th sulphurous fuel must be avoided. Baths of molten alloys of
lead and tin are used when very exact temperatures are required, and
when articles have thick and thin parts adjacent. But the gas furnaces
have the same advantages in a more handy form. Quenching is done in
water, oil, or in various hardening mixtures, and sometimes in solids.
Rain water is the principal hardening agent, but various saline
compounds are often added to intensify its action. Water that has been
long in use is preferred to fresh. Water is generally used cold, but in
many cases it is warmed to about 80 deg. F., as for milling cutters and
taps, warmed water being less liable to crack the cutters than cold. Oil
is preferred to water for small springs, for guns and for many cutters.
Mercury hardens most intensely, because it does not evaporate, and so
does lead or wax for the same reason; water evaporates, and in the
spheroidal state, as steam, leaves contact with the steel. This is the
reason why long and large objects are moved vertically about in the
water during quenching, to bring them into contact with fresh cold
water.

There is a good deal of mystery affected by many of the hardeners, who
are very particular about the composition of their baths, various oils
and salts being used in an infinity of combinations. Many of these are
the result of long and successful experience, some are of the nature of
"fads." A change of bath may involve injury to the steel. The most
difficult articles to harden are springs, milling cutters, taps,
reamers. It would be easy to give scores of hardening compositions.

Hardening is performed the more efficiently the more rapidly the
quenching is done. In the case of thick objects, however, especially
milling cutters, there is risk of cracking, due to the difference of
temperature on the outside and in the central body of metal. Rapid
hardening is impracticable in such objects. This is the cause of the
distortion of long taps and reamers, and of their cracking, and explains
why their teeth are often protected with soft soap and other substances.

The presence of the body of heat in a tool is taken advantage of in the
work of tempering. The tool, say a chisel, is dipped, a length of 2 in.
or more being thus hardened and blackened. It is then removed, and a
small area rubbed rapidly with a bit of grindstone, observations being
made of the changing tints which gradually appear as the heat is
communicated from the ho