process is a continuous one, and when once
started it is kept in operation for months or even years without
interruption.

It seems probable that the first product of combustion of the
carbon, at the point where the tuyers enter the furnace, is
carbon dioxide. This is at once reduced to carbon monoxide by
the intensely heated carbon present, so that no carbon dioxide
can be found at that point. For practical purposes, therefore,
we may consider that carbon monoxide is the first product of
combustion.

~Varieties of iron.~ The iron of commerce is never pure, but contains
varying amounts of other elements, such as carbon, silicon, phosphorus,
sulphur, and manganese. These elements may either be alloyed with the
iron or may be combined with it in the form of definite chemical
compounds. In some instances, as in the case of graphite, the mixture
may be merely mechanical.

The properties of iron are very much modified by the presence of these
elements and by the form of the combination between them and the iron;
the way in which the metal is treated during its preparation has also a
marked influence on its properties. Owing to these facts many kinds of
iron are recognized in commerce, the chief varieties being cast iron,
wrought iron, and steel.

~Cast iron.~ The product of the blast furnace, prepared as just described,
is called cast iron. It varies considerably in composition, usually
containing from 90 to 95% iron, the remainder being largely carbon and
silicon with smaller amounts of phosphorus and sulphur. When the melted
metal from the blast furnace is allowed to cool rapidly most of the
carbon remains in chemical combination with the iron, and the product is
called white cast iron. If the cooling goes on slowly, the carbon
partially separates as flakes of graphite which remain scattered through
the metal. This product is softer and darker in color and is called gray
cast iron.

~Properties of cast iron.~ Cast iron is hard, brittle, and rather easily
melted (melting point about 1100 deg.). It cannot be welded or forged into
shape, but is easily cast in sand molds. It is strong and rigid but not
elastic. It is used for making castings and in the manufacture of other
kinds of iron. Cast iron, which contains the metal manganese up to the
extent of 20%, together with about 3% carbon, is called spiegel iron;
when more than this amount of manganese is present the product is called
fe