Iron Vapour in Air-Coal Gas Flame. 223 



must be taken of the fact, as illustrated by the figure, that 

 the thickness of the stratum of mantle is a minimum near 

 the base of the cone and a maximum just above its apex. 



Fig. 1. 



Mantle 



Cone 



I 

 I 



% 



I 



Slit 

 I 



r 



i 



± 



JSymer 



Relative positions of flame image and slit. 



For this reason alone one would always expect the flame 

 lines to reach their maximum intensity just above the cone. 

 We also know from the results obtained by many experi- 

 menters — in particular, E. Bauer * — that the temperature of 

 a flame is highest just above the explosion region. 



a. Air-coal Gas Flame. (Plate II. No. 1.) 



The cone lines (Classes II. and III.) first become visible 

 at the base of the cone ; then, on passing upwards, they 

 gradually grow brighter and reach their maximum of 

 strength near the apex. After this there is an abrupt and 

 enormous decrease in their intensities. Class II. lines con- 

 tinue to be emitted as feeble lines by the flame above the 

 cone (example : triplet at 4046), but Class III. lines are no 

 longer emitted. With regard to Class I. lines, they behave 

 in a similar manner to the cone lines throughout the height 

 of the cone, but, though there is a slight reduction in their 

 intensities as they pass from the cone into the mantle, they 



* E. Bauer, Thlses de Doctorat, Paris, 1913. 

 R2 



