On the Electrical Properties of Different Kinds of Glass. 199 



XXX and XXXII lead-soda glasses, in which the amount of soda is 

 nearly equivalent chemically to the potash in XXIX and XXX respec- 

 tively. Reference to Table I will show that in both cases the potash 

 glasses have very much higher resistances than the soda glasses. 



Both XXI and XXVII were lead glasses ; but in the previous paper 

 a glass was discussed which was made by Messrs. Schott and Co., and 

 was composed mainly of barium oxide and alumina in combination 

 with silica and boron trioxide. This, which we shall call the Jena 

 barium glass, had a very high resistance, one quite unmeasurable, 

 indeed, within the range of temperatures covered by the experiments. 

 Moreover, it was found that the inductive capacity in a plate of this 

 glass was exceedingly low, and that the glass showed little or no 

 effects of dielectric polarisation. 



It was thought that it might be of interest to find whether the high 

 resistance of this glass was associated with the presence of the large 

 percentage of barium oxide. Accordingly, further experiments have 

 been made on flasks of barium-potash glass manufactured by Messrs. 

 Powell and Sons. This glass is numbered XXVIII of Table I. Its 

 resistance is very low in comparison with that of either the lead-potash 

 glass or the Jena barium glass, which, however, it must be remembered 

 contained no potash. It was found to be subject to a somewhat rapid 

 disintegration of its surface, and it probably differed in physical con. 

 stitution from the Jena glass. The Jena glass, moreover, contains a 

 considerable quantity of boron trioxide and alumina, which are absent 

 from the glass made by Messrs. Powell. The presence in glass, how- 

 ever, of a considerable percentage of potash with lead is, as shown by 

 XXIX and XXX, consistent with a high specific resistance. 



It is interesting also to compare XXVIII and XXVI, which contain 

 approximately the same percentage of barium and lead oxides respec- 

 tively, and are otherwise very similar. The lead glass, XXVI, contains 

 soda, from which the barium glass, XXVIII, is free ; but in spite of 

 this, the resistance of XXVI is about three times that of XXVIII at 

 the same temperature. 



Comparison with Ordinary Glass. Experiments were next made to 

 test how the glasses of specially prescribed chemical composition, 

 already experimented on, compared with the ordinary kinds of glass 

 used in the construction of apparatus. A common lime glass, XXV, 

 and common lead glass, XXVI, were examined. The resistances of 

 both of these were low ; that of XXV was low in comparison with the 

 resistance of any other glass in the table.* As has been pointed out, 



* It may be noticed here that the first experiments on the electrical properties 

 of specimens of glass, which were afterwards subjected to chemical analysis, seem 

 to be those described in Mr. T. Gray's paper, ' Roy. Soc. Proc.,' vol. 34, p. 199. 

 The analyses of the specimens (which were of glasses used for different purposes in 

 the arts) were made with great care by Professor Divers, of Tokio. 



