thin adhesive
layer of the latter metal. Such tinned plate commonly known as "tin" has
become the favorite material for pans and cans. But if the tin is
scratched the iron beneath rusts more rapidly than if the tin were not
there, for an electrolytic action is set up and the iron, being the
negative element of the couple, suffers at the expense of the tin.

With zinc it is quite the opposite. Zinc is negative toward iron, so
when the two are in contact and exposed to the weather the zinc is
oxidized first. A zinc plating affords the protection of a Swiss Guard,
it holds out as long as possible and when broken it perishes to the last
atom before it lets the oxygen get at the iron. The zinc may be applied
in four different ways. (1) It may be deposited by electrolysis as in
nickel plating, but the zinc coating is more apt to be porous. (2) The
sheets or articles may be dipped in a bath of melted zinc. This gives us
the familiar "galvanized iron," the most useful and when well done the
most effective of rust preventives. Besides these older methods of
applying zinc there are now two new ones. (3) One is the Schoop process
by which a wire of zinc or other metal is fed into an oxy-hydrogen air
blast of such heat and power that it is projected as a spray of minute
drops with the speed of bullets and any object subjected to the
bombardment of this metallic mist receives a coating as thick as
desired. The zinc spray is so fine and cool that it may be received on
cloth, lace, or the bare hand. The Schoop metallizing process has
recently been improved by the use of the electric current instead of the
blowpipe for melting the metal. Two zinc wires connected with any
electric system, preferably the direct, are fed into the "pistol." Where
the wires meet an electric arc is set up and the melted zinc is sprayed
out by a jet of compressed air. (4) In the Sherardizing process the
articles are put into a tight drum with zinc dust and heated to 800 deg. F.
The zinc at this temperature attacks the iron and forms a series of
alloys ranging from pure zinc on the top to pure iron at the bottom of
the coating. Even if this cracks in part the iron is more or less
protected from corrosion so long as any zinc remains. Aluminum is used
similarly in the calorizing process for coating iron, copper or brass.
First a surface alloy is formed by heating the metal with aluminum
powder. Then the temperature is raised to a high degree so as to cause
the