170 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The strongest copper lines show only very faintly, the weaker ones not 

 at all. 



Correlation of Effects. 



In the metals arranged in the order given above (tin, silver, copper, 

 magnesium, aluminium, zinc, and sodium) the following relations hold 

 roughly : — 



(1) The set of lines characteristic of the spectrum of each metal in an 

 atmosphere of hydrogen is stronger than that of the preceding metal of 

 the series ; (2) the hydrogen lines appearing in the spectrum of the me- 

 tallic arc of each metal are stronger than in that of the succeeding metal 

 of the series ; (3) the general working of the metallic arc is worse for the 

 metals at the first of the series than for those at the end. Briefly stated, 

 the intensities of the hydrogen lines coming out in the spectra of various 

 metals are roughly inversely proportional to the intensities of the char- 

 acteristic flutings of those metals. 



GENERAL EFFECTS OF THE HYDROGEN ATMOSPHERE. 



Historical. 



Liveing and Dewar * found the carbon arc to work badly in hydrogen, 

 and to give spectral lines of different relative intensities than in air. 

 Professor Crew f has given quantitative measurements of the changes of 

 intensities for the metallic arc spectra of magnesium, zinc, and iron. 



The general effects of the hydrogen atmosphere may be summarized 

 thus : — 



(1) The arc works poorly in hydrogen. (2) The intensity of the 

 whole spectrum is greatly reduced in hydrogen. (3) Those metallic lines 

 which belong to the series of Kayser and Runge are uniformly reduced 

 in intensity. (4) Other lines are reduced in intensity but not uniformly, 

 (o) Certain lines supposed to belong to the spark spectrum make their 

 appearance in the arc in hydrogen. 



Discussion. 



The radiations of the electric arc are generally admitted to be due to 

 three causes, — electrical, chemical, and thermal. The chemical cause 

 must depend upon the electrical cause in some way, for the chemical cause 



* Proc. Roy. Society, 33, 430 (1882). 

 t Phil. Mag., 50, 497 (1900). 



