22 Dr. Karl Heumann's Contributions to 



platinum tube was then heated by means of a Bunsen's burner 

 held not too near to the issuing vapour, whereupon the dis- 

 tance between flame and burner gradually diminished until the 

 two were in contact. On removing the Bunsen's lamp the 

 original distance was quickly regained. 



If the stream of gas be very rapid, the experiment carried out 

 as just described does not succeed, because the temperature of 

 the vapour in the tube is not sufficiently raised. I do not 

 doubt that, in the experiment described by Benevides, the dis- 

 tance between flame and burner would be greatly diminished, 

 if not actually removed, by passing the compressed gas through 

 a long tube maintained at a full red heat before igniting it. 



Although the explanation which I have given of the fact 

 that a space is noticed between flame and burner in the case 

 of quickly moving gases has taken into account all the points 

 which have been observed, and although I have not found any 

 facts opposed to this explanation, yet I must confess that I 

 am scarcely altogether satisfied with it. Thus the fact that 

 the approach of a small flame to the orifice whence the burn- 

 ing gas issues causes a diminution in the size of the observed 

 space, is not to be set down so much to the decreased with- 

 drawal of heat by the issuing cold gas (as was the case in the 

 experiment with the heated platinum tube*), but much more 

 to the carrying over of the combustion to the heated part. I 

 cannot look upon the cooling actions described above as alone 

 sufficient to cause all the observed circumstances. 



The second explanation already given of the cause of the 

 observed space in the case of compressed gases takes into 

 account the relation existing between the velocity of the 

 gaseous stream and the velocity of propagation of combustion. 



In order to gain a clear idea of the action of this factor, let 

 us suppose that the flame of a compressed gas issuing from a 

 tube is separated by a distance of several centimetres from the 

 orifice of the tube. The question suggests itself, Why does 

 not the flame make its way backwards towards the burner? or, 

 in other words, Why is not the combustion propagated back- 

 wards throughout the line of contact of gas and air towards 

 the burner ? 



The gaseous stream is evidently surrounded by a zone con- 

 sisting of a combustible mixture of air and gas molecules (I 

 use this expression on account of its shortness). As soon as 

 the temperature of a pair of molecules in one part of the zone 

 is raised to the ignition-point by means of a flame brought 

 near, chemical action occurs (combustion), and so much heat 

 is thereby evolved as suffices to raise the temperature of the 



* Supra, p. 17. 



