4 Dr. Karl Heumann's Contributions to 



luminosity. Wibel was led to this view by considering the 

 fact that a mixture of coal-gas and air, nitrogen, or carbon 

 dioxide, which burned with a blue flame, became luminous 

 and burned with a smoky flame when the tube from which 

 the mixture issued was strongly heated. In this case the 

 dilution must have been increased by the increase of tempera- 

 ture ; nevertheless the flame became luminous. 



In order to convince one's self of the justness of the conclu- 

 sions which are drawn from this observation, it is necessary to 

 examine somewhat closely the method adopted by Wibel in 

 his investigation. He says, "A tube of platinum, 8 to 10 

 centims. in length, is attached to an ordinary Bunsen's burner 

 which is closed at the bottom; the gas to be burned is brought 

 into the burner by means of a tube soldered to the lower part; 

 when the flame of the burning ,gas has been adjusted to the 

 proper point, the indifferent gas is admitted until the flame is 

 rendered non-luminous ; the platinum tube is then heated by 

 means of two non-luminous Bunsen flames held horizontally 

 on either side of the tube, so as to ensure that it be equally 

 heated .... The same appearance is noticed in the case of 

 the ordinary Bunsen flame, rendered non-luminous by ad- 

 mixed air, when the platinum tube is placed in the opening 

 of the lamp and is heated." 



The last mentioned experiment, as described by Wibel, must 

 be controlled before one can justly identify the diminution of 

 luminosity in the Bunsen burner with Knapp's experiments 

 upon diminution. Some time ago Barentin * showed that the 

 amount of luminous gas which enters a given space is very 

 different according as the gas is or is not ignited. Barentin 

 believed that the explanation of the smaller amount of gas 

 entering a burning lamp was to be found in the counter 

 pressure exercised by the burning, and therefore expanding 

 gas, upon the entering gas. Blochmannj showed that the 

 diminished consumption of gas was to be traced solely to the 

 increase of volume caused by the gas passing over the heated 

 upper part of the burner. 



The fact that a mixture of gases issuing from a Bunsen 

 lamp through a strongly heated tube burns with a luminous 

 flame, may therefore be due to causes other than that put 

 forward by Wibel, viz. rise in temperature of the flame ; for 

 it is evidently an improbable supposition that the consumption 

 of gas, and therefore also the quantity of air (or other gas) 

 drawn into the burner, will be unaffected by the passage of 

 the gas over a glowing tube ; and so also it cannot, a priori, 

 be expected that the proportion between air and gas will re- 

 * Pogg. Ann. cvii. 183. t J> fur Gasbeleuchtung, v. 355. . 



