6 Dr. Karl Heumann's Contributions to 



mixture, burning with a luminous flame in consequence of 

 the application of heat, has or has not been altered so that its 

 luminosity shall continue when it has been allowed to cool to 

 the ordinary temperature. In other words, if the increase in 

 luminosity is directly due to increase in temperature of the 

 flame, and is not brought about by a chemical change in the 

 gaseous mixture, then the flame which appeared luminous at 

 the point of the strongly heated tube should again become non- 

 luminous when the gaseous mixture is cooled, after having 

 passed through the heated tube, and is then ignited. 



This experiment may be carried out by connecting two glass 

 tubes by means of gypsum to the platinum tube, the outer 

 glass tube being Y-shaped and being surrounded by cold 

 water. If the mixture of gases be passed through this ar- 

 rangement, the platinum tube being strongly heated, and be 

 ignited at the orifice of the glass V-tube, a non-luminous flame 

 is noticed ; whereas if the V-tube be removed and the gases be 

 ignited at the orifice of the platinum tube, the flame becomes 

 luminous. More simply, the experiment may be carried out 

 by burning the gaseous mixture as it issues from a platinum 

 tube about 12 or 15 centims. in length : in heating this tube 

 near to its orifice the flame becomes luminous ; but on heating 

 the tube at a point further back the luminosity of the original 

 flame is not increased, because the heated gases are again 

 cooled by passing over the outer part of the platinum tube. 



In employing a mixture of air and coal-gas under certain 

 conditions, it is found, as Wibel has noticed, that " the gas 

 aspirated from the opening of the burner reveals — by the 

 amount of water and carbon dioxide which it contains, as also 

 by its burning with a luminous flame under the ordinary 

 conditions " — that a partial decomposition has taken place. - 



While Wibel noticed a not inconsiderable deposition of 

 carbon when air and coal-gas were passed through a red-hot 

 platinum tube, in my experiments, in which the air only was 

 passed through a heated platinum tube, no such deposition 

 was noticed in the glass tube, at the orifice of which the gases 

 burned with a luminous flame for a considerable length of 

 time. In Wibel's case the deposition of carbon was doubtless 

 due to a too great local heating of the platinum tube through 

 which the gases were passed. Such an intense heating is not 

 necessary in order to attain the aim of the experiment. From 

 that experiment in which the flame of a gas, previously ren- 

 dered non-luminous, was restored to luminosity by means of 

 heat, Wibel draws very far-reaching conclusions. He rejects 

 the deductions of previous experimenters ; but in doing so he 

 rushes too far to the opposite extreme. For example, he 

 believes himself justified in concluding: — 



