of Glass at different Temperatures, 233 



in the specific inductive capacity, and also that this quality is 

 much affected by change of temperature. 



The following numbers give the results of a corresponding 

 experiment on the resistance of the same tube. The numbers 

 in the left-hand column are proportional to the conductivities; 

 those on the right give the corresponding temperatures : — 



12 12° 



Heated to 240° and cooled slowly, 



9 23° 



300 57 



55 49 



Judging from these results, the resistance of the tube at 12° 

 at the end of the experiment must have been about three times 

 what it was at the beginning. This tube was then heated to 

 redness in a blowpipe-flame, and cooled quickly by blowing 

 air against it. The capacity deflection was then found to be 

 52 at 30° C, and the conductivity deflection 16 at the same 

 temperature. It was then heated to 250° and allowed to cool 

 slowly as before. The results were then found to be 52 divi- 

 sions for capacity and 18 for conductivity. This shows a 

 slight diminution of resistance, due to the heating to 250° im- 

 mediately after the glass had been heated to redness and cooled 

 quickly. It appears, therefore, that the permanent change is 

 not due to annealing. 



These results seem to point to the conclusion that slow 

 changes take place in glass which render it a better conductor 

 of electricity, and at the same time increase its specific induc- 

 tive capacity*. I am not sure that any considerable deterio- 

 ration in the insulating-power of glass stems and electrometer- 

 jars has been detected ; but in such a case these experiments 

 point to an at least temporary cure — keeping the glass at a 

 high temperature for a considerable time. This accords with 

 the treatment sometimes adopted for badly-insulating ebonite. 



* This conclusion is confirmed, so far as the capacity is concerned, by 

 the observations of Mr. J. E. II. Gordon on change of specific inductive 

 capacity in pieces of glass which had been kept carefully for a considerable 

 time after their specific inductive capacitv had been first determined 

 (< Nature,' Sept. 18, 1879). 



Phil Mag. S. 5. Vol. 10. No. 62. Oct. 1880. 



