n out of the solution than could possibly be dissolved from the
anodes by the chemicals set free on account of this insoluble scale or
furring up of the anodes, which sometimes reaches one-eighth of an
inch in thickness. To all intents and purposes the deposits obtained
from acid solutions under favourable circumstances are fairly adhesive
when great care has been exercised to thoroughly scale and clean the
surface to be coated, which is found to be the principal difficulty in
the application of any electro-chemical process for copper, lead, or
tin, as well as for zinc, and that renders even the application of
paint or other brush compounds to futile unless honestly complied
with. Unfortunately these acid zinc coatings are of a transitory
nature, Their durability being incomparable with hot galvanizing, as
the deposit is porous and retains some of the acid salts, which cause
a wasting of the zinc, and consequently the rusting of the iron or
steel. Castings coated with acid zinc rust comparatively quickly, even
when the porosity has been reduced by oxidation, aggravated no doubt
by some of the corroding agents--sal-ammoniac, for instance--being
forced into the pores of the metal. Other matters of serious moment in
the electro-zincing process, apart from the slowness of the operation,
were the uncertain nature, thickness, and extent of the coating on
articles of irregular shape, and the formation of loose, dark-coloured
patches on the work; the unhealthy and non-metallic look and want of
brilliancy and the lustre prevented engineers and the trade from
accepting the process or its results, except for the commoner articles
of use. To obviate any tendency of the paint to peel off from the zinc
surface, as it generally manifests a disposition to do, it is
recommended to coat all the zinc surfaces, previous to painting them,
with the following compound: 1 part chloride of copper, 1 part nitrate
of copper, 1 part sal-ammoniac, dissolved in 61 parts of water, and
then add 1 part commercial hydrochloric acid. When the zinc is brushed
over with this mixture it oxidizes the surface, turns black, and dries
in from twelve to twenty-four hours, and may then be painted over
without any danger of peeling. Another and more quickly applied
coating consists of, bi-chloride of platinum, 1 part dissolved in 10
parts of distilled water, and applied either by a brush or sponge. It
oxidizes at once, turns black, and resists the weak acids, rain,