846 Mr. G. A. Hemsalech and the Comte de Gramont on 



stages in the development of an arc flash between magnesium 

 poles in hydrogen : — 



Thickness of rarefied gas film 018 mm. 



Heights from stationary electrode : X 4481... 1"54 „ 



5184... 1-59 „ 



H a 6563... 0-89 „ 



Thus the hydrogen lines are emitted for a considerably 

 shorter period than the metal lines. The thickness of the 

 rarefied gas film is approximately of the same order of 

 magnitude as that of liquid films. Measurements made on 

 the spectrum of a blown arc in air gave 0'17 mm. as the 

 thickness of the rarefied air film. 



The following table contains a summary of the results of 

 our observations on the appearance of the various types 

 of magnesium lines as given by 80 volt arcs burning in 

 various gases : — 





Air. 



Nitrogen. 



Oxygen. 



Hydrogen. Coal-gas. 



Quality of arc 



Stable. 



Stable. 



Unstable. 



Unstable. Unstable. 



Flame line \4571. 



Normal. 



Normal. 



Normal. 



Normal. Normal. 



Series triplets 



Normal. 



Normal. 



Normal. 



Normal. Normal. 



Rydberg series ... 



Diffuse. 



Well-defined 

 and prominent 



Diffuse 

 and winged. 



Diffuse Diffuse 

 and winged. and winged. 



Spark line X 4481 . 



Diffuse, 

 traces 

 only. 



Relatively 



feeble, but 



narrow and 



sharply 



Strong and 

 symmetrically 

 broadened in 

 1st phase ; fairly 



Very intense and 

 symmetrically 

 broadened in 1st phase ; 

 narrower in 2nd phase. 



defined. strong but narrow 

 in 2nd phase. 



§ 6. Arc in liquid air. 



The electrodes, which consisted of metal rods, were com- 

 pletely immersed in the liquid, so that before the arc was 

 struck they were at a temperature of about —190° C. With 

 the current at 80 volts the discharge consisted in short 

 flashes, each one of which was accompanied by the evolution 

 of numerous air-bubbles caused no doubt by the momentary 

 rise in temperature. Since the luminous vapour in this 

 case is completely enclosed by liquid air its temperature 

 cannot possibly attain a high value, and this inference is 

 amply borne out by the relative behaviour of the flame line 



