the terms
_combustible_ and _supporter of combustion_ are merely relative may be
shown in the following way: A lamp chimney A is fitted with a cork and
glass tubes, as shown in Fig. 62. The tube C should have a diameter of
from 12 to 15 mm. A thin sheet of asbestos in which is cut a circular
opening about 2 cm. in diameter is placed over the top of the chimney.
The opening in the asbestos is closed with the palm of the hand, and gas
is admitted to the chimney through the tube B. The air in the chimney
is soon expelled through the tube C, and the gas itself is then
lighted at the lower end of this tube. The hand is now removed from the
opening in the asbestos, when the flame at the end of the tube at once
rises and appears at the end within the chimney, as shown in the figure.
The excess of coal gas now escapes from the opening in the asbestos and
may be lighted. The flame at the top of the asbestos board is due to the
combustion of coal gas in air, while the flame within the chimney is due
to the combustion of air in coal gas, the air being drawn up through the
tube by the escaping gas.

~Appearance of flames.~ The flame caused by the union of hydrogen and
oxygen is almost colorless and invisible. Chlorine and hydrogen combine
with a pale violet flame, carbon monoxide burns in oxygen with a blue
flame, while ammonia burns with a deep yellow flame. The color and
appearance of flames are therefore often quite characteristic of the
particular combustion which occasions them.

~Structure of flames.~ When the gas undergoing combustion issues from a
round opening into an atmosphere of the gas supporting combustion, as is
the case with the burning Bunsen burner (Fig. 63), the flame is
generally conical in outline. It consists of several distinct cones,
one within the other, the boundary between them being marked by
differences of color or luminosity. In the simplest flame, of which
hydrogen burning in oxygen is a good example, these cones are two in
number,--an inner one, formed by unburned gas, and an outer one, usually
more or less luminous, consisting of the combining gases. This outer one
is in turn surrounded by a third envelope of the products of combustion;
this envelope is sometimes invisible, as in the present case, but is
sometimes faintly luminous. The lower part of the inner cone of the
flame is quite cool and consists of unburned gas. Toward the top of the
inner cone the gas has become heated to a high temperatur