upon it or rubbed on in a
lump, or the iron is rubbed in the salt in powder. The iron is then
returned to the fire for a few minutes, and finally immersed in water. By
some persons the salt is supposed to act unequally, as if there were greasy
spots upon the iron which the salt refused to touch, and the effect under
any circumstances is exceedingly superficial; nevertheless, upon all parts
not exposed to wear, a sufficient coating of steel may be obtained by this
process.

715. _Q._--What kind of iron is most suitable for the working parts of an
engine?

_A._--In the malleable iron work of engines scrap iron has long been used,
and considered preferable to other kinds; but if the parts are to be case
hardened, as is now the usual practice, the use of scrap iron is to be
reprehended, as it is almost sure to make the parts twist in the case
hardening process. In case hardening, iron absorbs carbon, which causes it
to swell; and as some kinds of iron have a greater capacity for carbon than
other kinds, in case hardening they will swell more, and any such unequal
enlargement in the constituent portions of a piece of iron will cause it to
change its figure. In some cases, case hardening has caused such a twisting
of the parts of an engine, that they could not afterward be fitted
together; it is preferable, therefore, to make such parts as are to be case
hardened to any considerable depth of Lowmoor, Bowling, or Indian iron,
which being homogeneous will absorb carbon equally, and will not twist.

716. _Q._--What is the composition of the brass used for engine bearings?

_A._--The brass bearings of an engine are composed principally of copper
and tin. A very good brass for steam engine bearings consists of old copper
112 lbs., tin 12-1/2 lbs., zinc 2 or 3 oz.; and if new tile copper be used,
there should be 13 lbs. of tin instead of 12-1/2 lbs. A tough brass for
engine work consists of 1-1/2 lb. tin, 1-1/2 lb. zinc, and 10 lbs. copper;
a brass for heavy bearings, 2-1/2 oz. tin, 1/2 oz. zinc, and 1 lb. copper.
There is a great difference in the length of time brasses wear, as made by
different manufacturers; but the difference arises as much from a different
quantity of surface, as from a varying composition of the metal. Brasses
should always be made strong and thick, as when thin they collapse upon the
bearing and increase the friction and the wear.

717. _Q._--How is Babbitt's metal for lining the bushes of machinery
co