On the Variation of the Electrical Resistance of Glass, Sfc. 203 



hence the density in this case serves as a guide to the electrical quality 

 of the glass. A specimen of glass containing too much lead for a pure 

 silicate has not yet been experimented on, but the result of such an 

 experiment would be of great interest in furnishing evidence as to 

 whether purity of chemical composition and high electrical resistance 

 go together. 



When we turn to lime glass, however, we find that the density is 

 no guide to the electrical quality. Specimens having nearly the same 

 density vary enormously as to their electrical resistance. This, how- 

 ever, is to be expected, as the density may change but little, even 

 when the chemical composition is greatly altered. Lime glass gene- 

 rally contains both soda and potash, and the ratio of these two bases 

 may influence considerably the density, while the glass remaining a 

 good glass the electrical conductivity may not be much affected. 



So far as the results of chemical composition go, however, it appears 

 that in the case of lime glass also, a glass which would be pronounced 

 good from a chemical point of view is also relatively good from an 

 electrical point of view. On the other hand a glass which would be 

 pronounced bad chemically is also bad electrically. 



In the following tables the resistance at various temperatures of six 

 specimens of lime glass and two specimens of lead glass are given. 

 The first column contains the temperature, the second the resistance 

 in ohms of a cubic centimetre, the third the density, and the fourth 

 the chemical composition in those cases where it was determined. 



Specimen I. 



60° C 



100 



130 



160 



174 



(Bohemian glass tubing.) 



605 x 10 11 



20 x 10 n 

 20 x 10 10 

 24 x 10 9 

 87 x 10 8 



2-43 



37 c 

 59 

 73 

 101 

 131 



Specimen IT. (Test-tube.) 



229 x 10 10 

 306 xlO 9 

 612 x 10 8 

 56 x 10 8 

 62 x 107 



2 -458 



Specimen III. (Japanese lime glass tubing.) 



10° C. 



30 



52 



75 



85 



670 x 10 10 

 199 x 10 10 

 300 x 10 9 

 450 x 10 8 

 220 xlO 8 



2-55 



Silica 61-3 



Potash 22*9 



Lime, &c, by diff. 15 *8 



100 -o 



