It has been shown, however, in Chapter VI. that the range over which mixtures of acetylene and air are explosive depends on the size of the vessel, or more particularly on the diameter of the tube, in which they are stored; so that if the burner tube between the air inlets and the point of ignition can be made small enough in diameter, a normally explosive mixture will cease to exhibit explosive properties. Manifestly, if a tube is made very small in diameter, it will only pass a small volume of gas, and it may be useless for the supply of an atmospheric burner; but Le Chatelier's researches have proved that a tube may be narrowed at one spot only, in such fashion that the explosive wave refuses to pass the constriction, while the virtual diameter of the tube, as far as passage of gas is concerned, remains considerably larger than the size of the constriction itself. Moreover, inasmuch as the speed of propagation of the explosion is strictly fixed by the conditions prevailing, if the speed at which the mixture, of acetylene and air travels from the air inlets to the point of ignition is more rapid than the speed at which the explosion tends to travel from the point of ignition to the air inlets, the said mixture of acetylene and air will burn quietly at the orifice without attempting to fire backwards into the tube. By combining together these two devices: by delivering the acetylene to the injector jet at a pressure sufficient to drive the mixture of gas and air forward rapidly enough, and by narrowing the leading tube either wholly or at one spot to a diameter small enough, it is easy to make an atmospheric burner for acetylene which behaves perfectly as long as it is fairly alight, and the supply of gas is not checked; but further difficulties still remain, because at the instant of lighting and extinguishing, i.e., while the tap is being turned on or off, the pressure of the gas is too small to determine a flow of acetylene and air within the tube at a speed exceeding that of the explosive wave; and therefore the act of lighting or extinguishing is very likely to be accompanied by a smart explosion severe enough to split the mantle, or at least to cause the burner to fire back. Nevertheless, after several early attempts, which were comparative failures, atmospheric acetylene burners have been constructed that work quite satisfactorily, so that the gas has become readily available for use under the mantle, or in heating stoves. Sometimes success has been obtained by the employment of more than one small tube leading to a common place of ignition, sometimes by the use of two or more fine wire- gauze screens in the tube, sometimes by the addition of an enlarged head to the burner in which head alone thorough mixing of the gas and air occurs, and sometimes by the employment of a travelling sleeve which serves more or less completely to block the air inlets.

DUTY OF INCANDESCENT ACETYLENE BURNERS.--Granting that the petty troubles and expenses incidental to incandescent lighting are not considered prohibitive--and in careful hands they are not really serious-- and that mantles suitable for acetylene are employed, the gas may be rendered considerably cheaper to use per unit of light evolved by consuming it in incandescent burners. In Chapter VIII. it was shown that the modern self-luminous, l/2-foot acetylene burner emits a light of about 1.27 standard English candles per litre-hour. A large number of incandescent burners, of German and French construction, consuming from 7.0 to 22.2 litres per hour at pressures ranging between 60 and 120 millimetres have been examined by Caro, who has found them to give lights of from 10.8 to 104.5 Hefner units, and efficiencies of from 2.40 to 5.50 units per litre-hour. Averaging his results, it may be said that incandescent burners consuming from 10 to 20 litres per hour at pressures of 80 or 100 millimetres yield a light of 4.0 Hefner units per litre- hour. Expressed in English terms, incandescent acetylene burners consuming 0.5 cubic foot per hour at a pressure of 3 or 4 inches give the duties shown in the following table, which may advantageously be compared with that printed in Chapter VIII., page 239, for the self-luminous gas:

ILLUMINATING POWER OF INCANDESCENT ACETYLENE. HALF-FOOT BURNERS.

1 litre = 3.65 candles | 1 candle = 0.274 litre.
1 cubic foot = 103.40 candles. | 1 candle = 0.0097 cubic foot.

A number of tests of the Güntner or Schimek incandescent burners of the 10 and 15 litres-per-hour sizes, made by one of the authors in 1906, gave the following average results when tested at a pressure of 4 inches:

_________________________________________________________________
| | | | |
| Nominal size | Rate of Consumption per | Light in | Duty |
| of Burner. | Hour | Candles | Candles per |
| | | | Cubic Foot |
|______________|_________________________|__________|_____________|
| | | | | |
| Litres. | Cubic Foot | Litres | | |
| 10 | 0.472 | 13.35 | 46.0 | 97.4 |
| 15 | 0.663 | 18.80 | 70.0 | 105.5 |
|______________|____________|____________|__________|_____________|

These figures indicate that the duty increases slightly with the size of the burner. Other tests showed that the duty increased more considerably with an increase of pressure, so that mantles used, or which had been previously used, at a pressure of 5 inches gave duties of 115 to 125 candles per cubic foot.

It should be noted that the burners so far considered are small, being intended for domestic purposes only; larger burners exhibit higher efficiencies. For instance, a set of French incandescent acetylene burners examined by Fouché showed:

_________________________________________________________________
| | | | | |
| Size of Burner | Pressure | Cubic Feet | Light in | Candles per |
| in Litres. | Inches. | per Hour. | Candles. | Cubic Feet. |
|________________|__________|____________|__________|_____________|
| | | | | |
| 20 | 5.9 | 0.71 | 70 | 98.6 |
| 40 | 5.9 | 1.41 | 150 | 106.4 |
| 70 | 5.9 | 2.47 | 280 | 113.4 |
| 120 | 5.9 | 4.23 | 500 | 118.2 |
|________________|__________|____________|__________|_____________|

By increasing the pressure at which acetylene is introduced into burners of this type, still larger duties may be obtained from them: