188 
ELECTRO-METALLURGY 
-A. L TJ UVE I IsT I TJ JS/L . 
BY 
OAPITAINE D’ARTILLERIE J. ROUSSEAU. 
Precis of a Paper published in the “ Revue d' Artillerie 
BY 
F. E. B. L., late R.A. 
I.—PRINCIPAL APPLICATIONS OF ELECTRICITY TO METALLUR¬ 
GICAL OPERATIONS. 
WORKING OF METALS HOT. 
The heating of a considerable mass of metal is an operation not free from 
difficulty, and which should be carried out with care : the injurious effect of 
molecular tensions which would be enhanced by rapid or intense action must be 
guarded against. The phenomena which occur within the mass of a metallic 
block submitted to heat are not purely physical; it has been recently discovered, 
in raising the temperature to the point of fusion, that there are a certain number 
of stages characteristic of each metal, which are real points of chemical change. 
When the temperature of the metal passes these critical points the molecular 
arrangement is modified, and we find produced by heat alone, and apart from the 
influence of the surrounding medium, internal disturbances, dissociations, or re¬ 
combinations. These facts recall the allotropic modifications of sulphur and 
phosphorus under the action of heat. Simple metals, like aluminium, appear to 
have but one critical point. The malleability of zinc, for instance, is at its 
maximum at 200° cent. (392° Fahr.), but in more complex metals, such as cast 
and wrought-iron, steel, and bronze, there are several critical points, and each 
represents a particular modification : thus one of the three critical temperatures 
of iron is 750° cent. (1392° Fahr.) which corresponds to the disappearance of 
magnetism. In dealing with heat therefore, according as we obtain these critical 
points with more or less rapidity, the mechanical properties of the metal in its 
final state vary in a sensible degree. The operations of annealing, hardening, 
and forging should accordingly be based on a knowledge of the critical tempera¬ 
tures characteristic of each metal. 
4, VOL. xxi. 
