nd may be finished with fine
sandpaper, then pumice dust and water, applied on felt. After
cleaning the pumice off by means of water and a rag, the final touch
may be given by means of vaseline, applied on cloth or on ebonite
shavings.

Sec. 106. Mica.

A great variety of minerals go under this name. Speaking generally,
the Russian micas coming into commerce are potash micas, and mica
purchased in England may be taken to be potash mica, especially if it
is in large sheets.

At ordinary temperatures "mica" of the kind found in commerce is an
excellent insulator. Schultze (Wied. Ann. vol. xxxvi. p. 655) comes
to the conclusion that both at high and at low temperatures mica (of
all kinds?) is a better insulator than white "mirror glass," the
composition of which is not stated. The experiments of the author
referred to were apparently left unfinished, and altogether too much
stress must not be laid on the results obtained, one of which was that
mica conducts electrolytically to some extent at high temperatures.

Bouty (Journal de Physique, 1890 [9], 288) and J. Curie (These de
Doctorat, Paris, 1888) agree in making the final conductivity of the
mica used in Carpentier's condensers exceedingly small--at all events
at ordinary temperatures. Bearing in mind that for such substances
the term specific resistance has no very definite meaning, M. Bouty
considers it is not less than 3.19 x 1028 E.M. units at ordinary
temperatures. M. Bouty gives a note or illustration of what such
numbers mean--a precaution not superfluous in cases where magnitudes
are denoted logarithmically. Referring to the value quoted, viz.
3.19 x 1028, M. Bouty says, "Ce serait la resistance d'une colonne de
mercure de 1mmq de section et de longueur telle que la lumiere se
propageant dans le vide, mettrait plus de 3000 ans A se transmettre
d'une extremite a I'autre de la colonne."

M. Bouty returns to the study of mica (muscovite) in the Journal de
Physique for 1892, p. 5, and there deals with the specific inductive
capacity, which for a very small period of charge he finds has the
value 8--an enormous value for such a good insulator, and one that it
would be desirable to verify by some totally distinct method. This
remark is enforced by the fact that M. Klemencic finds the number 6
for the same constant. The temperature coefficient of this constant
was too small for M. Bouty to determine. The electric intensity was
of the order of 100 vo