90 Dr. Karl Heumann's Contributions to 



this respect immediately it is introduced into the flame ; as its 

 own temperature increases, it loses its power of cooling the 

 surrounding particles of heated gas. A thick wire also will 

 evidently cause a diminution of luminosity for a longer period 

 than a thin wire. 



It is generally a matter of indifference in what part of the 

 flame the cooling body is placed ; hence the burner itself may 

 play a not unimportant part in cooling the burning gas in its 

 neighbourhood, and so in aiding in the production of the dark 

 zone of flame which is noticed just over the orifice of the 

 burner. 



Before studying the action of the burner in detail, however, 

 it will be well that we should inquire more narrowly into the 

 cooling action exercised by a cold object brought into the flame 

 upon the luminosity of that flame. If a luminous gas-flame 

 be slightly pressed down by means of a porcelain basin, the 

 luminosity of the flame is somewhat diminished, and the basin 

 is covered with a deposit of soot. If, however, the basin be 

 at the beginning of the experiment deeply depressed within 

 the flame, the luminosity is at once decreased ; but there is no 

 deposit of soot on the basin. 



I have already shown that the decrease of luminosity is to 

 be traced in this experiment to the cooling action of the cold 

 object placed in the flame. If, as may easily be done, an equal 

 area of the porcelain basin be brought into the flame in each 

 case, the cooling action of this cold surface will be approxi- 

 mately the same in each* ; nevertheless when the basin is 

 brought into the upper part of the flame it is covered with 

 soot, while no such deposition occurs when the basin is 

 brought into the lower part of the flame. 



The following experiments will throw light upon these phe- 

 nomena. 



A porcelain rod was brought into the lower part of a gas- 

 flame burning at a round orifice 8 millims. in width ; the 

 flame throughout a considerable area became blue. (By 

 cutting off the still luminous portion of the flame by means 

 of a shade, the action of the porcelain rod is rendered more 

 apparent.) No soot was deposited on the porcelain. The 

 same rod was held in the upper part of the luminous flame: 

 it was gradually covered with a tolerably thick deposit of 

 soot ; at the same time diminution in the luminosity of the 

 flame was noticed. 



In order to explain these facts, I put forward the following 



* Rather more beat will "be withdrawn from the upper hot flame, on 

 account of the greater difference of temperature between it and the basic, 

 than from the lower part of the flame, in equal time-intervals. 



