of Electrolytic Decomposition. 39 



its apparent metallic conduction results when a fine thread of 

 metallic silver has formed between the terminals of the sul- 

 phide of silver bar. 



Hittorf (I. c.) has discovered that cuprous sulphide also con- 

 ducts electrolytically " und besitzt entweder gar keine oder 

 ganz geringe metallische Leitung." Unlike sulphide of silver 

 this body has a high resistance, but, like it, the conductivity 

 increases with increased temperature. Cuprous sulphide melts 

 at a white heat, and may be cast in the form of a rod which 

 at the ordinary temperature is very brittle, although at a 

 " higher " temperature it may be bent, 



Hittorf (I. c.) points out the difficulty of proving experi- 

 mentally that the conduction of Cu 2 S and Ag 2 S is entirely 

 electrolytic, because the formation of copper or silver by the 

 action of the current in the substance of the bars places 

 their ends in true metallic communication with the battery- 

 terminals. 



Cuprous selenicle and argentic selenide closely resemble 

 their corresponding sulphur compounds to which reference 

 has just been made, and are described as being "soft" and 

 " slightly malleable" respectively (Watts's 'Dictionary of 

 Chemistry '). Selenides ordinarily closely resemble sulphides 

 in their physical and chemical relations, and are consequently 

 regarded as being possessed of similar molecular constitutions. 

 Hittorf, however, says of cuprous and of argentic selenide, 

 that u beide sind gute metallische Leiter."" Their electrical 

 behaviour is therefore of an exactly opposite nature to that of 

 their corresponding sulphur compounds. 



Faraday has noticed that conduction commences in heated 

 electrolytes at very different degrees of liquefaction and 

 softening. 



Plumbic chloride * conducts very appreciably at a tempera- 

 ture far below that at which it fuses, and at which it is not 

 noticeably soft Beetz (I. c.) has shown that glass begins to 

 conduct between 200° C. and 220° C, and Dr. Lodge reminds 

 me that this has recently been shown to take place at 100° C. 



It appears from the behaviour of these bodies that — 



(i.) In some solid electrolytes there is a sufficient mobility of 

 the molecules at the ordinary temperature to enable electrolytic 

 conduction to take place ; in others it is conferred by a rise of 

 temperature which is insufficient to render the solid liquid or even 



* E. Wiedemann, Ber. d. Kgl. Sachs. Gesellschaft der Wissenschaften, 

 1874. I have unfortunately been unable to refer to this paper ; but since 

 the above was written the author has kindly referred me to l Die Elektri- 

 citatslehrer ' (G. Wiedemann), Bd. i. S. 558, wherein it is stated that the 

 iodide and bromide of lead behave similarly. 



