248 Barns and Iddings — Electric conductivity, etc. 



The table also shows that the conductivity of rock magmas 

 at high temperatures is considerable even when they have 

 passed from a liquid to a solid state, that is, when they exist 

 as highly heated glasses. 



In general, therefore, a thorough change of chemical struc- 

 ture through ionic diffusion, whether directed by an electrical 

 field or otherwise, must be an easy possibility in a sufficiently 

 temperatured but otherwise solid magma. 



§ 6. The second point gained is more specific. Kohlrausch* 

 has shown that the contribution of a single ion in promoting 

 electrical conductivity decreases with the number of ions pres- 

 ent. In other words in concentrated solutions the ions are 

 stopped en route by what may be termed friction, but what is 

 probably temporary intercombination with each other. Hence 

 their mean velocity, and consequently the specific molecular 

 conductivity of the solution falls short of the proportionate 

 quantity. Moreover, it is perfectly well knownf that many 

 solutions (H 2 S0 4 , HC1, ZnS0 4 etc.) show maxima of electric 

 conductivity for gradual changes of concentration. The case 

 of an aqueous electrolyte decreasing in electric conductivity 

 with increasing concentration is therefore given by many ex- 

 amples, and doubtless their number could be indefinitely 

 increased by employing aqueous solutions at high temperatures 

 under pressure. 



Now this is exactly what we observe in our series of three 

 silicate solutions. Conduction increases markedly from (3) to 

 (1), that is, from basic to acid, and it seems altogether probable 

 that the anticipative maximum will correspond to a degree of 

 silicic dilution greater than 75 per cent of silica. 



Tracing this inference further we come to the conclusion 

 that pure silica is probably an insulator, or that it occupies a 

 position in siliceous electrolysis very closely analogous to that 

 of water in aqueous electrolysis. It is to be remarked that we 

 here refer to the electrolytic solvent and leave the question of 

 chemical solvent temporarily out of consideration. This is in 

 accord with the observations of Warburg,:}: who has elaborately 

 investigated the insulation of a film of pure silica at about 300,° 

 the film having been produced by the electrolysis of glass. 

 The insulator was found to be so perfect that the capacity of 

 the condenser was measureable. The insulation of quartz 

 relative to glass has to some extent (as far as 224°) been tested 

 by Warburg and Tegetmeier.§ though the conditions were 

 complicated in this instance both by crystalline structure and 

 the impurity of the mineral. 



* Kohlrausch, 1. c. p. 183. fl. c. p. 43. 



% Warburg, Wied Ann., xx\, p. 622, 1884. 



gWarburg and Tegetmeier, Wied. Ann., xxxv, p. 463, 466, 1888. 



