lts per centimetre, and the experiments seem to
indicate that the specific inductive capacity would be only slightly
less if referred to a period of charge indefinitely short.

I have found that the residual charge in a mica condenser, made
according to Carpentier's method (to be described below), is about 1
per cent of the original charge under the following circumstances.

Voltage 300 volts on a plate 0.2 mm. thick, duration of charge ten
minutes, temperature about 20 deg. C. To get this result the mica must be
most carefully dried. This and other facts indicate that the
so-called residual charge on ordinary condensers is, to a very large
extent, due to the creeping of the charge from the armatures over the
more or less conducting varnished surfaces of the mica, and its slow
return on discharge.

This source of residual charge was carefully guarded against by
Rowland and Nichols (Phil. Mag. 1881) in their work on quartz, and is
referred to by M. Bouty, who adduces some experiments to show that his
own results are not vitiated by it. On the other hand, M. Bouty shows
that a small rise in temperature enormously affects the state of a
mica surface, and that the surface gets changed in such a way as to
become very fairly conducting at 300 deg. C. Also anybody can easily try
for himself whether exposing a mica condenser plate which has been
examined in presence of phosphorus pentoxide to ordinary air for five
minutes will not enormously increase the residual charge, as has
always been the case in the writer's experience, and if so, it is open
to him to suggest some cause other than surface creeping as an
explanation.

M. Bouty, using less perfectly dried mica, did not get so good a
result as to smallness of residual charge as the one above quoted.

The chief use of mica for laboratory purposes depends on the ease with
which it can be split, and also upon the fact that it may be
considerably crumpled and bent without breaking. It therefore makes
an excellent dielectric in so far as convenience of construction is
concerned in the preparation of condensers, and lends itself freely to
the construction of insulating washers or separators of any kind. Its
success as a fair insulator at moderate temperatures has led to its
use in resistance thermometers, where it appears to have given
satisfaction up to, at all events, 400 deg. C.

It is worth a note that according to Werner Siemens, who had immense
experience (Wied