614 
MR. J. HOPKINSON ON THE 
possessing small insulating power, but exhibit very great return charge. The values of 
the function — B for the three glasses agree almost within the limits of these 
roughly approximate experiments. 
At the opposite extreme are the flints or potash-lead glasses, which have great specific 
resistance. The experiment does not prove that No. 7 conducts electricity at all ; for it 
is not certain that the very slight loss of charge may not be due to conduction over the 
surface of the glass ; but it is certainly not less than 100,000 times as resistant as No. 1. 
The flints also have very similar values of \}/(£) — B, much smaller than the soda-lime 
glasses. 
IV. It is known that glass at a moderately high temperature conducts electricity 
electrolytically. The following experiment shows that with the more conductive glasses 
electrolytic conduction occurs at the ordinary temperature of the air. 
A flask of blue glass, No. 2, was very carefully insulated with strong sulphuric acid 
within the flask, and was placed in a vessel of caustic potash. Platinum wires dipping 
in the two liquids communicated with the quadrants of the electrometer. On insulation 
the acid developed a positive charge as follows : — 
In 
1 
2 
minute 
15 
divisions of the scale. 
55 
1 
55 
55 
55 55 
55 
2 
minutes 33| 
55 
55 v 5 5 
55 
5 
55 
47 
55 
55. 55 
55 
10 
55 
55 
„ 
55 55 
55 
15 
55 
57 
55 
55 55 
one Daniell’s element giving 68 divisions of the scale. 
The experiment was repeated after the flask had stood some days with the two liquids 
connected by a platinum wire ; the potential developed much more slowly, and in 
50 minutes was stationary at 38^- divisions. 
Summary . — These experiments are subject to many causes of error. Deducing ^ 
from an observation of dx t in a period of many seconds or even minutes gives values of 
•vf/(£) — B necessarily too low, in some cases very much too low. No attempt was made 
to keep the glass at a constant temperature ; the temperature of the room was occa- 
sionally noted, but is not given here, as no conclusion is based upon it. The experi- 
ments were performed irregularly at such times as other circumstances permitted. It 
will be observed that the discords of the experiments of verification are considerable, 
but they are irregular. It may, perhaps, be assumed that they are within the limits of 
error, and we may infer that the fundamental hypothesis is verified, viz. that the effects 
on a dielectric of past and present electromotive forces are superposable. Ohm’s law 
asserts the principle of superposition in bodies in which conduction is not complicated 
by residual charge. Conduction and residual charge may be treated as parts of the 
