42 Profs. A. Gray and J. J. Dobbie. Connection between 



The resistance was taken after five minutes' electrification in each 

 case. The "Jena" glass (XXIV of the table), in both resistance 

 and capacity experiments, showed very considerable effects of 

 dielectric polarisation, which were a very long time in disappearing, 

 though the conducting coatings of the flask were kept short-circuited. 

 The dielectric polarisation of the lead glass made by Messrs. Powell 

 was also considerable. 



On- the other hand it is very remarkable that the barium glasses 

 XXIII, in the capacity experiments, showed little or no sign of 

 polarisation effects ; and we propose to make some further investi- 

 gations of the physical properties of this glass. 



In our previous paper the results obtained with eight different 

 samples of lead glass were compared, and it was shown that the 

 electrical conductivity fell off almost quite regularly as the amount 

 of lead oxide increased, and increased with an increase in the amount 

 of soda. The glass which possessed the highest specific resistance 

 (8400 x 10 10 ohms) contained 4O5 per cent, of lead oxide, 7'5 per 

 cent, of potassium oxide, and 2*1 per cent, of sodium oxide. Both 

 of the lead glasses dealt with in this paper contained a still higher 

 percentage of lead oxide, and were almost free from soda, and the 

 electrical resistance was so great as not to be measurable. It is, of 

 course, impossible to say how far this result is due to the increase of 

 lead oxide, and how far to the elimination of soda. With the view 

 of definitely settling this point, Messrs. Powell and Sons have kindly 

 undertaken to prepare for us a glass exactly similar to XXI, but 

 having the potash replaced as nearly as possible by the equivalent 

 amount of soda. It should be noticed that the amount of foreign 

 matter (i.e., of ingredients other than silica, lead oxide, and alkali) 

 present in glasses XXI and XXII is very small, and is less than one- 

 fourth of the amount present in the purest glass previously tested, 

 which was also, it may be mentioned, the glass possessing the high- 

 est resistance. 



It is noteworthy that the barium glass XXIII has a very high 

 resistance, and in this respect behaves more like lead than lime 

 glasses, which have usually a low resistance. It is impossible, how- 

 ever, in view of the somewhat complicated composition of this glass, 

 to say how far the high resistance is due to the presence of the 

 barium, and how far ifc may be influenced by other ingredients, 

 especially the boric acid, which was not present in any of the glasses 

 previously examined by ourselves or others. 



The " Jena " glass XXIV has a low resistance, as was to be anti- 

 cipated, from its high percentage of soda and complex composition. 



The chemical composition of glass XXI is capable of being 

 expressed with tolerable accuracy by a simple chemical formula, and 

 this is also in accord with previous experience, which pointed to the 



