Cast SteeL 
397 
being heated are placed at a greater distance from each 
other than before, and in proportion as this heat is again 
abstracted, the attraction subsisting between them will 
become efficacious, and they will approach nearer to ac- 
tual contact ; but the impetus with which this takes place 
will be in proportion to the difference of temperature, and 
therefore when red-hot steel is plunged in ice-cold mercu- 
ry, the force or resilient spring of its particles will be 
greater than if mercury at 200° had been made use of, 
and consequently its hardness will also be greater. But 
this theory however ingenious is opposed by certain facts 
which perhaps may be found more consonant with the 
following explanation of them. 
If we take the specific gravity of a piece of steel both 
when hardened, and after it has been softened by heating 
again and gradually cooling, we shall find that its bulk 
in the former case is greater than in the latter ; whereas if 
the hardness of steel was owing to the rapidity and energy 
with which its particles collapsed on cooling, directly the 
reverse of this ought to take place, the state of greatest 
hardness should be that of the greatest specific gravity. So 
in like manner we find to be the case with glass ; if a little 
of this in a melted state is dropped into cold water it will 
prove very hard and brittle ; but if the same piece is again 
heated red (without however in any degree softening it) 
and afterwards allowed to cool gradually, its specific gra 
vity will have very notably increased, and it will have be- 
come tough and elastic. We may therefore consider the 
hardening of steel to be caused by the contemporaneous 
expulsion of part of its heat and the fixation of its parti- 
cles before they have had time to arrange themselves and 
contract Upon each other. Hence on the impression of 
any external force, the particles that are struck are not 
able to slide on each others surfaces, and thus distribute 
the impetus which they have received over the contiguous 
