2 Dr. Karl Heumann's Contributions to 



I have generally in my remarks adopted a chronological 

 arrangement of the various researches, because it is only thus 

 that the course of thought which I have followed can be 

 logically represented. A systematic arrangement would, it 

 is true, place the simpler before the more complex conditions ; 

 but in the present case the latter, which occur chiefly in the 

 case of luminous carbon-containing flames, are the more im- 

 portant and have been more studied. 



The experience gained from the study of flames burning 

 under complex conditions is tested and applied in the case of 

 simpler flames, and so becomes a guide to the explanation of 

 the conditions affecting luminous flames in general. 



Diminution and Restoration of Luminosity in Hydrocarbon- 

 flames. 



In the greater number of researches which have hitherto 

 been conducted upon the luminosity of flames, those flames 

 have been principally examined the luminosity of which is to 

 be ascribed to the presence of carbon, and methods of re- 

 search in which the luminosity has been either increased or 

 diminished have, for the most part, been adopted. 



Such methods are open to many objections, the principal 

 of which (viz. the introduction of various agencies acting 

 now in this way, now in the opposite, during the same ex- 

 periment) has been too much overlooked. By reason of 

 this oversight, researches, leading to diametrically opposed 

 conclusions, have been published ; and since Davy's theory 

 is no longer of universal application, we have been left with- 

 out any means of bringing the facts concerning luminous 

 flames into harmony with one another. 



Frankland has broached the hypothesis that the luminosity 

 of flame is not due to particles of suspended carbon, but is 

 caused by the vapours of heavy hydrocarbons which radiate 

 white light. Strong positive evidence in support of a view 

 so much at variance with the generally accepted theory, could 

 hardly be expected ; and Frankland has relied principally 

 upon the fact that we are acquainted with many luminous 

 flames in which we cannot suppose that solid matter is 

 present. 



To the instances already known Frankland has added the 

 interesting observation that hydrogen and carbon monoxide 

 when burned in oxygen under a pressure of 10 to 20 at- 

 mospheres, yield a luminous flame affording a continuous 

 spectrum, and also that the faintly luminous flame of alcohol 

 becomes as bright as that of a candle when the pressure is in- 

 creased to 18 or 20 atmospheres*. These experiments are 

 * Compare L. Cailletet, Compt, Bend, clxxx. 487. 



