nd
hard; but it is expedient also to make them tough, so as to approach as
nearly as possible to the state of malleable iron. This may be done by
mixing in the furnace as many different kinds of iron as possible; and it
may be set down as a general rule in iron founding, that the greater the
number of the kinds of metal entering into the composition of any casting,
the denser and tougher it will be. The constituent atoms of the different
kinds of iron appear to be of different sizes, and the mixture of different
kinds maintains the toughness, while it adds to the density and cohesive
power. Hot blast iron was at one time generally believed to be weaker than
cold blast iron, but it is now questioned whether it is not the stronger of
the two. The cohesive strength of unmixed iron is not in proportion to its
specific gravity, and its elasticity and power to resist shocks appear to
become greater as the specific gravity becomes less. Nos. 3 and 4 are the
strongest irons. In most cases, iron melted in a cupola is not so strong as
when remelted in an air furnace, and when run into green sand it is not
reckoned so strong as when run into dry sand, or loam. The quality of the
fuel, and even the state of the weather, exerts an influence on the quality
of the iron: smelting furnaces, on the cold blast principle, have long been
known to yield better iron in winter than in summer, probably from the
existence of less moisture in the air; and it would probably be found to
accomplish an improvement in the quality of the iron if the blast were made
to pass through a vessel containing muriate of lime, by which the moisture
of the air would be extracted. The expense of such a preparation would not
be considerable, as, by subsequent evaporation, the salt might be used over
and over again for the same purpose.

701. _Q._--Will you explain the process of casting cylinders?

_A._--The mould into which the metal is poured is built up of bricks and
loam, the loam being clay and sand ground together in a mill, with the
addition of a little horse-dung to give it a fibrous structure and prevent
cracks. The loam board, by which the circle of the cylinder is to be swept,
is attached to an upright iron bar, at the distance of the radius of the
cylinder, and a cylindrical shell of brick is built up, which is plastered
on the inside with loam, and made quite smooth by traversing the
perpendicular loam board round it. A core is then formed in a similar