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 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