transferring at once to the plating bath. In the case
where the copper plates require to be weighed they are dipped into
very hot distilled water after scratch-brushing, and then dried at
once by means of a clean glass cloth.

(4) Aluminium (which, however, does not readily lend itself to plating
operations [Footnote: This difficulty has now been overcome. See
note, section 138.] ) is best treated by alkali rubbed on with a cork,
or by a hot alkaline carbonate where rubbing is inexpedient. The
clean aluminium is scratch-brushed under water, and at once
transferred to the plating bath.

(5) Iron for Nickel-plating. According to Dr. Gore
(Electra-metallurgy, p. 319) the best bath for cleaning iron is made
as follows: "One gallon of water and one pound of sulphuric acid are
mixed with one or two ounces of zinc (which of course dissolves); to
this is added half a pound of nitric acid." The writer has been
accustomed to clean iron by mechanical means, to deprive it of grease
by caustic alkali, and to finish it off by, means of a hard scratch
brush. This process has always worked satisfactorily.

(6) Articles soldered with soft solder containing lead and tin do not
readily lend themselves to electrolytic processes, the solder
generally becoming black and refusing to be coated with the
electro-deposit. Moreover, if soldered articles are boiled for any
length of time in caustic alkali during the preliminary cleansing,
enough tin will dissolve to form a solution of stannate of potash or
soda--strong enough to deposit tin on brass or copper. A method of
coppering soldered articles will be described later on.

Sec. 130. Scratch-brushing.

This process is generally indispensable, and to its omission is to be
traced most laboratory failures in electroplating. Scratch-brushes
may be bought at those interesting shops where "watchmakers' supplies"
are sold. It will be well, therefore, to purchase a selection of
scratch brushes, for they are made to suit particular kinds of work.
They are all made of brass wire, and vary both in hardness and in the
fineness of the wire. The simplest kind of scratch brush consists
merely of a bundle of wires bound up tightly by another wire, and
somewhat "frizzed" out at the ends (Fig. 90). A more useful kind is
made just like a rotating brush, and has to be mounted on a lathe
(Fig. 91).

Fig. 90. Fig. 91.

The scratch brush is generally, if not al