plated are pickled in dilute sulphuric
acid, and polished with fine sand and a scratch-brush, rinsed in
water, loosely wound round with zinc wire or tape, and immersed in the
bath for ten or fifteen minutes at ordinary temperatures. The coating
is finished with the scratch-brush and whiting. By this process
cast-or wrought-iron, steel, copper, brass, and lead can be tinned
without a separate battery. The only disadvantage of the process is
that the bath soon becomes clogged up with zinc chloride, and the tin
salt must be frequently removed. In Hern's process a bath composed
of--

Tartaric acid 2 oz.
Water 100 "
Soda 3 "
Protochloride of tin 3 "

is employed instead of the preceding. It requires a somewhat longer
exposure to properly tin articles in this than in Weigler's bath.
Either of these baths may be used with a separate battery.

SECTION VII.

GALVANIZING.

Galvanizing, as a protecting surface for large articles, such as enter
into the construction of bridges, roofs, and shipwork, has not quite
reached the point of appreciation that possibly the near future may
award to it. Certain fallacies existed for a long time as to the
relative merits of the dry or molten and the wet or electrolytical
methods of galvanizing. The latter was found to be costly and slow,
and the results obtained were erratic and not satisfactory, and soon
gave place to the dry or molten bath process, as in practice at the
present day; but the difficulty of management in connexion with large
baths of molten material, and the deterioration of the bath, and other
mechanical causes, limit the process to articles of comparatively
small size and weight. The electro deposition of zinc has been subject
to many patents, and the efforts to introduce it have been lamentable
in both a mechanical and financial sense. Most authorities recommend a
current density of 18 or 20 amperes per square foot of cathode
surface, and aqueous solutions of zinc sulphate, acetate or chloride,
ammonia, chloride or tartrate, as being the most suitable for
deposition. Electrolytes made by adding caustic potash or soda to a
suitable zinc salt have been found to be unworkable in practice on
account of the formation of an insoluble zinc oxide on the surface of
the anode and the resultant increased electrical resistance; the
electrolytes are also constantly getting out of order, as more metal
is take