l. The
last polishing is by the use of jewellers' rouge on the mould, now
lined with a fine textile.

_Filing Glass._--If a new file, thoroughly lubricated with a solution of
camphor in oil of turpentine, is used, there is but little difficulty in
filing the softer glasses. A slow movement of the file, without
excessive pressure but without allowing the file to slip, is desirable.
After a time the cutting edges of the file teeth will wear down and it
will be necessary to replace the file by another.

_Grinding Stoppers._--This is, perhaps, the most common form of grinding
that the laboratory worker will need to perform, and for that reason,
rather full details of the procedure are desirable.

A very crude form of ground-in stopper may be made by drawing out the
neck and the mass of glass which is intended to form the stopper to
approximately corresponding angles, wetting the surfaces with a mixture
of the abrasive material and water, and grinding the stopper in by hand.
Frequent lifting of the stopper is necessary during grinding, in order
to allow fresh supplies of abrasive material to reach the contacts. When
an approximate fit is obtained, the coarse abrasive should be washed
off, care being taken that the washing is complete, and a finer abrasive
substituted. After a while, this is replaced in its turn by a still
finer grinding material.

Such a method of grinding may give a satisfactory stoppering if the
angles of the plug and socket correspond very closely before grinding is
commenced; but if there is a wide difference in the original angles,
then no amount of grinding by this method will produce a good result.
The reason for this is that the plug will become so worn in the
preliminary grinding as to assume the form of a highly truncated cone;
the socket will assume a reverse form, and the end result will be a
loose-fitting plug and socket.

Satisfactory grinding may be carried out by the use of copper or
type-metal cones for the preliminary shaping. Such cones should be
mounted on a mandrel which will fit into the chuck of the American
hand-drill and turned on the lathe to the desirable angle for
stoppering. A number of these cones will be necessary. A number of
similar moulds, that is to say blocks of type-metal or hard lead in
which is a hole corresponding in size and angle to the plug desired,
should be made also. These must be rotated, either in the lathe or by
other means, and are used for the prelimin