and
the elements generally.

Zinc surfaces, after a brief exposure to the air, become coated with a
thin film of oxide--insoluble in water--which adheres tenaciously,
forming a protective coating to the underlying zinc. So long as the
zinc surface remains intact, the underlying metal is protected from
corrosive action, but a mechanical or other injury to the zinc coating
that exposes the metal beneath, in the presence of moisture causes a
very rapid corrosion to be started, the galvanic action being changed
from the zinc positive to zinc negative, and the iron, as the positive
element in the circuit, is corroded instead of the zinc. When
galvanized iron is immersed in a corrosive liquid, the zinc is
attacked in preference to the iron, provided both the exposed parts of
the iron and the protected parts are immersed in the liquid. The zinc
has not the same protective quality when the liquid is sprinkled over
the surface and remains in isolated drops. Sea air, being charged with
saline matters, is very destructive to galvanized surfaces, forming a
soluble chloride by its action. As zinc is one of the metals most
readily attacked by acids, ordinary galvanized iron is not suitable
for positions where it is to be much exposed to an atmosphere charged
with acids sent into the air by some manufactories, or to the
sulphuric acid fumes found in the products of combustion of rolling
mills, iron, glass, and gas works, etc., and yet we see engineers of
note covering-in important buildings with corrugated and other sheets
of iron, and using galvanized iron tie rods, angles, and other
constructive shapes in blind confidence of the protective power of the
zinc coating; also in supreme indifference as to the future
consequences and catastrophes that arise from their unexpected
failure. The comparative inertia of lead to the chemical action of
many acids has led to the contention that it should form as good, if
not a better, protection of iron than zinc, but in practice it is
found to be deficient as a protective coating against corrosion. A
piece of lead-coated iron placed in water will show decided evidences
of corrosion in twenty-four hours. This is to be attributed to the
porous nature of the coating, whether it is applied by the hot or wet
(acid) process. The lead does not bond to the plate as well as either
of the other metals--zinc, tin, copper, or any alloys of them. The
following table gives the increase in weight of different