to the object, and a certain amount of
pressure is necessary. The mark produced is a thin scratch which,
although fairly definite, lacks breadth, and this is a disadvantage
where the marking has to be read at a distance. This disadvantage may to
some extent be overcome by making a number of parallel scratches.

_The Abrasive Pencil._--A rod of carborundum composition may be ground
or filed to a point, and this forms a very useful pencil for general
work. The marking produced is rather less definite than that produced by
a writing diamond, but has the advantage of being broader.

_The Cutting Wheel._--"Cutting" in this case is scarcely the ideal
expression, it should rather be "grinding," but "cutting" is more
commonly used. Exceedingly good graduations may be made by the edge of a
small, thin, abrasive wheel which is mounted on the end of a small
mandrel and driven by a flexible shaft from an electric motor or any
other convenient source of power. The depth of the mark can be
controlled, and very light pressure will suffice.

_Etching._--This is often the quickest and easiest way of marking glass
apparatus. The object to be marked should first be warmed and coated
very thoroughly with a thin film of paraffin wax. When cold, the marking
is made through the paraffin wax by means of a needle point, and the
object is then exposed to the action of hydrofluoric acid. If a shallow
but clearly visible marking is desired, it is well to use the vapour of
the acid; this may be done by bending up a sheet-lead trough on which
the object can rest with the marked surface downwards. A little of the
commercial hydrofluoric acid, or a mixture of a fluoride and sulphuric
acid, is distributed over the bottom of the trough, and the whole
arrangement is allowed to stand for about an hour. The object is washed
thoroughly and the paraffin wax removed, either by melting and wiping
off or by the use of a solvent, and the marking is finished.

If a deep marking is desired, in order that it may afterwards be filled
with some pigment, a better result is obtained by the use of liquid
commercial hydrofluoric acid, which is a solution of hydrogen fluoride
in water. The acid is mopped on to the object after the markings have
been made on the paraffin wax film, and allowed to remain in contact for
a few minutes. It is advantageous to repeat the mopping-on process at
intervals during the etching.

In all cases where hydrofluoric acid is used, or sto