FLAMES,--ILLUMINANTS
~Conditions necessary for flames.~ It has been seen that when two
substances unite chemically, with the production of light and heat, the
act of union is called combustion. When one of the substances undergoing
combustion remains solid at the temperature occasioned by the
combustion, light may be given off, but there is no flame. Thus iron
wire burning in oxygen throws off a shower of sparks and is brilliantly
incandescent, but no flame is seen. When, however, both of the
substances are gases or vapors at the temperature reached in the
combustion, the act of union is accompanied by a flame.
~Flames from burning liquids or solids.~ Many substances which are liquids
or solids at ordinary temperatures burn with a flame because the heat of
combustion vaporizes them slowly, and the flame is due to the union of
this vapor with the gas supporting the combustion.
~Supporter of combustion.~ That gas which surrounds the flame and
constitutes the atmosphere in which the combustion occurs is said to
support the combustion. The other gas which issues into this atmosphere
is said to be the combustible gas. Thus, in the ordinary combustion of
coal gas in the air the coal gas is said to be combustible, while the
air is regarded as the supporter of combustion. These terms are entirely
relative, however, for a jet of air issuing into an atmosphere of coal
gas will burn when ignited, the coal gas supporting the combustion.
Ordinarily, when we say that a gas is combustible we mean that it is
combustible in an atmosphere of air.
[Illustration: Fig. 62]
~Either gas may be the supporter of combustion.~ That 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 temperature by the
burning envelope surrounding it. On reaching the supporter of combustion
on the outside it is far above its kindling temperature, and combustion
follows with the evolution of much heat. The region of combustion just
outside the inner cone is therefore the hottest part of the flame.
[Illustration: Fig. 63]
~Oxidizing and reducing flames.~ Since the tip of the outside cone
consists of very hot products of combustion mixed with oxygen from the
air, a substance capable of oxidation placed in this part of the flame
becomes very hot and is easily oxidized. The oxygen with which it
combines comes, of course, from the atmosphere, and not from the
products of combustion. This outer tip of the flame is called the
_oxidizing flame_.
At the tip of the inner cone the conditions are quite different. This
region consists of a highly heated combustible gas, which has not yet
reached a supply of oxygen.
If a substance rich in oxygen, such as a metallic oxide, is placed in
this region of the flame, the heated gases combine with its oxygen and
the substance is reduced. This part of the flame is called the _reducing
flame_. These flames are used in testing certain substances, especially
minerals. For this purpose they are produced by blowing into a small
luminous Bunsen flame from one side through a blowpipe. This is a tube
of the shape shown in Fig. 64. The flame is directed in any desired way
and has the oxidizing and reducing regions very clearly marked (Fig.
65). It is non-luminous from the same causes which render the open
Bunsen burner flame non-luminous, the gases from the lungs serving to
furnish oxygen and to dilute the combustible gas.
[Illustration: Fig. 64]
[Illustration: Fig. 65]
~Luminosity of flames.~ The luminosity of flames is due to a number of
distinct causes, and may therefore be increased or diminished in several
ways.
1. _Presence of solid matter._ The most obvious of these causes is the
presence in the flame of incandescent solid matter. Thus chalk dust
sifted into a non-luminous flame renders it luminous. When hydrocarbons
form a part of the combustible gas, as they do in nearly all
illuminating gases and oils, some carbon is usually set free in the
process of combustion. This is made very hot by the flame and becomes
incandescent, giving out light. In a well-regulated flame it is
afterward burned up, but when the supply of oxygen is insufficient it
escapes from the flame as lampblack or soot. That it is temporarily
present in a well-burning luminous flame may be demonstrated by holding
a cold object, such as a small evaporating dish, in the flame for a few
seconds. This cold object cools the carbon below its kindling
temperature, and it is deposited on the object as soot.
2. _Pressure._ A second factor in the luminosity of flames is the
pressure under which the gases are burning. Under increased pressure
there is more matter in a given volume of a gas, and the chemical action
is more energetic than when the gases are rarefied. Consequently there
is more heat and light. A candle burning on a high mountain gives less
light than when it burns at the sea level.
If the gas is diluted with a non-combustible gas, the effect is the same
as if it is rarefied, for under these conditions there is less
combustible gas in a given volume.
3. _Temperature._ The luminosity also depends upon the temperature
attained in the combustion. In general the hotter the flame the greater
the luminosity; hence cooling the gases before combustion diminishes the
luminosity of the flame they will make, because it diminishes the
temperature attained in the combustion. Thus the luminosity of the
Bunsen flame is largely diminished by the air drawn up with the gas.
This is due in part to the fact that the burning gas is diluted and
cooled by the air drawn in. The oxygen thus introduced into the flame
also causes the combustion of the hot particles of carbon which would
otherwise tend to make the flame luminous.
~Illuminating and fuel gases.~ A number of mixtures of combustible gases,
consisting largely of carbon compounds and hydrogen, find extensive use
for the production of light and heat. The three chief varieties are coal
gas, water gas, and natural gas. The use of acetylene gas has already
been referred to.
~Coal gas.~ Coal gas is made by heating bituminous coal in large retorts
out of contact with the air. Soft or bituminous coal contains, in
addition to large amounts of carbon, considerable quantities of
compounds of hydrogen, oxygen, nitrogen, and sulphur. When distilled the
nitrogen is liberated partly in the form of ammonia and cyanides and
partly as free nitrogen gas; the sulphur is converted into hydrogen
sulphide, carbon disulphide, and oxides of sulphur; the oxygen into
water and oxides of carbon. The remaining hydrogen is set free partly
as hydrogen and partly in combination with carbon in the form of
hydrocarbons. The most important of these is methane, with smaller
quantities of many others, some of which are liquids or solids at
ordinary temperatures. The great bulk of the carbon remains behind as
coke and retort carbon.
~The manufacture of coal gas.~ In the manufacture of coal gas it is
necessary to separate from the volatile constituents formed by the
heating of the coal all those substances which are either solid or
liquid at ordinary temperature, since these would clog the gas pipes.
Certain gaseous constituents, such as hydrogen sulphide and ammonia,
must also be removed. The method used to accomplish this is shown in
Fig. 66. The coal is heated in air-tight retorts illustrated by A. The
volatile products escape through the pipe X and bubble into the tarry
liquid in the large pipe B, known as the _hydraulic main_, which runs
at right angles to the retorts. Here is deposited the greater portion of
the solid and liquid products, forming a tarry mass known as _coal tar_.
Much of the ammonia also remains dissolved in this liquid. The partially
purified gas then passes into the pipes C, which serve to cool it and
further remove the solid and liquid matter. The gas then passes into
D, which is filled with coke over which a jet of water is sprayed. The
water still further cools the gas and at the same time partially removes
such gaseous products as hydrogen sulphide and ammonia, which are
soluble in water. In E the gas passes over some material such as lime,
which removes the last portions of the sulphur compounds as well as much
of the carbon dioxide present. From E the gas passes into the large
gas holder F, from which it is distributed through pipes to the places
where it is burned.
[Illustration: Fig. 66]
One ton of good gas coal yields approximately 10,000 cu. ft. of
gas, 1400 lb. of coke, 120 lb. of tar, and 20 gal. of
ammoniacal liquor.
Not only is the ammonia obtained in the manufacture of the gas
of great importance, but the coal tar also serves as the source
of many very useful substances, as will be explained in Chapter
XXXII.
~Water gas.~ Water gas is essentially a mixture of carbon monoxide and
hydrogen. It is made by passing steam over very hot anthracite coal,
when the reaction shown in the following equation takes place:
C + H_{2}O = CO + 2H.
When required merely to produce heat the gas is at once ready for use.
When made for illuminating purposes it must be enriched, that is,
illuminants must be added, since both carbon monoxide and hydrogen burn
with non-luminous flames. This is accomplished by passing it into
heaters containing highly heated petroleum oils. The gas takes up
hydrocarbon gases formed in the decomposition of the petroleum oils,
which make it burn with a luminous flame.
Water gas is very effective as a fuel, since both carbon monoxide and
hydrogen burn with very hot flames. It has little odor and is very
poisonous. Its use is therefore attended with some risk, since leaks in
pipes are very likely to escape notice.
~Natural gas.~ This substance, so abundant in many localities, varies much
in composition, but is composed principally of methane. When used for
lighting purposes it is usually burned in a burner resembling an open
Bunsen, the illumination being furnished by an incandescent mantle. This
is the case in the familiar Welsbach burner. Contrary to statements
frequently made, natural gas contains no free hydrogen.
TABLE SHOWING COMPOSITION OF GASES
=====================+================+========+========+==========
| PENNSYLVANIA | COAL | WATER | ENRICHED
| NATURAL | GAS | GAS | WATER
| GAS | | | GAS
---------------------+----------------+--------+--------+----------
Hydrogen | | 41.3 | 52.88 | 30.00
Methane | 90.64 | 43.6 | 2.16 | 24.00
Illuminants | | 3.9 | | 12.05
Carbon monoxide | | 6.4 | 36.80 | 29.00
Carbon dioxide | 0.30 | 2.0 | 3.47 | 0.30
Nitrogen | 9.06 | 1.2 | 4.69 | 2.50
Oxygen | | 0.3 | | 1.50
Hydrocarbon vapors | | 1.5 | | 1.50
=====================+================+========+========+==========
These are analyses of actual samples, and may be taken as about
the average for the various kinds of gases. Any one of these
may vary considerably. The nitrogen and oxygen in most cases is
due to a slight admixture of air which is difficult to exclude
entirely in the manufacture and handling of gases.
~Fuels.~ A variety of substances are used as fuels, the most important of
them being wood, coal, and the various gases mentioned above. Wood
consists mainly of compounds 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° can be easily reached, whereas the hottest oxyhydrogen
flame is not much above 2000°. 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 furnace is that
the temperature can be regulated to any desired intensity.
EXERCISES
1. Why does charcoal usually burn with no flame? How do you account for
the flame sometimes observed when it burns?
2. How do you account for the fact that a candle burns with a flame?
3. What two properties must the mantle used in the Welsbach lamp
possess?
4. (a) In what respects does the use of the Welsbach mantle resemble
that of lime in the calcium light? (b) If the mantle were made of
carbon, would it serve the same purpose?
5. Would anthracite coal be suitable for the manufacture of coal gas?
6. How could you prove the formation of carbon dioxide and water in the
combustion of illuminating gases?
7. Suggest a probable way in which natural gas has been formed.
8. Coal frequently contains a sulphide of iron. (a) What two sulphur
compounds are likely to be formed when gas is made from such coal? (b)
Suggest some suitable method for the removal of these compounds.
9. Why does the use of the bellows on the blacksmith's forge cause a
more intense heat?
10. What volume of oxygen is necessary to burn 100 l. of marsh gas and
what volume of carbon dioxide would be formed, all of the gases being
measured under standard conditions?
11. Suppose a cubic meter of Pennsylvania natural gas, measured under
standard conditions, were to be burned. How much water by weight would
result?