PURE SUBSTANCES, FUSED SALTS, SOLID ELECTROLYTES 363 



at the anode is remarkably good. The applicability of Faraday's Law 

 has been further verified by Tubandt and Eggert. 15 There can be little 

 question but that, in the case of these salts, Faraday's Law holds true. 



By employing solid silver iodide above its transition point in contact 

 with a silver cathode, Tubandt 16 has found it possible to test Faraday's 

 law in the case of other electrolytes than the silver salts and, further- 

 more, has been able to carry out transference measurements in order to 

 determine to what extent the conductance in solid electrolytes is due to 

 the positive and to what extent it is due to the negative carrier. It has 

 been shown that for silver iodide, silver bromide, silver chloride, silver 

 sulphide, above its transition point, and copper sulphide (Cu 2 S), Fara- 

 day's Law holds and that in these salts the current is carried entirely by 

 the positive ion. These results are very significant in that they show 

 that one set of ions in these solids forms a fixed framework through which 

 the other ions move with considerable facility. In the above salts, the 

 negative ions form the framework through which the positive ions move. 

 In lead chloride, however, the current is carried by the negative ion; 

 the positive ions form the framework through which the negative ions 

 move. These facts have an important bearing on the theory of the 

 structure of solid salts. 



Silver sulphide has a transition point at 179. Above the transition 

 temperature, as was shown by actual electrolysis of the salt, Faraday's 

 Law holds and the current is carried entirely by the positive ions. Below 

 the transition temperature, the (3 form of silver sulphide appears to con- 

 duct in part metallically. In the (3 form of silver sulphide, Faraday's 

 Law does not hold, only about 80 per cent of the current being carried 

 by the silver ion. The negative ion in this case is apparently not in- 

 volved in the conduction process, the remainder of the current being 

 carried by a metallic process of conduction. Apparently, therefore, solid 

 electrolytes exist in which the current is carried partly metallically and 

 partly electrolytically. As we shall see in a subsequent chapter, solu- 

 tions of the alkali metals in liquid ammonia likewise conduct the cur- 

 rent by a mixed process. 



The conductance of a heterogeneous mixture of two solid electrolytes 

 is approximately a linear function of the composition of the mixture. 

 When two solid electrolytes form mixed crystals, however, the conduc- 

 tance of the homogeneous mixture is often much greater than that of 

 the pure constituents. In the following table are given values of the 



"Tubandt and Eggert, Ztschr. J. anorg. Cliem. 110, 196 (1920) 

 "Tubandt, Ztschr. f. Electroch. 26, 358 (1920). 



