nds of carbon, hydrogen, and oxygen. The
composition of coal and the fuel gases has been given. Since these fuels
are composed principally of carbon and hydrogen or their compounds, the
chief products of combustion are carbon dioxide and water. The practice
of heating rooms with portable gas or oil stoves with no provision for
removing the products of combustion is to be condemned, since the carbon
dioxide is generated in sufficient quantities to render the air unfit
for breathing. Rooms so heated also become very damp from the large
amount of water vapor formed in the combustion, and which in cold
weather condenses on the window glass, causing the glass to "sweat."
Both coal and wood contain a certain amount of mineral substances which
constitute the ashes.

~The electric furnace.~ In recent years electric furnaces have come into
wide use in operations requiring a very high temperature. Temperatures
as high as 3500 deg. can be easily reached, whereas the hottest oxyhydrogen
flame is not much above 2000 deg.. These furnaces are constructed on one of
two general principles.

[Illustration: Fig. 67]

1. _Arc furnaces._ In the one type the source of heat is an electric arc
formed between carbon electrodes separated a little from each other, as
shown in Fig. 67. The substance to be heated is placed in a vessel,
usually a graphite crucible, just below the arc. The electrodes and
crucible are surrounded by materials which fuse with great difficulty,
such as magnesium oxide, the walls of the furnace being so shaped as to
reflect the heat downwards upon the contents of the crucible.

[Illustration: Fig. 68]

2. _Resistance furnaces._ In the other type of furnace the heat is
generated by the resistance offered to the current in its passage
through the furnace. In its simplest form it may be represented by Fig.
68. The furnace is merely a rectangular box built up of loose bricks.
The electrodes E, each consisting of a bundle of carbon rods, are
introduced through the sides of the furnace. The materials to be heated,
C, are filled into the furnace up to the electrodes, and a layer of
broken coke is arranged so as to extend from one electrode to the other.
More of the charge is then placed on top of the coke. In passing through
the broken coke the electrical current encounters great resistance. This
generates great heat, and the charge surrounding the coke is brought to
a very high temperature. The advantage of this type of fu