S66 Mr, G. A. Hemsalech and the Comte de Gramont on 



the magnesium sulphate arc between graphite poles the carbon 

 bands do not show in the spectrum, and, accordingly, the value 



found for the electric force, namely 519 - — s , is near the 



' J cm. ' 



average. In the third case, magnesium chloride between 

 graphite poles, the spectrum of the arc shows also no trace 

 of the carbon bnnds, but its predominant feature is the strong 

 development of the lines and bands of calcium. Now it is 

 well known that an arc passes particularly well through 

 calcium vapour, and therefore the latter must be a good 

 conductor of electricity. Hence we venture to ascribe the 

 high value obtained in this case for the applied electric force 

 to the presence of calcium vapour which lowered the resist- 

 ance of the arc. Thus it, would seem that all our values are 

 to some extent influenced by the resistance prevailing in the 



arc. Our lowest value, namely 404 -^-, for the blown arc 



would therefore indicate that this arc possessed the highest 

 resistance of all the arc flashes examined; this inference 

 seems likely to be true, since air was blown through the arc 

 from the commencement till the end of the flash. As a 

 result of these considerations, we venture to conclude that 

 the emission of the spark line X 4481 is controlled by electric 

 forces and, further, that there exists a definite critical minimum 

 value of this force below which this particular radiation is no 

 longer excited. 



With regard to the permanent emission of X4481 in a 

 stable arc burning in nitrogen, it is evident that the electric 

 forces alone would not suffice to cause its excitation, since 

 the average value of the acting electric field was only about 



150 y — -. Nor can the higher temperature of the stable arc 



cm. ° L 



account for it, because the line shows only the merest traces 

 along the path of a stable arc in air and no sign of it has been 

 detected at the very high temperature prevailing in the fringe 

 zone of a single plate resistance furnace. But, as we have 

 already suggested in § 5, it is quite possible that the emission 

 of X4481 by a nitrogen arc is brought about by the chemical 

 reaction between the magnesium vapour and the surrounding- 

 gas. Since, however, this line is not emitted in the explosion 

 region of the air-coal gas flame, where a similar reaction 

 occurs, it would seem that in the case of the arc in nitrogen 

 the emission is finally realized with the help of the additional 

 effects of the higher temperature and of the prevailing electric 

 forces. 



