8 Dr. Karl Heumann's Contributions to 



In experiments upon decrease of luminosity caused by com- 

 pletely indifferent gases free from oxygen there will, of course, 

 be a considerable decrease in temperature, because a fixed 

 quantity of beat must be divided throughout a larger volume 

 of gas. 



W. Stein *, however, has pointed out that in these cases 

 a cause other than lowering of temperature is at work. He 

 shows that a flame rendered non-luminous by means of nitro- 

 gen yet possesses so high a temperature that it is able to 

 decompose, with deposition of carbon, coal-gas conducted in a 

 glass tube through it ; he also observes that an inflammable 

 gas, carbon monoxide, whose pyrometric effect is nearly as 

 great as that of coal-gas, causes the flame of the latter gas to 

 become non-luminous. In order to bring about the complete 

 non-luminosity of 1 volume of coal-gas, there is required 

 1*6 volume in Bunsen's burner, and 0*9 volume in Bronner's 

 burner, of carbon monoxide. 



In this case decrease in luminosity is not accompanied by 

 a real decrease in the temperature of the flame ; and we are 

 obliged to allow that dilution of the burning gas plays an 

 important part, and may of itself, independently of any ab- 

 sorption of heat (which often takes place simultaneously) 

 cause decrease in luminosity. 



Wibel's experiment does not prove, as that author supposes 

 it to do, that cooling of the interior of the flame is the sole 

 cause of decreased luminosity, because the flame is simul- 

 taneously altered in its composition, i. e. it is largely diluted 

 by the entering gas. 



We find, then, some of those observers who have been 

 already mentioned tracing decreased luminosity, brought about 

 by admixed gases, solely to the diluting action of these gases ; 

 we find Wibel, on the other hand, tracing this decrease 

 solely to the cooling action of these gases ; but it appears to 

 me that the truth lies between these two conflicting views. 



It is difficult to devise experiments in which two or more 

 causes tending to decrease luminosity are not simultaneously 

 at work ; and yet every thing depends upon our being able 

 sharply to distinguish between these various causes. It will 

 only be possible to gain a clear knowledge of the processes 

 going on in flames when we are able to separate these pro- 

 cesses and to study each alone. 



It appeared to me necessary to devise an experiment in 

 support of the well-known statement — the luminosity of a 

 flame is diminished by cooling — which should admit of no 

 other interpretation than this. 



By the following method I have been able to show that 

 * /. pract. Chem. ix. 183. 



