848 Mr. G. A. Hemsalech and the Comte cle Gramont on 



of striking the arc and during the further course of the 

 arc flash. 



In order to account for the absence of any marked dif- 

 ferences between the first and second phase spectra of 

 magnesium under these conditions, we venture to suggest 

 that possibly chemical actions intervene in this case. We 

 have already shown in § 5 that nitrogen is effective in 

 bringing out the spark line X 4481 and in strengthening 

 the lines of the Rydberg series. Remembering then that 

 the boiling-point of nitrogen ( — 196° C.) is lower than that 

 of oxygen ( — 183° C), we may be justified in presuming 

 that when the temperature of the liquid air in the immediate 

 vicinity of the arc rises nitrogen will be given off first, 

 so that the first chemical reaction to start along the arc flash 

 will be one between magnesium vapour and nitrogen. Thus 

 the peculiar character of the magnesium spectrum as observed 

 in the arc under liquid air may be the result of the cooperation 

 of the following factors : — s 



1. A low temperature keeping down the degree of ionization 



of the vapours in the arc gap and in this way preventing 

 the formation of a stable arc. 



2. Electric forces acting strongly during the first phase 



and causing emission of spark lines. Feebler electric 

 forces acting during second phase but unable by them- 

 selves to keep up strong emission of spark lines. 



3. A chemical reaction between nitrogen and magnesium 



setting in at the end of the first phase and helping (in 

 cooperation with the feebler electric forces) to continue 

 during the second phase the emission of the spark lines 

 initiated by the strong electric forces in the first phase. 



§ 7. Cause of broadening of the hydrogen lines during 

 the first phase. Stark effect. 



From the results of our preliminary experiments we had' 

 concluded that the emission of spark lines by the arc depended 

 upon the existence of strong electric fields, and we pointed out 

 that for an electrode distance of 0'05 mm. the intensity of 

 the field set up in the gap with a potential difference of only 



110 volts is equal to 22,000 ^ S *. An electric field of this 



strength should reveal itself by a manifestation of the Stark 

 effect. As will be remembered, the arc flash is produced 

 between a fixed lower electrode A and an upper movable 

 one B, a diop of liquid (water or oil) being placed between 

 * Hemsalech and de Gramont, loc. cit. § 12. 



