200 



Mr. T. Gray. 



which the resistance was measured; in the fourth, the density of 

 the glass; and in the fifth, the date at which the resistance was- 

 measured. 



Date. 



May 3, 1880. 

 December 9, 1880. 

 May 3, 1880. 

 December 10, 1880. 

 May 3, 1881. 

 May 17, 1880. 

 November 17, 1880. 



These results show a very considerable increase of conductivity 

 with age, and also show a marked difference in the variation of 

 different specimens. The number of experiments is not sufficient to 

 give much information regarding this time change, but the fact that 

 they give evidence that such a change takes place seems to warrant 

 the publication of these preliminary results. 



The measurements of resistance described in this paper were, like 

 those in the previous paper above referred to, for the most part made 

 by means of an astatic galvanometer of high resistance and great 

 sensibility. The galvanometer used had an internal resistance of 

 10,000 ohms, and one Daniell's element produced a deflection of one 

 division when a resistance of about 10 11 ohms was in the circuit. The 

 great advantage of the galvanometer over the electrometer method of 

 measurement is its simplicity ; the deflection being independent of the 

 capacity of the circuit, provided no change is taking place in that 

 capacity. In many cases, however, the resistance of glass at low 

 temperatures cannot be measured by the galvanometer, and in these 

 cases, the most convenient instrument is a Thomson's quadrant elec- 

 trometer. 



The method adopted in the galvanometer measurements was that of 

 direct deflection, the current being produced by fifty Daniell's ele- 

 ments, placed on a table well insulated with ebonite supports, 

 kept dry by being enclosed in boxes containing sulphuric acid. 

 The main difficulty in this method is to ensure absence of leakage 

 currents through the galvanometer. The test used for the absence 

 of such currents was to insulate the electrode of the inside coating 

 of the glass vessel, and then close the key. If there was no deflection, 

 it was assumed that the circuit was sufficiently insulated. 



Several measurements were made by means of the quadrant elec- 

 trometer, and in that case the method adopted was to connect one- 

 coating of the glass, one pair of quadrants, and the case of the elec- 



No. of 



specimen. 



Specific resistance 

 in ohms per 

 cub. centim. 



Tempera- 

 ture. 





146 x 10 10 



40° C. 



■ { 



122 x 10 10 



40° C. 





24 x 10 10 



■{■ 



17 x 10 10 



55 





12 x 10 10 







41 x 10 11 



140° C. 



• { 



17 x 10 u 





Density. 



2-57 



2 ! 53 



55 



3 -07 



