_A._--Rust joints are not now much used in engines of any kind, yet it is
necessary that the engineer should be acquainted with the manner of their
formation. One ounce of sal-ammoniac in powder is mingled with 18 ounces or
a pound of borings of cast iron, and a sufficiency of water is added to wet
the mixture thoroughly, which should be done some hours before it is wanted
for use. Some persons add about half an ounce of flowers of brimstone to
the above proportions, and a little sludge from the grindstone trough. This
cement is caulked into the joints with a caulking iron, about three
quarters of an inch wide and one quarter of an inch thick, and after the
caulking is finished the bolts of the joints may be tried to see if they
cannot be further tightened. The skin of the iron must, in all cases, be
broken where a rust joint is to be made; and, if the place be greasy, the
surface must be well rubbed over with nitric acid, and then washed with
water, till no grease remains. The oil about engines has a tendency to
damage rust joints by recovering the oxide. Coppersmiths staunch the edges
of their plates and rivets by means of a cement formed of pounded
quicklime, with serum of blood, or white of egg; and in copper boilers such
a substance may be useful in stopping the impalpable leaks which sometimes
occur, though Roman, cement appears to be nearly as effectual.

714. _Q._--Will you explain the method of case hardening the parts of
engines?

_A._--The most common plan for case hardening consists in the insertion of
the articles to be operated upon among horn or leather cuttings, hone dust,
or animal charcoal, in an iron box provided with a tight lid, which is then
put into a furnace for a period answerable to the depth of steel required.
In some cases the plan pursued by the gunsmiths may be employed with
convenience. The article is inserted in a sheet iron case amid bone dust,
often not burned; the lid of the box is tied on with wire, and the joint
luted with clay; the box is heated to redness as quickly as possible and
kept half an hour at a uniform heat: its contents are then suddenly
immersed in cold water. The more unwieldy portions of an engine may be case
hardened by prussiate of potash--a salt made from animal substances,
composed of two atoms of carbon and one of nitrogen, and which operates on
the same principle as the charcoal. The iron is heated in the fire to a
dull red heat, and the salt is either sprinkled