Having now brought before you the various methods by which ordinary
coal gas can be enriched, so as to give an increased luminosity to the
flame, I wish now to discuss the methods by which the gas can be
burnt, in order to yield the greatest amount of light, and also the
compounds which are produced during combustion.
In the first lecture, while discussing the theory of luminous flames,
I pointed out that, in an atmospheric burner, it was not the oxygen of
the air introduced combining with and burning up the hydrocarbons, and
so preventing the separation of incandescent carbon, which gave the
non-luminous flame, but the diluting action of the nitrogen, which
acted by increasing the temperature at which the hydrocarbons are
broken up, and carbon liberated, a fact which was proved by
observation that heating the mixture of gas and air again restored the
luminosity of the flame. This experiment clearly shows that
temperature is a most important factor in the illuminating value of a
flame, and this is still further shown by a study of the action of the
diluents present in coal gas, the non-combustible ones being far more
deleterious than the combustible, as they not only dilute, but
withdraw heat.
Anything which will increase the temperature of the flame will also
increase the illuminating power, provided, of course, that the
increase in temperature is not obtained at the expense of the too
rapid combustion of the hydrocarbons.
As has been shown in the experiments relating to the action of
diluents on flame, already quoted, oxygen, when added to coal gas,
increases its illuminating value to a marked and increasing degree,
until a certain percentage has been added, after which the
illuminating power is rapidly decreased, until the point is reached
when the mixture becomes explosive. This is due to the fact that the
added oxygen increases the temperature of the flame by doing the work
of the air, but without the cooling and diluting action of the
nitrogen; when, however, a certain proportion is added, it begins to
burn up the heavy hydrocarbons, and although the temperature goes on
increasing, the light-giving power is rapidly diminished by the
diminution of the amount of free carbon in the flame.
It has been proposed to carburet and enrich poor coal gas by
admixture with it of an oxy-oil gas made under Tatham's patents, in
which crude oils are cracked at a comparatively low temperature, and
are there mixed with from 12 to 24 per cent. of oxygen gas. Oil gas
made at low temperatures, _per se_, is of little use as an illuminant,
as it burns with a smoky flame, and does not travel well, but when
mixed with a certain amount of oxygen, it gives a very brilliant white
light, and no smoke, while as far as experiments have at present gone,
its traveling powers are much improved.
At first sight it seems a dangerous experiment to mix a heavy
hydrocarbon gas with oxygen, but it must be remembered that although
hydrogen and carbon monoxide only need to be mixed with half their own
volume of oxygen to give a most explosive mixture, yet as the number
of carbon and hydrogen atoms in the combustible gas increase, so does
the amount of oxygen needed to give explosion. Thus coal gas needs
rather more than its own volume, and ethylene three times its volume,
to give the maximum explosive results, while these mixtures begin to
be explosive when 10 per cent. of oxygen is mixed with hydrogen or
water gas, 30 per cent. with coal gas, and over 50 per cent. of oil
gas of the character used. It is claimed that if this gas was used as
an enricher of coal gas, 5 per cent. of it would increase the
luminosity of 16-candle gas by about 40 per cent.
Oxygen has been obtained for some time past from the air on a
commercial scale by the Brin process, and at the present time there
seems every prospect of our being able to obtain oxygen at a rate of
about 3s. 6d. per 1,000 cubic feet. Another process by which this
important result can also be obtained was first introduced by Tessie
du Mothay, and has now just been revived. It consists of passing
alternate currents of steam and air over sodic manganate heated to
dull redness in an iron tube; the process has never been commercially
successful, for the reason that the contents of the tube fused, and
flowing over the surface of the iron rapidly destroyed the tubes or
retorts, and also as soon as fusion took place, the mass became so
dense that it had little or no action on the air passing over it. Now,
however, this difficulty has been partly overcome by so preparing the
manganate as to prevent fusion, and to keep it in a spongy state,
which gives very high results, and the substance being practically
everlasting, the cost of production is extremely low.
It is proposed to feed this by a separate system of pipes to small gas
jets, and by converting them into practically oxyhydrogen blow pipes,
to raise solid masses of refractory material to incandescence, and
also by supplying oxygen in the same way to oil lamps of particular
construction, to obtain a very great increase in illuminating power.
Whether these methods of employing cheap oxygen would be successful or
not, I do not wish to discuss at the present time, but there is no
doubt but that cheap oxygen would be an enormous boon to the gas
manager, as by mixing 0.8 per cent. of oxygen with his coal gas before
purification, he could not only utilize the method so successfully
introduced by Mr. Valon at Ramsgate, but could also increase the
illuminating value of his gas.
In speaking of the structure of flame, I pointed out that close to the
burner from which the gas giving the flame is issuing, a space exists
in which no combustion is going on--in other words, a flame is never
in contact with the rim of the burner. This is best seen when the gas
is turned low--with a batswing burner, for instance--turned so low
that only a small non-luminous flame is left, the space between burner
and flame will appear as great as the flame itself, while, if the gas
is mixed with an inert diluent like carbon dioxide, the space can be
very much increased.
Several theories have been brought forward to explain this phenomenon,
but the true one is that the burner abstracts so much heat from the
flame at that point that it is unable to burn there, and this can be
proved by the fact that where a cold object touches the flame, a
dividing space, similar to that noticed between flame and burner, will
always be observed, and the colder the object and the more diluted the
gas the greater is the observed space. If a cold metal wire or rod is
held in a non-luminous flame, it causes an extinction of the gas for
some considerable space around itself; but as the temperature of the
rod rises, this space becomes smaller and smaller until the rod is
heated to redness, and then the flame comes in contact with the rod.
In the same way, if the burner from which the gas is issuing be heated
to redness, the space between burner and flame disappears. It has
already been shown that cooling the flame by an inert diluent reduces
the illuminating value, and finally renders it more luminous; and we
are now in a position to discuss the points which should be aimed at
in the construction of a good gas burner.
In the first place, a sensible diminution in light takes place when a
metal burner is employed, and the larger the surface and thickness of
the metal the worse will be its action on the illuminating power of
the flame; but this cooling action is only influencing the bottom of
the flame, so that with a small flame the total effect is very great,
and with a very large flame almost _nil_.
The first point, therefore, to attend to is that the burner shall be
made of a good non-conductor. In the next place, the flow of the gas
must be regulated to the burner, as, if you have a pressure higher
than that for which the burner is constructed, you at once obtain a
roaring flame and a loss of illuminating power, as the too rapid rush
of gas from the burner causes a mingling of gas and air and a
consequent cooling of the flame. The tap also which regulates the
flame is better at a distance from the burner than close to it, as any
constriction near the burner causes eddies, which give an unsteady
flame.
These general principles govern all burners, and we will now take the
ordinary forms in detail. In the ordinary flat flame burner, given a
good non-conducting material, and a well regulated gas supply, little
more can be done, while burning it in the ordinary way, to increase
its luminosity; and it is the large surface of flame exposed to the
cooling action of the air which causes this form of burner to give the
lowest service of any per cubic foot of gas consumed. Much is done,
moreover, by faulty fittings and shades, to reduce the already poor
light given out, because the light-yielding power of the flame largely
depends upon its having a well rounded base and broad, luminous zone;
and when a globe with a narrow opening is used with such a flame--as
is done in 99 out of 100 cases--the updraught drags the flame out of
shape, and seriously impairs its light-giving powers, a trouble which
can be got over by having the globe with an opening at the bottom not
less than 4 inches in diameter, and having small shoulders fixed to
the burner, which draw out the flame and protect the base from the
disturbing influence of draughts.
The Argand burner differs from the flat flame burners in that a
circular flame is employed. The air supply is regulated by a
cylindrical glass, and this form of burner gives a better service than
the flat flame burner, as not only can the supply of gas and air be
better adjusted, but the air being slightly warmed by the hot glass
adds to the temperature of the flame, which is also increased by
radiation from the opposite side of the flame itself.
The chief loss of light in such a burner depends upon the fact that,
being circular, the light from the inner surface has to pass through
the wall of flame, and careful photometric experiments show that the
solid particles present in the flame so reduce its transparency that a
loss amounting to about 25 per cent. of light takes place during its
transmission.
The height of the flame also must be carefully adjusted to the size of
the flame, as too long a chimney, by increasing the air supply unduly,
cools, and so lowers the illuminating power of the flame. Experiments
with carbureted water gas gave the following results, with a
consumption of 5 cubic feet per hour:
-----------------------------------------------------
Size of Chimney. | Height of Flame. | Candle Power. |
------------------+------------------+---------------|
6 X 1-7/8 | 2-1/2 | 21 |
7 X 1-7/8 | 2-1/4 | 21.3 |
8 X 1-7/8 | 2-1/8 | 20.8 |
9 X 1-7/8 | 1-7/8 | 18.2 |
------------------+------------------+---------------+
For many years no advance was made upon these forms of burner, but
when, ten years ago, it was recognized that anything which cools the
flame reduces its value, while anything which increases its
temperature raises its illuminating power, then a change took place in
the forms of burner in use, and the regenerative burners, introduced
by such men as Siemens, Grimston, and Bower, commenced what was really
a revolution in gas lighting.
By utilizing the heat contained in the escaping products of combustion
to raise the temperature of the gas and air which are to enter into
combination in the flame, an enormous increase in the temperature of
the solid particles of carbon in the flame is obtained, and a far
greater and whiter light is the result.
The Bower lamp, in which (at any rate in the later forms) the flame
burns between a downward and an upward current of air, was one of the
first produced, and so well has it been kept up to date that it still
holds its own; while as types of the "inverted cone" regenerative
burner, we may also take the Cromarty and Wenham lights, which have
been followed by a host of imitators, and so closely are the original
types adhered to that one begins seriously to wonder what the use of
the Patent Office really is.
The Schulke, and the last form of Siemens regenerative burner,
however, stand apart from all the others by dealing with flat and not
conical flames, and in both regeneration is carried on to a high
degree. The only drawback to the regenerative burner is that it is by
far the best form of gas stove as well as burner, and that the amount
of heat thrown out by the radiant solid matter in the flame is, under
some circumstances, an annoyance. But, on the other hand, we must not
forget that this is the form best adapted for overhead burners, and
that nearly every form of regenerative lamp can be adapted as a
ventilating agent, and that with the withdrawal of the products of
combustion from the air of the room, the great and only serious
objection to gas as an illuminant disappears.
When coal gas is burned, the hydrogen is supposed to be entirely
converted into water vapor, and the carbon to finally escape into the
air as carbon dioxide; and if this were so, every cubic foot of gas
consumed would produce approximately 0.52 cubic foot of carbon dioxide
and 1.34 cubic feet of water vapor, while the illuminating power
yielded by the cubic foot of gas will, of course, vary with the kind
of burner used.
Roughly speaking, the ordinary types of burner give the following
results:
------------------------------------------------------------
| Illuminating | Products of Combustion
| Power in | per
Name of Burner. | Candles per | Candle Power.
| c.f. of gas |------------------------
| Consumed. | Carbon | Water
| | Dioxide. | Vapor.
-----------------+-----------------+------------+-----------
Batswing. | 2.9 | 0.18 c.f. | 0.46 c.f.
Argand. | 3.3 | 0.16 c.f. | 0.40 c.f.
Regenerative. | 10.0 | 0.05 c.f. | 0.13 c.f.
-----------------+-----------------+------------+------------
So that the regenerative forms of burner, by giving the greatest
illuminating power per cubic foot of gas consumed, yield a smaller
amount of vitiation to the air per candle of light emitted.
An ordinary room, say 16' X 12' X 10', would not be considered
properly illuminated unless the light were at least equal to 32 candle
power; and in the table below the amount of the oxygen used up and the
products of combustion formed by each class of illuminant and burner
in attaining this result are given, the number of adults who would
exhale the same amount during respiration being also stated.
From these data it appears, according to rules by which the degree of
vitiation of the air in any confined space is measured by the amount
of oxygen used up and carbon dioxide formed, that candles are the
worst offenders against health and comfort. Oil lamps come next, and
gas least. This, however, is an assumption which practical experience
does not bear out. Discomfort and oppression in a room lighted by
candles or oil are less felt than in one lighted by any of the older
forms of gas burner; and the partial explanation of this is to be
found in the fact that, when a room is illuminated with candles or
oil, people are contented with a feebler and more local light than
when using gas. In a room of the size described, the inmates would be
more likely to use two candles placed near their books, or on a table,
than thirty-two scattered about the room.
Moreover, the amount of water vapor given off during the combustion of
gas is greater than in the case of the other illuminants. Water vapor
having a great power of absorbing radiant heat from the burning gas
becomes heated, and diffusing itself about the room, causes great
feeling of oppression; the air also being highly charged with
moisture, is unable to take up so rapidly the water vapor which is
always evaporating from the surface of our skin, whereby the functions
of the body receive a slight check, resulting in a feeling of
_malaise_.
Added to these, however, is a far more serious factor which has, up to
the present, been overlooked, and that is that an ordinary gas flame,
in burning, yields distinct quantities of carbon monoxide and
acetylene, the prolonged breathing of which in the smallest traces
produces headache and general physical discomfort, while its effect
upon plant life is equally marked.
AMOUNT OF OXYGEN REMOVED FROM THE AIR, AND CARBON DIOXIDE AND WATER
VAPOR GENERATED TO GIVE AN ILLUMINATION EQUAL TO 32 CANDLE POWER.
(The amount of light required in a room 16' X 12' x 10'.)
|Quantity of | | Products of Combustion| |
| Materials | Oxygen | | Carbon | |
Illuminant | Used | Removed |Water Vapor| Dioxide |Adults|
--------------+------------+----------+-----------+-----------+------+
Sperm Candles |3,840 grains|19.27 c.f.|13.12 c.f. |13.12 c.f. | 21.8 |
Paraffin Oil |1,984 " |12.48 c.f.| 7.04 c.f. | 8.96 c.f. | 14.9 |
Gas (London)--| | | | | |
Burners: | | | | | |
Batswing | 11 c.f. |13.06 c.f.|14.72 c.f. | 5.76 c.f. | 9.6 |
Argand | 9.7 c.f. |11.52 c.f.|12.80 c.f. | 5.12 c.f. | 8.5 |
Regenerative| 3.2 c.f. | 3.68 c.f.| 4.16 c.f. | 1.60 c.f. | 2.6 |
Ever since the structure of flame has been noted and discussed, it has
been accepted as a fact beyond dispute that the outer almost invisible
zone which is interposed between the air and the luminous zone of the
flame is the area of complete combustion, and that here the unburnt
remnants of the flame gases, meeting the air, freely take up oxygen
and are converted into the comparatively harmless products of
combustion, carbon dioxide and water vapor, which only need partial
removal by any haphazard process of ventilation to keep the air of the
room fit to support animal life. I have, however, long doubted this
fact, and at length, by a delicate process of analysis have been able
to confirm my suspicions. The outer zone of a luminous flame is not
the zone of complete combustion; it is a zone in which luminosity is
destroyed in exactly the same way that it is destroyed in the Bunsen
burner; that is the air penetrating the flame so dilutes and cools
down the outer layer of incandescent gas that it is rendered
non-luminous, while some of the gas sinks below the point at which it
is capable of burning, with the result that considerable quantities of
the products of incomplete combustion carbon monoxide and acetylene
escape into the air, and render it actively injurious.
I have proved this by taking a small platinum pipe, with a circular
loop on the end, the interior of the loop being pierced with minute
holes, and by making a circular flame burn within the loop so that the
non-luminous zone of the flame just touched the inside of the loop,
and then by aspiration so gentle as not to distort the shape of the
flame, withdrawing the gases escaping from the outer zone. On
analyzing these by a delicate process, which will be described
elsewhere, I arrived at the following results: