which purpose the sheets may be
suspended in a paraffined wood rack, so as to lie horizontally in the
silvering solution, a space of about half an inch being allowed
between the sheets. The silvering being finished, the sheets are
dipped along two parallel edges in 75 per cent nitric acid. With
regard to the third and fourth edges of the sheet, the silver is
removed on one side only, using a spun glass brush; if we agree to
call the two surfaces of the mica A and B respectively, and the two
edges in question C and D, then the silver is removed from the A side
along edge C, and from the B side along edge D. The silvered part is
shown shaded in Fig. 84. By this arrangement the silver and mica
plates may be built up together so as to form the same mutual
arrangement of contacts as in an ordinary mica tin-foil condenser.

Fig. 84.

It need hardly be said that the sheets require very complete washing
after treatment with nitric acid, followed by a varnishing of the
edges as already described in the case of glass, and baking at a
temperature of 140 deg. C. in air free from flame gases, till the shellac
begins to emit its characteristic odour and is absolutely hard when
cold.

The plates are then built up so as to connect the sheets which require
to be connected, and to insulate the other set. General contact is,
if necessary, secured by means of a little silver leaf looped across
from plate to plate--a part of the construction which requires
particular attention and clean hands, for it is by no means so easy to
make an unimpeachable contact as might at first appear.

The condenser, having been built up, may be clamped solid and placed
in its case; the capacity will not depend appreciably on the
tightness of the clamp screws--a great feature of the construction.
Such a condenser will not give its best results unless absolutely dry.
I have kept one very conveniently in a vacuum desiccator over
phosphorus pentoxide, but if of any size, the condenser deserves a box
to itself, and this must be air-tight and provided with a drying
reagent, so arranged that it can be removed through a manhole of some
sort.

Contact to the brass-work on the lid may be made by pressing spring
contacts tightly down upon the ends of the silver foils and carrying
the connections through the lid. This also serves to secure the
condenser in position.

Sec. 108. Micanite.

This substance, though probably comparing somewhat unfavourably