RESIDUAL CHARGE OF THE LEYDEN JAR. 
615 
same phenomenon, viz. an after-effect, as regards electric displacement, of electromotive 
force. The experiments appear to show, though very roughly, that the principle of 
Ohm’s law is applicable to the whole phenomenon of conduction through glass. 
V. Effect of Temperature. 
The purpose of the previous experiments being to examine generally the applicability 
of the formulae and to compare the values of \{/(£) for different glasses of known compo- 
sition, no account was taken of temperature, and no attempt made to maintain it 
constant, although it is well known that changes of temperature greatly affect both 
conduction and polarization in glass*. It appeared, however, desirable to compare the 
same glass at different temperatures in the same manner as different glasses at the same 
temperature. 
The flask, carefully filled with sulphuric acid as before, was placed in an earthenware 
jar containing sulphuric acid, which was in its turn placed in a double cylindrical shell 
of copper, with oil or water between the cylinders. The jar was covered by two disks 
of wood, through holes in the centre of which the neck of the flask projected. A 
cap of sealing-wax, carrying a small cup of mercury for making electrical connexions 
with the interior, closed the flask. A thermometer dipped into the acid outside the 
flask for reading the temperature of the glass, whilst a second thermometer was inserted 
between the cylinders in the oil or water to help the observer in regulating the tempe- 
rature by means of a spirit-lamp. In the two experiments below freezing-point the 
earthenware jar was removed from the oil-bath and placed in a freezing-mixture of 
hydrochloric acid and sulphate of soda. In all cases the temperature was maintained 
approximately constant for some time before observing. It will be remarked that, as 
the acid was not stirred, the temperature-readings are subject to a greater probable 
error than that due to the thermometer itself. But as the changes of temperature of 
the acid were always very slow, the error thus introduced cannot seriously affect the 
results. All temperatures are Centigrade. The actual readings are given, and also 
the temperature, roughly corrected when necessary, for the exposed portion of the stem 
of the thermometer. The times in these and in most of the previous experiments were 
taken by ear from a dead-beat seconds clock, the eye being fixed on the image and the 
scale. In the intervals between the short insulations to determine the flask was 
either connected with the battery or discharged. In all cases the registered time of 
observation is taken at the middle of the period of insulation ; thus, in the experiment 
at 39^° below, insulation was made one second before the minute, and the reading one- 
second after. Two glasses were examined, Nos. 2 and 7, selected as extreme cases. The 
whole of the observations made are given, excepting three manifestly in error, although 
* Vide Mr. Perry, Proceedings of the Royal Society, 1875, p. 468 ; Prof. Clerk Maxwell, “ Electricity and 
Magnetism,” Art. 271. 
MDCCCLXXVII. 4 S 
