retort into
a receiver, the latter must be drawn down from a larger piece, few
beginners being able to widen a tube by the method explained with
sufficient ease and certainty. The other operations are similar to
the operations above described.

Sec. 43. Funnels often require to be ground to an angle of 60 deg.. For
this purpose it is well to keep a cast-iron cone, tapering from
nothing up to four inches in diameter. This may be mounted on a
lathe, and will be found of great use for grinding out the inside of
funnels. Care must be taken to work the funnel backwards and
forwards, or it will tend to grind so as to form rings, which
interfere with filtering. A rough polish may be given on the lines
explained in the next section.

Sec. 44. A rough polish may be easily given to a surface which has been
finished by washed flour emery, in the following manner. Turn up a
disc of soft wood on the lathe, and run it at the highest wood-turning
speed. Rub into the periphery a paste of sifted powdered pumice stone
and water.

Any fairly smooth ground glass surface may be more or less polished by
holding it for a moment against the revolving disc. Exact means of
polishing will be described later on. Meanwhile this simple method
will be found both quick and convenient, and is often quite sufficient
where transparency, rather than figure, is required. I daresay a fine
polish may be got on the same lines, using putty powder or washed
rouge (not jewellers' rouge, which is too soft, but glass-polishers'
rouge) to follow the pumice powder, but I have not required to try
this.

Sec. 45. It is sometimes required to give to ground glass surfaces a
temporary transparency. This is to be done by using a film of oil of
the same refractive index as the glass. Cornu has employed a varnish
consisting of a mixture of turpentine and oil of cloves, but the
yellow-brown colour of the latter is often a disadvantage. It will be
found that a mixture of nut oil and oil of bitter almonds, or of
bromo-napthalene and acetone, can be made of only a faint yellow
colour; and by exact adjustment of the proportions will have the same
refractive index for any ray as crown glass (ordinary window glass).

Procure a sample of the glass and smash it up to small fragments in an
iron mortar. Sift out the fine dust and the larger pieces; bits
about as large as small beads--say one-sixteenth inch every way--do
very well. Boil the sifted glass with