THE HARDENING AND TEMPERING OF STEEL. 
253 
Committee have boldly struck out an idea of their own. This 
has been suggested by Edison’s experiments on platinum wire, 
communicated to the American Association for the Advance- 
ment of Science at Saratoga in 1879. These experiments showed 
that incandescent platinum wire became covered with minute 
fissures, due to the expiration of the occluded gases under the 
action of the heat : and that when the wire was allowed to cool 
in vacuo , these cracks closed up again and disappeared. By a 
succession of heatings and coolings in vacuo , the whole of the 
occluded gases were expelled, and the metal was then greatly 
altered in character, becoming much more dense and hard, and 
remaining perfectly rigid under the most intense incandescence. 
The Committee suggest that the same action may take place in 
steel ; that the heating of the metal expels the gases which 
remain occluded at ordinary temperatures, and that the sudden 
contraction in rapid cooling prevents their re-absorption (perhaps 
actually assists their expulsion) ; the particles of the metal are 
thus brought closer together, and their force of cohesion is 
increased. When the metal is gently heated, as in tempering, 
re-absorption begins ; and the characteristic colours are due to 
changes in the surface ( e.g ., the gradual opening of minute 
fissures) which are produced by this re-absorption. 
We do not propose at present to criticize this theory. 
The Committee suggest the carrying out of a series of experi- 
ments, with the special view of testing its truth. But since 
the Report was published, the subject has been discussed at a 
General Meeting of the Institution, and Prof. D. E. Hughes 
then gave it as his opinion, supported by recent researches, 
that soft steel was a mere coarse mixture of iron and carbon, 
while in hardened steel part of the carbon at least was in the 
form of an actual alloy with the iron, and that the qualities of 
the steel depend upon the constitution of this alloy. This was 
illustrated by the fact that hard steel was readily attacked by 
dilute sulphuric acid, while soft steel was not, which he 
accounted for by supposing that in the former case the com- 
bination was so intimate, that local galvanic circuits were set 
up, each molecule forming a minute carbon and iron battery. 
It is to be hoped that this theory, taken in conjunction with 
the opposing theory of occlusion, will be fully and carefully 
investigated, especially as such an investigation, whether it 
confirms either theory or not, can hardly fail to throw con- 
siderable light upon the very interesting phenomena we have 
here attempted to describe. 
