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



observed for H^ in the water film are. Thus in the case of 

 a 200 volt water arc between magnesium poles the separation 

 of the outer components of H^ at a distance of about o 0'05 mm. 

 from the stationary electrode was found to be 20*3 Angstrom 

 units. Hence the shift of each component from the initial 

 line was approximately 10 units. According to Stark an 



electric field of 10*4 x 10 4 — - produces a shift of about 



cm. * 



19*4 units for the outer parallel component of H^. Con- 

 sequently, since the displacement is proportional to the 

 strength of the electric fields a shift of 10 Angstrom units 



would correspond roughly to 50,000 — 8 . On calculating 



the field for a gap of 0*05 mm. with an applied potential of 



200 volts we obtain 40,000 ——a value which is of the 



cm. 



same order of magnitude as that derived from the amount 

 of separation of the components through the Stark effect. 

 Thus it would seem that the broadening of the hydrogen 

 lines observed during the initial stages of an arc flash is 

 caused by the strong electric field set up momentarily when 

 the electrodes separate. 



§ 8. Detailed results of observations. 



The photographic records of the spectral phenomena 

 exhibited by electric arcs under various conditions, which 

 served as basis for our measurements and observations, were 

 obtained in the manner already described in § 9 of our first 

 paper. Also the numbers and signs used for expressing 

 relative intensities and characters are the same as those 

 adopted there. For reasons already stated in § 1 we have 

 made a more exhaustive examination of the arc spectra of 

 magnesium. In the case of zinc and cadmium we give the 

 results only for the liquid air and water arc respectively, in 

 addition to that of the ordinary arc. The spectra of these 

 two elements do not play an important role in astrophysical 

 problems, and the few results we give will suffice to 

 illustrate the general behaviour of their spark lines under 

 arc conditions. 



Most of the photographs were taken on Wratten's pan- 

 chromatic plates. A few spectra, however, were obtained 

 on ordinary plates and they are therefore deficient beyond 

 X 5500. In these cases the space in the tables reserved for the 

 relative intensity of the line is left blank. Lines marked S 

 belong to the Rydberg nebulous series, and those marked H 

 to Paschen's series. 



