R. W. Wood, Jr. — Combustion of Gas Jets. 479 



Pressure 12 cms — Flame beyond the contraction went out, and 

 there was left only a short tube of blue fire, as sharply defined 

 as if made of rigid matter (fig. 2 E). Through this tube a vast 

 quantity of unconsumed gas flowed, which was so cold that the 

 eye could be brought directly over the tube, when it was seen 

 to be distinctly hollow, with thin sharply defined walls. 

 Pressure 20 cms — Tube shortened to one-half its former length, 

 and the walls became thinner (fig. 2 f). Pressure 23 cms — The 

 tube went out. 



The formation of the tube is a rather curious phenomenon. 

 It appears to be due to the fact that the gas molecules on the 

 outside are moving slower than those within, the speed being 

 reduced by friction with the walls of the orifice. What we 

 have then is a jet of gas moving at high speed surrounded, 

 below the contracted place (where the air mixes with it), by a 

 shell of gas moving at a velocity so low that it will remain 

 ignited ; with an increase of pressure we should expect the 

 shell to become thinner, and this is exactly what happens. 

 Burning the gas in oxygen, the jet will stand a far greater 

 pressure without forming a tube or blowing itself out. In fact 

 it is difficult to raise the pressure high enough to bring this 

 about. Were we dealing with a flame of pure hydrogen, 

 burning in an atmosphere of oxygen, it would be impossible. 

 Any one who has used the oxy-hydrogen blowpipe knows that 

 it is impossible to give a velocity to the mixed gases great 

 enough to extinguish the jet. The rate at which flame will run 

 down such a jet, or rather the rate at which combustion takes 

 place through a mixture of two parts of hydrogen and one of 

 oxygen is 2500 meters (more than a mile and a half) a second. 

 That is to say, if we had a tube a mile and a half long filled 

 with the mixture, the flame would traverse it in one second. 

 The explosion would be practically instantaneous. Contrast 

 this with coal gas and air. Fill a glass tube a meter long and 

 an inch in diameter with a mixture consisting of 1 part of coal 

 gas to 10 of air. Apply a flame to the open end, and a disc of 

 blue fire will descend the tube with a whistling noise at the 

 rate of about 18 inches a second! The reason why the jet of 

 coal gas blows itself out is very obvious ; its velocity is greater 

 than 18 inches per second, consequently it carries the flame 

 away, so to speak. To cause an oxy-hydrogen jet to act in a 

 similar manner we should have to give it a velocity greater 

 than 2500 meters a second. To determine the exact form of 

 the jet when not ignited, the gas was passed through two 

 bulbs, each filled with asbestus, that in one bulb soaked with 

 ammonia, in the other with hydrochloric acid. The gas, 

 passing through these bulbs, became charged with dense white 

 fumes of ammonic chloride, which rendered the jet plainly 



