200 Profs. A. Gray and J. J. I)oMi-. 



however, the resistance of XXVI considerably exceeded that of the 

 barium glass, XXVIII. 



Effect of Annealing. The last four specimens mentioned in Table I 

 (XXIX XXXII) were experimented on to find the effect of an- 

 nealing on the resistance of glass. It is well known that in most 

 conductors, especially pure metals, the effect of annealing is to diminish 

 the specific resistance. XXIX and XXX were specimens of Messrs. 

 Powell's lead-potash glass. Of these, XXIX and XXXI were carefully 

 annealed in the usual way by Messrs. Powell : XXX and XXXII were 

 left in the unannealed state. 



The results are given side by side in Table I, and show that the 

 effect of annealing glass is very greatly to increase its specific resist- 

 ance. In the case of XXX and XXXII, the lead-soda glasses, the 

 specific resistance has been raised to three times its former value. 

 Annealed glass is therefore a much better insulator than unannealed 



Variation of Resistance with Time. The question of variation of 

 resistance with time has been investigated by testing flasks, which had 

 been set apart for the purpose, at intervals of about six months. The 

 results are shown in Table II. XXI, XXVII, and XXVIII have had 

 their resistances determined three times, and so far no change has dis- 

 closed itself. As noticed above, the surface of the glass XXVIII seems 

 to become disintegrated in course of time ; for the surface, which was 

 originally cleaned till quite clear, has gradually acquired a milky 

 appearance, and is now quite opaque. 



TJte Capacity Experiment*. 



The specific inductive capacity of the glass of these flasks was 

 determined by the method described in our former paper, and the 

 results are shown in Table I. It will be seen that the specific inductive 

 capacity of Powell's lead-soda glass, XXVII, is rather low in com- 

 parison with that of the corresponding lead-potash glass, XXI, which 

 was about 8. 



It is noteworthy, however, that the lead-soda glass was free from 

 the dielectric polarisation effects which were so troublesome in the case 

 of XXI. Of the glasses discussed in the present paper, XXVI, one of 

 the ordinary glasses experimented on, was the only one that showed 

 dielectric polarisation conspicuously. 



No trouble from dielectric polarisation was experienced with Nos. 

 XXIX XXXII, so that annealing does not cause any marked differ- 

 ences in the " electric absorption " of the glass. 



