y damp air will,
however, enormously reduce the insulating power of the glass, so that
unvarnished glass surfaces must be kept for apparatus which is
practically air-tight.

For outside or imperfectly protected uses the glass does better when
varnished. It is a fact, however, that varnished glass is rarely if
ever so good as unvarnished glass at its best. Too much care cannot
be taken over the preparation of the varnish; French polish, or
carelessly made shellac varnish, is likely to do more harm than good.

The best orange shellac must be dissolved in good cold alcohol by
shaking the materials together in a bottle. The alcohol is made
sufficiently pure by starting with rectified spirit and digesting it
in a tin flask over quick-lime for several days, a reversed condenser
being attached. A large excess of lime must be employed, and this
leads to a considerable loss of alcohol, a misfortune which must be
put up with.

After, say, thirty hours' digestion, the alcohol may be distilled off
and employed to act on the shellac. In making varnish, time and
trouble are saved by making a good deal at one operation--a
Winchester full is a reasonable quantity. The bottle may be filled
three-quarters full of the shellac flakes and then filled up with
alcohol; this gives a solution of a convenient strength.

The solution, however, is by no means perfect, for the shellac
contains insoluble matter, and this must be got rid off.`' One way of
doing this is to filter the solution through the thick filtering paper
made by Schleicher and Schuell for the purpose, but the filtering is a
slow process, and hence requires to be conducted by a filter paper
held in a clip (not a funnel) under a bell jar to avoid evaporation.

Another and generally more convenient way in the laboratory is to
allow the muddy varnish to settle--a process requiring at least a
month--and to decant the clear solution off into another bottle,
where it is kept for use. The muddy residue works up with the next
lot of shellac and alcohol, which may be added at once for future use.

The glass to be varnished is warmed to a temperature of, say, 50 deg. C,
and the varnish put on with a lacquering brush; a thin uniform coat is
required. The glass is left to dry long enough for the shellac to get
nearly hard and to allow most of the alcohol to evaporate. It is then
heated before a fire, or even over a Bunsen, till the shellac softens
and begins to yield its frag