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.