hous varieties of sulphur is obtained. This
mixture is very much stronger and tougher than the purely crystalline
substance, and may be worked with ordinary hardwood tools into fairly
permanent plates, rods, etc. Sheets of pure thick filter paper may
also be dipped into sulphur at 170 deg. C, at which temperature air and
moisture are mostly expelled, and such sheets show a very considerable
insulating power. The sulphur does not penetrate the paper, which
therefore merely forms a nucleus.

Cakes of the crystalline or mixed varieties may be made by grinding up
some purified sulphur, moistening it with redistilled carbon
bisulphide, or toluene, or even benzene (C6H6), and pressing it in a
suitable mould under the hydraulic press. The plates thus formed are
porous, but are splendid insulators, especially if made from the
crystalline variety of sulphur, and they appear to keep their shape
very well, and do not crack with ordinary temperature changes.

The metals which resist the action of sulphur best are gold and
aluminium; while platinum and zinc are practically unacted upon at
temperatures below a red heat--in the former case,--and below the
boiling-point of sulphur in the latter.

A very convenient test of the purity of sulphur is the colour assumed
by it when suddenly cooled from the temperature at which it is
viscous. Quite pure sulphur remains of a pale lemon yellow under this
treatment, but the slightest trace of impurity, such as arises from
dust containing organic matter, stains the sulphur, and renders it
darker in colour.

Sec. 103. Fused Quartz.

This is on the whole the most reliable and most perfect insulator for
general purposes. No exact numerical data have been obtained, but the
resistivity must certainly be of the same order as that of pure
sulphur at its best. The influence of the moisture of the air also
reaches its minimum in the case of quartz, as was originally observed
by Boys.

As yet, however, the material can only be obtained in the form of rods
or threads. For most purposes rods of about one-eighth of an inch in
diameter are the most convenient. These rods may be used as
insulating supports, and succeed perfectly even if they interpose less
than an inch of their length to electrical conduction. The sketch
(Figs. 81 and 81A) shows (to a scale of about one-quarter full size)
a complete outfit for elementary electrostatic experiments, such as
has been in use in the writer's lab