the Theory of Luminous Flames. 9 



cooling a flame is of itself capable of bringing about decrease 

 of luminosity, and that luminosity may be then restored by 

 simply applying heat ; the result cannot be called in question 

 by supposing dilution or oxidation to have taken place. 



A luminous gas-flame, 3 to 4 centims. in length, proceeding 

 from the point of a blowpipe or other narrow tube, is allowed 

 to play horizontally upon a platinum basin suspended in a 

 vertical position, so that the flame may broaden out and be- 

 come blue. In this well-known experiment decrease of 

 luminosity must not be traced solely to withdrawal of heat 

 by means of the metal, inasmuch as the broadening out of the 

 flame enables oxidation and dilution, as well as cooling, to 

 influence the result. 



If the platinum basin be now heated, on the side opposite 

 to that on which the flame impinges, by means of a Bunsen's 

 lamp held horizontally, the gas-flame becomes more and more 

 luminous as the temperature of the basin increases, until it 

 finally is restored to its original degree of luminosity. Of 

 course the metal must be perfectly pure, and must not be 

 touched with the fingers before the experiment ; else the flame 

 will be coloured yellow. 



It is here shoion that luminosity of the flame, which had been 

 diminished by the use of the platinum basin, is restored solely 

 by raising the temperature. 



If the Bunsen lamp be removed, the flame quickly decreases 

 in luminosity until it becomes blue. 



In this experiment, in which decrease of luminosity is 

 brought about by lowering the temperature, the objection 

 formerly praised — viz. that the broadening out of the flame 

 complicated the result — can no longer be maintained, inas- 

 much as the small decrease of volume consequent upon the 

 cooling would tend to produce an opposite result. It is there- 

 fore experimentally proved that cooling a flame is itself 

 sufficient to cause a decrease in the luminosity of that flame. 



Reasons have been already given which oblige us to ac- 

 knowledge that dilution of a flame by admixed gases is of 

 itself sufficient to cause decreased luminosity* (Bunsen's 

 flame, decreased luminosity by carbon dioxide) ; and inas- 

 much as the admission of a cold gas into a flame must with- 

 draw heat from that flame, it is concluded that the decrease in 

 the luminosity of carbon-containing flames brought about by 



* Frankland lias observed that decrease of luminosity of carbon-con- 

 taining- flames is a consequence of dilution by lowering of atmospheric 

 pressure ; and be bas concluded that tbe decrease of luminosity is con- 

 nected with tbe decrease of pressure. I have not cited this experiment 

 in proof of the effect of dilution in decreasing luminosity, because lower- 

 ing of temperature is associated with lowering of pressure, and this must 

 have an influence in decreasing the intensity of the light. 



