though
this prevents the glass from being so much strained, it reduces the
adherence of the solder. It is a comfort to be able to solder
aluminium after working for so many years by way of electroplating, or
filing under solder. An alternative method of soldering aluminium
will be described when the electroplating of aluminium is discussed, Sec.
138.

Gilding Glass. In looking over some volumes of the Journal fuer
praktische Chemie, I came across a method of gilding glass due to
Boettger (Journ. f. prakt. Chem. 103, p. 414). After many trials
I believe I am in a position to give definite instructions as to the
best way of carrying out this rather troublesome operation. The films
of gold obtained by the process are very thick, and the appearance of
the gold exceedingly fine. The difficulty lies in the exact
apportionment of the reducing solution. If too much of the reducing
solution be added, the gold deposits in a fine mud, and no coating is
obtained. If, on the other hand, too little of the reducing solution
be added, little or no gold is deposited. The secret of success turns
on exactly hitting the proper proportions.

The reducing solution consists of a mixture of aldehyde and glucose,
and the difficulty I have had in following Boettger's instructions
arose from his specifying "commercial aldehyde" of a certain specific
gravity which it was impossible to reproduce. I did not wish to
specify pure aldehyde, which is not very easily got or stored, and
consequently I have had to determine a criterion as to when the
proportion of reducing solution is properly adjusted.

The aldehyde is best made as required. I employed the ordinary
process as described in Thorpe's Dictionary of Applied Chemistry, by
distilling alcohol, water, sulphuric acid, and manganese dioxide
together. The crude product is mixed with a large quantity of calcium
chloride (dry--not fused), and is rectified once. The process is
stopped when the specific gravity of the product reaches 0.832 at 60 deg.
F. The specific gravity of pure aldehyde is 0.79 nearly.

The following is a modification of Boettger's formula:-

Solution I

1 gram of pure gold is converted into chloride--got acid free--i.e.
to the state represented by AuCl3, and dissolved in 120 cc. of water.

This solution is the equivalent of one containing 6.5 grains of
trichloride to the ounce of water.

Solution II.

6 grams sodium hydrate.

100 grams wat