398 
Cast Steel. 
ones ; or in other words the mass becomes harder than 
it was before, hence also the whole force of a blow is borne 
by a comparatively small number of insulated particles, 
and these entirely giving way before a degree of percus- 
sion that might easily be sustained by the whole when 
combined, thus produce the quality of brittleness. 
Ii highly carburetted steel is made nearly as hot as it 
can bear without melting, and is then plunged in very cold 
water, it is apt to fly to pieces, and even if this does not 
take place, the metal is not applicable to any use in this state 
of extreme hardness, for the particles are placed so far a- 
sunder, that the whole has a strong tendency to become e| 
crumbly, and will not bear a fine, even, sharp edge. In 
the practice of the best manufacturers, the hardening heat 
even for files, which are the hardest of all steel instruments, 
is not greater than a red visible by day-light ; and all cut- 
ting and elastic instruments require to be much softer. 
The various degrees of hardness necessary for different ar- 
ticles are not however given, as might at first be supposed, 
by the simple process of hardening at the requisite tempe- 
rature, but by the compound method of first giving to > 
every article nearly a file hardness, and then, by the subse- 
quent process of tempering, reducing the hardness to the ; 
particular degree necessary for each article. 
Tempering consists in softening hardened steel by the 
application of a heat not greater than that which was em- 
ployed in hardening it ; for this purpose it is gradually 
heated more or less according to the temper required, and 
cooled again either gradually or rapidly, diis making no 
difference ; after which the steel is found to be softened or 
tempered exactly in proportion to the heat which it has 
undergone. While the steel is tempering, its surface dis- 
plays a succession of colours (supposed to arise from a 
commencing oxydation) in proportion as it becomes 
