er.

Solution III.

0.2 grams glucose (bought as pure).

12.6 cubic centimetres 95 per cent alcohol.

12.6 cubic centimetres water.

2.0 cubic centimetres aldehyde, sp. gr. 0.832.

To gild glass these solutions are used in the following proportions by
volume:-

16 parts of No. I.

4 parts of No. II.

0.8 parts of No. III.

The glass is first cleaned well with acid and washed with water: it
is then rinsed with Solution No. III. If it is desired to gild the
inside of a glass vessel, Solution No. III. may be placed in the
vessel first, and the walls of the vessel rinsed round carefully.
Solutions I. and II. are mixed separately and then added
to III.--after about two minutes the whole is well shaken up.

If it be desired to gild a mirror of glass, the glass-plate is
suspended face downwards in a dish of the mixed solutions--care being
taken to rinse the glass with Solution III. first.

If the mixture darkens in from 7' to 10' in diffuse daylight and at
60 deg.F. it will gild well, and it generally pays to make a few trials
in a test tube to arrive at this. If too much reducing solution is
present the liquid will get dark more rapidly, and vice versa. The
gilding will require several hours--as much as twelve hours may be
needed.

The reaction is one of great chemical interest, being one of that
class of reactions which is greatly affected by capillarity. Thus it
occasionally happens that when the reducing solution is not in the
right proportion, gold will be deposited at the surface of the liquid
(so as to form a gilt ring on the inside of a test tube), the
remainder of the gold going down as mud. The gold deposited is at
first transparent to transmitted light and is deeply blue. I thought
this might be due to a trace of copper or silver, but on carefully
purifying the gold no change of colour was noted. If the reducing
solution is present in slightly greater proportions than that given in
the formula, the gold comes down with a richer colour, and has a
tendency to form a mat surface and to separate from the glass. The
gold which is deposited more slowly has a less rich colour but a
brighter surface. The operation should be interrupted when a
sufficient deposit has been obtained, because it is found that the
thicker the deposit, the more lightly is it held to the glass surface.

Sec. 75. The Use of the Diamond-cutting Wheel.

A matter which is n