proportion of its combustible constituent which lies between the lower and the upper explosive limit of that constituent escapes from an open- ended pipe and a light is applied, the mixture will fire and the flame will pass back into the pipe, there to produce an explosion, unless the orifice of the said pipe is so small as to prevent the explosive wave passing (as is the case with a proper acetylene burner), or unless the pipe itself is so narrow as appreciably to alter the range of explosibility by lowering the upper explosive limit from its normal value. By far the most potent factor in altering the range of explosibility of any gas when mixed with air is the diameter of the vessel containing or delivering such mixture. Le Chatelier has investigated this point in the case of acetylene, and his values are reproduced overleaf; they are comparable among themselves, although it will be observed that his absolute results differ somewhat from those obtained by Eitner which are quoted later: _Explosive Limits of Acetylene mixed with Air._--(Le Chatelier.) ___________________________________________________________ | | | | | | Explosive Limits. | | | Diameter of Tube |_______________________| Range of | | in Millimetres. | | | Explosibility. | | | Lower. | Upper. | | |__________________|___________|___________|________________| | | | | | | | Per Cent. | Per Cent. | Per Cent. | | 40 | 2.9 | 64 | 61.1 | | 30 | 3.1 | 62 | 58.9 | | 20 | 3.5 | 55 | 51.5 | | 6 | 4.0 | 40 | 36.0 | | 4 | 4.5 | 25 | 20.5 | | 2 | 5.0 | 15 | 10.0 | | 0.8 | 7.7 | 10 | 2.3 | | 0.5 | ... | ... | ... | |__________________|___________|___________|________________| Thus it appears that past an orifice or constriction 0.5 mm. in diameter no explosion of acetylene can proceed, whatever may be the proportions between the gas and the air in the mixture present. With every gas the explosive limits and the range of explosibility are also influenced by various circumstances, such as the manner of ignition, the pressure, and other minor conditions; but the following figures for mixtures of air and different combustible gases were obtained by Eitner under similar conditions, and are therefore strictly comparable one with another. The conditions were that the mixture was contained in a tube 19 mm. (3/4-inch) wide, was at about 60° to 65° F., was saturated with aqueous vapour, and was fired by electric spark.