are made of ordinary sheet window glass, but as the insulating
property of this material appears to vary, it is generally necessary
to clean, dry, and test a sheet before using it. With regard to hard
Bohemian glass, this is stated by Koeller (Wien Bericht) to insulate
ten times as well as the ordinary Thuringian soft soda glass.

On the whole the most satisfactory laboratory practice is to employ
good white flint glass. The only point requiring attention is the
preparation of the glass by cleaning and drying. Of course all grease
or visible dirt must be removed as described in an earlier chapter (Sec.
13), but this is only a beginning. The glass after being treated as
described and got into such a state as to its surface that clean water
no longer tends to dry off unequally, must be subjected to a further
scrub with bibulous paper and a clear solution of oleate of soda. The
glass is then to be well rinsed with distilled water and allowed to
drain on a sheet of filter paper.

A very common cause of failure lies in the contamination of the glass
with grease from the operator's fingers. Before setting out to clean
the glass the student will do well to wash his hands with soap and
water, then with dilute ammonia and finally with distilled water.

In the case of an electrometer jar which has become conducting but is
not perceptibly dirty, rubbing with a little oleate of soda and a silk
ribbon, followed, of course, by copious washing, does very well. If
there is any tin-foil on the jar, great care must be taken not to
allow the glass surface to become contaminated by the shellac varnish
or gum used to stick the tin-foil in position.

Finally, the glass should be dried by radiant heat and raised to a
temperature of 100 deg. C. at least, and kept at it for at least half an
hour. Before drying it is of course advisable to allow the water to
drain away as far as possible, and if the water is only the ordinary
distilled water of the laboratory, the glass is preferably wiped with
a clean bit of filter paper; any hairs which may be left upon the
glass will brush off easily when the glass is dry.

In order to obtain satisfactory results the glass must be placed in
dry air before it has appreciably cooled. This is easily done in the
case of electrometer jars, and so long as the air remains perfectly
dry through the action of sulphuric acid or phosphorus pentoxide, the
jar will insulate. The slightest whiff of ordinaril