214 Mr. G. A. Hemsalech : A Comparative Study 



§ 3. The Luminous Phenomena exhibited by the Furnace 

 as the Temperature gradually rises. 



Up to a temperature of about 2500° C. the aspect presented 

 by the interior of the tube containing metallic iron is the same 

 as when no iron is present. At low temperatures the space 

 inside the furnace is filled with fumes or vapours giving out 

 a strong continuous spectrum, which obliterates all but the 

 strongest lines of the iron emission. It is not certain whether 

 this continuous spectrum is actually emitted by these vapours, 

 or whether it is merely reflected light from the inner surface' 

 of the white-hot carbon tube. It is, however, possible to 

 greatly reduce the obnoxious effects of these vapours by 

 passing a slow current of air or hydrogen through the tube. 

 The velocity of the gas should be such as to produce only a 

 very small flame at the opening of the carbon observation 

 tube; this precaution is of special moment in the case of long 

 exposures, because the too generous supply of fresh gas 

 rapidly wears away the inner surface of the furnace, owing 

 to chemical combination of the gases with the carbon 

 (see § 10) . As the temperature rises these luminous vapour 

 clouds gradually dissipate and the tube appears fairly clear 

 even without being constantly washed out by a current of gas. 

 Up to a temperature of about from 2400° C. to 2500° C. the 

 interior of the furnace, when free from clouds, glows in a 

 beautiful purple tint. With cobalt metal in the tube, at 

 about this temperature, a long luminous cloud of approxi- 

 mately cylindrical shape was observed to remain suspended 

 in a position along the axis of the tube, as though held there 

 in equilibrium by something expelled from all round the wall 

 of the furnace-tube ; its spectrum was continuous. The free 

 space between this cloud cylinder and the furnace wall 

 remained perfectly clear of mist and was of the same purple 

 tint as before. When the temperature is raised to about 

 2500° C. the whole interior space gives out a white light 

 showing strong continuous spectrum. I presume that this 

 white light is caused by incandescent carbon particles shot 

 off en masse in consequence of the more rapid disintegration, 

 through the higher temperature, of the inner walls of the 

 furnace. At 2700° O. the interior of the furnace is a blaze 

 of dazzling white light, and the spectrum now shows the 

 carbon bands in the green and blue; this band emission I take 

 to indicate that an electric current now actually passes through 

 conducting carbon vapour. 



As already stated, the phenomena observed inside the 

 furnace when the latter is charged with metallic iron, are the 

 same as those described above when no metal is present, up 



