88 Assistant-Prof. J. W, Clark on certain Cases 



sions respecting their nature, beyond regarding a free motion 

 of the particles resulting either from fusion or from solution 

 as necessary for electrolytic conduction. Even this is not 

 without noteworthy exceptions ; for Faraday has described 

 some binary compound liquids (SnCl 4 , AsCl 3 , &c,*) which 

 neither conduct nor decompose ; whilst, on the other hand, 

 some compound solids are known which conduct metallically 

 (Cu 2 Set, Ag 2 Set, SnS 2 , CuSf, to which perhaps must be added 

 Pb0 2 , Mn0 2 , and Ag 2 0), and some bodies which are solid 

 and yet conduct electrolytically (Cu 2 S$, Ag 2 S§). Further, 

 zincic iodide§ neither conducts nor is decomposed when ren- 

 dered fluid by heat ; whilst others (Hgl 2 §, HgCl 2 §, PbFlJ) 

 have been considered as conducting without decomposition 

 under the same circumstances. The nature of the conduction 

 of the metallic sulphides is very imperfectly knownlT. 



Whilst thinking over these facts about a year and a half 

 ago, it seemed to me very probable that a careful study of 

 these exceptions to the general laws of electrolytic decompo- 

 sition might result in more definite conjectures respecting the 

 condition of internal or molecular structure required for con- 

 duction and decomposition. To make such a study complete 

 requires the determination and comparison of a number of 

 physical constants (e. g. colour, conductivity, expansion, spe- 

 cific and latent heat, refractive index, specific inductive 

 capacity, &c.) for substances which are normal and abnormal 

 in their electrolytic behaviour. This cannot be completely 

 done at present for want of data; but before passing to the 

 consideration of those bodies to which my own investigation 

 refers, I wish to briefly refer to a few previous papers relating 

 to substances of remarkable electrolytic behaviour from this 

 point of view. 



Hittorf states that sulphide of silver fuses at a clear red 

 heat, but at a temperature of 180° C. it is sufficiently soft to 

 adapt its shape slowly to that of the surface upon which it 

 rests, and at the ordinary temperature it is malleable : a cast 

 stick of it can be slightly bent without fracture, and it can be 

 cut with a knife or turned in a lathe. This substance has a 

 very low resistance, and even at the ordinary temperatures 

 is electrolytically decomposable by a feeble current. Hittorf 

 (7. c.)has concluded, from a long and careful series of expe- 

 riments upon it, that it conducts electrolytically only, and that 



* Faraday, ' Experimental Researches, ' vol. i. 



t Hittorf, Pogg. Ann. Bd. lxxxiv. 1851. J Hittorf (I. c). § Faraday. 



|| Faraday, Beetz (Pogg. Ann. Bd. xcii. 1854) has since shown that 

 conduction takes place in a normal electrolytic way. Faraday discovered 

 that the solid Plumbic Fluoride began to conduct below a red heat. 



1} See Faraday, 'Experimental Researches ' \ and Hittorf (7. c). 





