IRON. 
The increase of dimensions acquired by 
steel in hardening is such, that in general 
such pieces of work as are finished soft 
will not fit their places when hardened. 
Rinman found, that bars of steel six inches 
long, six lines wide, and half an inch thick, 
were lengthened at least one line after 
hardening by a whitish red heat, which 
is about one seventieth of the linear dimen- 
sions; and supposing the dilatation to be 
proportional, Guyton Morveau computes 
the bulks to be as 48 to 49. But the cubes 
of 72 lines and 73 lines are in proportion 
more nearly as 47 to 50. Various kinds of 
steel at different hardness must no doubt 
greatly differ in this respect. The specific 
gravities, as given by Brisson, p. 366, art. 
Gravity (specific), afford a much less ratio. 
Rinman found by his experiments on two 
different kinds of fine cemented steel, the 
specific gravity of which was 7.991, that one 
after hardening gave only 7.553, and the 
other 7.708. These numbers agree suffi- 
ciently near with the experiment of the six- 
inch bar. Yet he once found Styrian steel 
augmented in density by hardening in the 
ratio of 7.822 to 7.782. Morveau found, 
with pieces of steel 28 lines long, that the 
increase of length by hardening was about 
the 350th part. 
The fineness of grain in hard steel, as 
exhibited in its fracture, is various, accord- 
ing to the quality of the metal, and the 
temper it has received. The harder the 
steel the coarser the grain. But in like 
circumstances, fine steel has the closest 
grain, and is ever the most uniform in its 
appearance. Workmen avail themselves 
much of this indication. In general a neat 
curve lined fracture, and even grey tex- 
ture, denote good steel; and the appear- 
ance of threads, cracks, or brilliant specks, 
denotes the contrary. But the manage- 
ment of the forging and other circumstances 
of manufacturing will modify these indica- 
tions ; and the steel that is good for some 
purposes may be less suited to fillers. 
The fluid into which ignited steel is 
plunged is of great consequence. All the 
facts seem reducible to these general con- 
clusions. The hardness will be greater, 
1. The hotter the steel is made, provided 
it be hot decomposed : 2. The more consi- 
derably its temperature is lowered in the 
cooling : 3. The shorter the time of cooling : 
and 4. The more favourable the fire or the 
cooling material may be to the steel-mak'ing 
process. But the most useful combination 
of hardness and tenacity will be at a me- 
dium temperature in each kind of steel. 
With regard to the first particular, little 
need be said, but that the decomposition 
of steel in heating will be prevented, and 
its surface somewhat improved, if it be 
bedded in charcoal, or the cementing com- 
pound, during the application of the heat. 
The second and third, namely the quantity 
and suddenness of cooling, require an at- 
tention to the doctrine of Caloric, as ex- 
plained under that article. The cooling 
will be more sudden and effectual the 
greater the quantity of heat absorbed in 
the same time. There are three circum- 
stances which favour this effect, namely, 
a very low temperature of the body to 
which the hot steel is applied; that it 
should be a good conductor of heat ; or that 
it shall assume either the fluid or elastic 
state, which always demand a supply of 
heat for their maintenance. Thus it is 
found, that steel is more effectually harden- 
ed in cold than in warm water, and at like 
temperatures more effectually in mercury 
than in water. It may also be remarked, 
that these two fluids cool the steel by 
different energies. The water is partly 
converted into vapour, which carries off the 
heat, and leaves the fluid much less altered 
in temperature than mercury, which acts by 
its conducting property. This last fluid, 
not having evaporated in the process, is 
found to have acquired a much more ele- 
vated temperature by the immersion. Oil 
is found to harden the surface of steel much 
more than its internal part, so that it resists 
the file, buf is much less easily broken by 
the hammer. This effect arises from its 
imperfect conducting quality, and the ele- 
vated temperature it demands to be con- 
verted into the vaporous state; to which 
we may also add, that a stratum of coal is 
formed round the steel from the burned 
oil, which still more effectually prevents 
the transmission of the heat. A remark- 
able instance, of this nature presented itself 
to our observation in hardening a small 
piece of steel two inches long, and a quar- 
ter of an inch diameter. At the time of 
ignition, the water nearest at hand had 
been used with soap. The steel made very 
little noise when plunged into the water, 
and remained hot for a considerable time ; 
but when taken out was found to be scarce- 
ly at all hardened. It was covered with 
coally matter ; which being cleared off, and 
the process repeated with clean water, it 
