Y. Electrostatic Capacity of Glass, II., and of Liquids. 
By J. Hopkinson, M.A., D.Sc., F.It.S. 
I. Electrostatic Capacity of Glass. —II. 
Received November 3,—Read December 16, 1880. 
In 1877* I had the honour of presenting to the Boyal Society the results of some 
determinations of the specific inductive capacity of glass, the results being obtained 
with comparatively low electromotive forces and periods of charge and discharge of 
sensible duration. 
In 1878 Mr. Gordon! presented to the Royal Society results of experiments, some 
of them upon precisely similar glasses, by a quite different method, with much greater 
electromotive forces and with very short times of charge and discharge. Mr. Gordon’s 
results and my own are compared in the following table :—- 
Gordon-. 
Hopkinson, 
Christmas, 
July and Aug., 
1877. 
1877. 
1879. 
Double extra-dense flint .... 
3T64 
3-838 
lo-l 
Extra-dense flint. 
3053 
3-621 
Light flint. 
3-013 
3-443 
6-85 
Hard crown. 
3-108 
3-310 
It is quite clear that such enormous differences cannot be due to mere errors of 
observation ; they must arise from a radical defect in one method or the other, or from 
some property of the material under investigation. I have now repeated my own 
experiments with greater battery power, and with a new key for effecting the con¬ 
nexions of the condensers, and have obtained substantially the same results as before. 
Two hypotheses suggest themselves as to the physical properties of glasses which 
might, if true, account for the diversity of results :—(i.) In my own earlier experiments 
a considerable time elapsed, during which some have thought residual charge might 
flow from the glass condenser and go to swell the capacity determined. Sir W. 
* Phil. Trans., 1878, Part I. 
f Phil. Trans., 1879, Part I. 
MDCCCLXXXI. 3 A 
