NICHOLS. 



THE VISIBLE RADIATION FROM CARBON. 



97 



These values having beeu plotted with relative cross-sections of the 

 ires as abscissae, and temperatures as ordinates, were found to lie 



1800 



1600 



1400 



1200 



100 200 300 



CROea-SECTION OF WIRE 



Figure 14. 



400 



upon a smooth curve {g) as shown in Figure 14. This curve, when ex- 

 tended to the line corresponding to zero cross-section, gave for the tem- 

 perature of the flame 1780", a temperature sufficient to account for the 

 success of Smithells's experiment, already described, in which platinum 

 wires of small diameter were melted in the outer sheath of such a flame. 

 I found it easy, by holding a wire of the size used in junction IV. in a 

 plane parallel to that of the flame, and moving it gradually toward the 

 latter to verify his statement. The wire was readily melted. 



It was not thought necessary to make further experiments upon this 

 flame. The region selected was, so far as one could judge from the 

 brightness of the luminous sheath, the hottest portion of flame. My 

 measurements upon this region would lead to the conclusion that the 

 luminous sheath of ordinary gas flames is at least one hundred and twenty 

 degrees lower than the corresponding region in the acetylene flame. 

 Luminous flames of ordinary illuminating gas would perhaps repay 

 further study, but owing to the fact that such gas is an ever varying 

 mixture and that it is burned under conditions of pressure, etc., such as 

 to give a fluctuating character to the flame, the problem would have at 



VOL. XXXVII. — 7 



