rough certain materials, like brick
walls, tile, stone, and also iron; this is _conductive_ heat, as
afforded by stoves, etc.

Or, the heat is _conveyed_ by means of air, water, or steam from one
place to another, as in the hot-water, hot-air, and steam systems of
heating; this we call _convected_ heat.

There is no strict line of demarcation differentiating the three
methods of heating, as it is possible that a radiant heat may at the
same time be conductive as well as convective--as is the case in the
Galton fireplace, etc.

=Materials of Combustion.=--The materials of combustion are air, wood,
coal, oil, and gas. Air is indispensable, for, without oxygen, there
can be no combustion. Wood is used in many places, but is too bulky
and expensive. Oil is rarely used as a material of combustion, its
principal use being for illumination. Coal is the best and cheapest
material for combustion. The chief objection against its use is the
production of smoke, soot, and of various gases, as CO, CO2, etc. Gas
is a very good, in fact, the best material for heating, especially if,
when used, it is connected with chimneys; otherwise, it is
objectionable, as it burns up too much air, vitiates the atmosphere,
and the products of combustion are deleterious; it is also quite
expensive. The ideal means of heating is electricity.

=Chimneys.=--All materials used for combustion yield products more or
less injurious to health. Every system of artificially heating houses
must therefore have not only means of introducing fresh air to aid in
the burning up of the materials, but also an outlet for the vitiated,
warmed air, partly charged with the products of combustion. These
outlets are provided by chimneys. Chimneys are hollow tubes or shafts
built of brick and lined with earthen pipes or other material inside.
These tubes begin at the lowest fireplace or connection, and are
carried up several feet above the roof. The thickness of a chimney is
from four to nine inches; the shape square, rectangular, or,
preferably, circular. The diameter of the chimney depends upon the
size of the house, the number of fire connections, etc. It should be
neither too small nor too large. Square chimneys should be twelve to
sixteen inches square; circular ones from six to eight inches in
diameter for each fire connection. The chimney consists of a _shaft_,
or vertical tube, and _cowls_ placed over chimneys on the roof to
prevent down draughts and the falling