THE CHEMISTRY OF FLAME.
141. Candle Flame.
Experiment 82.--Examine a candle flame, holding a dark object
behind it. Note three distinct portions: (1) a colorless interior
about the wick, (2) a yellow light-giving portion beyond that,
(3) a thin blue envelope outside of all, and scarcely
discernible. Hold a small stick across the flame so that it may
lie in all three parts, and observe that no combustion takes
place in the inner portion.
142. Explanation.--A candle of paraffine, or tallow, is chiefly
composed of compounds of C and H, in the solid state. The burning
wick melts the solid; the liquid is then drawn up by the wick
till the heat vaporizes and decomposes it, and O of the air comes
in contact with the outer heated portion of gas, and burns it
completely. Air tends to penetrate the whole body of the flame,
but only N can pass through uncombined, for the O that is left
after combustion in the outer portion seizes upon the compounds
of C and H in the next, or yellow, part. There is not enough O
here for complete combustion; at this temperature H burns before
C, and the latter is set free. In that state it is of course a
solid. Now an incandescent solid, or one glowing with heat, gives
light, while the combustion of a gas gives scarcely any light,
though it may produce great heat. While C in the middle flame is
glowing, during the moment of its dissociation from H, it gives
light. In the outer flame the temperature is high enough to burn
entirely the gaseous compounds of C and H together, so that no
solid C is set free, and hence no light is given except the faint
blue. No combustion takes place in the inner blue cone, because
no O reaches there.
By packing a wick into a cylindrical tin cup 5 or 10 cm high and
4 cm in diameter, containing alcohol, and lighting it, gunpowder
can be held in the middle of the flame in a def. spoon, without
burning. This shows the low temperature of that portion. Burning
P will also be extinguished, thus showing the exclusion of O.
143. Bunsen Flame.
Experiment 83.--Examine a Bunsen burner. Unscrew the top, and
note the orifices for the admission of gas and of air. Make a
drawing. Replace the parts; then light the gas at the top,
opening the air-holes at the base. Notice that the flame burns
with very little color. Try to distinguish the three parts, as in
the candle flame. These parts can best be seen by allowing direct
sunlight to fall on the flame and observing its shadow on a white
ground. Make a drawing of the flame. Hold across it a Pt wire and
note at what part the wire glows most. Also press down on the
flame for an instant with a cardboard or piece of paper; remove
before it takes fire, and notice the charred circle. Put the end
of a match into the blue cone, and note that it does not burn.
Put the end of a Pt wire into this blue cone, and observe that it
glows when near the top of the cone. What do these experiments
show? Ascertain whether this inner portion contains a combustible
material, by holding in it one end of a small d.t., and trying to
ignite any gas escaping at the other end. It should burn. This
shows that no combustion takes place in the interior of the
flame, because sufficient free O is not present.
Next, close the air-holes, and note that the flame is yellow and
gives much light. From this we infer the presence of solid
particles in an incandescent state. But these could not come from
the air. They must be C particles which have been set free from
the C and H compounds of the gas, just as in the candle flame.
The smoke that rises proves this. Hold an e.d. in the flame and
collect some C. Try the same with the air-holes open. 144. Light
and Heat of Flame.--Which of the two flames is hotter, the one
with the air-holes open, or that with them closed? Evidently the
former; for air is drawn in and mixes with the gas as it rises in
the tube, and, on reaching the flame at the top, the two are well
mingled, and the gaseous compounds of C and H burn at so high a
temperature that solid C is not freed; hence there is little
light. On closing the air-holes, no O can reach the flame except
from the outside, and the heat is much less intense.
(Fig 33.) (Fig 34.)
The H burns first, and sets the C free, which, while glowing,
gives the light. This again illustrates the facts (1) that flame
is caused by burning gas; (2) that light is produced by
incandescent solids. Charcoal, coke, and anthracite coal burn
without flame, or with very little, because of the absence of
gases.
145. Temperature of Combustion.
Experiment 84.--Light a Bunsen flame, with the basal orifices
open, and hold over it a fine wire gauze. Notice that the flame
does not rise above the gauze. Extinguish the light, and try to
ignite the gas above the gauze, holding the latter within 5 or 6
cm of the burner tube. Notice that it does not burn below the
gauze (Fig. 33).
Gas and O are both present. Evidently, then, the only condition
wanting for combustion is a sufficiently high temperature. The
gauze cools the gas below its kindling- point.
This principle is made use of in the miner's lamp of Davy (Fig.
34). In coal mines a very inflammable gas, CH4, called fire-damp,
issues from the coal. If this collects in large quantities and
mixes with O of the air, a kindling-point is all that is needed
to make a violent explosion. An ordinary lamp would produce this,
but the gauze lamp prevents it; for, though the inside may be
filled with burning gas, CH4, the flame cannot communicate with
the outside.
(Fig 35.) (Fig 36.)
a, reducing flame b, oxidizing flame
146. Oxidizing and Reducing Flames.--The hottest part of a Bunsen
flame is just above the inner blue cone (b, Fig. 36). Evidently
there is more O at that point. If a reducing agent, i.e. a
substance which takes up O, be put into this part of the flame,
the latter will remove the O and appropriate it, forming an
oxide. Cu heated there would become copper oxide. This part is
called the oxidizing flame. The inner blue part of the Bunsen
flame is devoid of O. It ought to remove O from an oxidizing
agent, i.e. a substance which supplies O. If copper oxide be
heated there (a, Fig. 36) by means of a mouth blow-pipe (Fig.
35), the flame will appropriate the O and leave the copper. This
is called the reducing flame. Only the upper part of this blue
central cone has heat enough to act in this way. By using a
prepared piece of metal, to make the flame thin and to shut off
the air, and then blowing the flame with a blow-pipe, greater
strength can be obtained in both oxidizing and reducing flames
(Fig. 36).
147. Combustible and Supporter Interchangeable.-- H was found to
burn in O. H was the combustible, O the supporter. Would O itself
burn in H?--i.e. would the combustible become the supporter, and
the supporter the combustible? As illuminating gas consists
largely of H, and as air is part O, we may try the experiment
with gas and air. Gas will burn in air. Will air burn in gas?
Experiment 85.--Fit a cork with two holes in it to the large end
of a lamp chimney. Through each hole pass a short piece of
tubing, and connect one of these with a rubber tube leading to a
gas-jet. Pass a metallic tube, long enough to reach the top of
the chimney, through the other, so that it will move easily up
and down. Turn on the gas, and light it at the top of the
chimney. Hold the end of the tube passing through the cork in the
flame for a minute, then draw it down to the middle of the
chimney (Fig. 37, a) and finally slowly remove it (b). Note that
O from the air is burning in the gas. Which is the supporter, and
which the combustible in this case? O will burn equally well in
an atmosphere of H, as can be shown by experiment.
148. Explosive Mixture of Gases.
Experiment 86.--Slowly turn down the burning gas of a Bunsen
lamp, having the orifices open, and notice that it suddenly
explodes and goes out at the top, but now burns at the base. As
the gas was gradually turned off, more air became mixed with it,
until there was the right proportion of each gas for an
explosion. Figure 38 shows the same thing. Light the gas at the
top a, when the tube c covers the jet b. Then gradually raise the
tube c. At a certain place there is the same explosion as with
the lamp.
149. Generalizations.--These experiments show (1) that three
conditions are necessary for combustion,--a combustible, a
supporter, and a burning temperature which varies for different
substances. Given these, "a fire" always results. The conditions
for "spontaneous combustion" do not differ from those of any
combustion. See Experiments 34, 112, 113, 114. (2) That
combustible and supporter are interchangeable. If H burns in O, O
will burn in H, the product, being the same in each case. (3) For
any combustion there must be a certain proportion of combustible
and of supporter. Twenty per cent of CO2 in the air dilutes the O
to such an extent that C will not burn. Hence the utility of the
chemical engine for putting out fires. (4) When two
gases, a combustible and a supporter, are mixed in the requisite
proportion, they form an explosive mixture, needing only the
kindling temperature to unite them.
Chemical combination is always accompanied by disengagement of
heat. Chemical dissociation is always accompanied by absorption
of heat. The disengagement, or the absorption, is not always
evident to the senses.
Combustion is the chemical combination of two or more substances
with the self-evident disengagement of great heat, and usually of
light.
The temperature of ignition varies greatly with different
substances. PH3 burns spontaneously at the usual temperatures of
the air. P takes fire at 60 degrees, but even at 10 degrees it
oxidizes with rapidity enough to produce phosphorescence. The
vapor of CS2 may be set on fire by a glass rod heated to 150
degrees, but a red-hot iron will not ignite illuminating gas.
Spontaneous combustion often takes place in woolen or cotton rags
which have been saturated with oil. The oil rapidly absorbs O,
and sets fire to the cloth. This is thought to be the origin of
some very destructive fires.