 342° C. It is an interesting substance on account 

 of the ease with which it volatilizes at temperatures much 

 below its melting-point ; and it is not unlikely that the ease 

 with which the molecules are thus shown to be leaving the 

 solid substance may be connected with a high diffusive rate 

 when it is fused, and this may partially explain the readiness 

 with which the products of its electrolytic decomposition often 

 mix and recombine, thus simulating conduction without de- 

 composition. The mercuric iodide which I have used in the 

 following experiments was prepared either by the precipitation 

 of recrystallized and sublimed mercuric chloride with pure 

 potassic iodide and sublimation of the product, or by the 

 sublimation of the commercially pure substance. 



It may not perhaps be out of place to point out, in the first 

 instance, the effect of heat upon mercuric iodide ; as it is 

 sometimes stated in text-books of Chemistry that, when heated, 

 it undergoes partial dissociation with liberation of iodine, where 

 the edge of the liquid is in contact with the hot glass vessel. 

 I have, however, convinced myself that this statement applies 

 only to the commercially pure substance, which on sublima- 

 tion leaves a little impure oxide of iron, and which, on the 



