aluminum on the surface to diffuse into the metal and afterwards it
is again baked in contact with aluminum dust which puts upon it a
protective plating of the pure aluminum which does not oxidize.

[Illustration: PHOTOMICROGRAPHS SHOWING THE STRUCTURE OF STEEL MADE BY
PROFESSOR E.G. MARTIN OF PURDUE UNIVERSITY

1. Cold-worked steel showing ferrite and sorbite (enlarged 500 times)

2. Steel showing pearlite crystals (enlarged 500 times)

3. Structure characteristic of air-cooled steel (enlarged 50 times)

4. The triangular structure characteristic of cast steel showing ferrite
and pearlite (enlarged 50 times)]

[Illustration: Courtesy of E.G. Mahin

THE MICROSCOPIC STRUCTURE OF METALS

1. Malleabilized casting; temper carbon in ferrite (enlarged 50 times)

2. Type metal; lead-antimony alloy in matrix of lead (enlarged 100
times)

3. Gray cast iron; carbon as graphite (enlarged 500 times)

4. Steel composed of cementite (white) and pearlite (black) (enlarged 50
times)]

Another way of protecting iron ware from rusting is to rust it. This is
a sort of prophylactic method like that adopted by modern medicine where
inoculation with a mild culture prevents a serious attack of the
disease. The action of air and water on iron forms a series of compounds
and mixtures of them. Those that contain least oxygen are hard, black
and magnetic like iron itself. Those that have most oxygen are red and
yellow powders. By putting on a tight coating of the black oxide we can
prevent or hinder the oxidation from going on into the pulverulent
stage. This is done in several ways. In the Bower-Barff process the
articles to be treated are put into a closed retort and a current of
superheated steam passed through for twenty minutes followed by a
current of producer gas (carbon monoxide), to reduce any higher oxides
that may have been formed. In the Gesner process a current of gasoline
vapor is used as the reducing agent. The blueing of watch hands, buckles
and the like may be done by dipping them into an oxidizing bath such as
melted saltpeter. But in order to afford complete protection the layer
of black oxide must be thickened by repeating the process which adds to
the time and expense. This causes a slight enlargement and the high
temperature often warps the ware so it is not suitable for nicely
adjusted parts of machinery and of course tools would lose their temper
by the heat.

A new method of rust proofing which is free from