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 1-7/82-1/221
7 X 1-7/82-1/421.3
8 X 1-7/82-1/820.8
9 X 1-7/81-7/818.2

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:

Products of Combustion per Candle Power.
Name of Burner.Illuminating Power in Candles per c.f. of gas Consumed.Carbon Dioxide.Water Vapor.
Batswing.2.90.18 c.f.0.46 c.f.
Argand.3.30.16 c.f.0.40 c.f.
Regenerative.10.00.05 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.