PORTER. — ON CERTAIN ARC SPECTRA. 377 



The two very strong spark lines at A 3351 and A 3282, appearing 

 also in the spectrum of the rotating arc, have their relative intensity de- 

 creased by nitrogen. But by ammonia they are relatively enhanced at 

 least twenty times. In the arc in air, these two lines are barely visible ; 

 but in ammonia they become two of the most prominent lines of the 

 spectrum. These two lines are similarly affected by hydrogen The 

 spark line at A 2368 is also intensified in ammonia but less than the two 

 preceding. The spark lines of wave-lengths 2449, 3471, 3539, 3574, 

 appear plainly in ammonia and hydrogen but not at all in air or nitrogen. 

 They are more enhanced by hydrogen than by ammonia. At A 3360 

 and A 3370 appear two unidentified lines of intensity 6 and 3 (on a 

 scale increasing from 1 to 10), respectively, in ammonia, and not quite 

 so intense in nitrogen, which can scarcely be detected in air and 

 hydrogen. These same lines appear also in the magnesium and zinc 

 arcs in air, nitrogen, and ammonia ; but oxygen has the effect of im- 

 mensely weakening both of them in the tin and magnesium arcs. A 3370 

 appears in the tin spark, but A 3360 does not. 



Zinc. 



The average intensity of the lines of the zinc arc spectrum is reduced 

 approximately one-half by nitrogen. The width of reversed lines is not 

 affected. 



The only zinc lines that suffer a disproportionate reduction by an 

 atmosphere of nitrogen are A 2558 and A 2502. These, with A 5182, are 

 the lines which are enhanced by hydrogen. They are strong spark lines. 

 In ammonia their intensity is two or three times as great as it is in air, 

 although the average intensity of the zinc spectrum is diminished perhaps 

 five times by ammonia. 



Iron. 



The substitution of nitrogen for air about the iron arc produces very 

 little change in the spectrum. The general intensity is not altered. 

 Compared with the number of lines relatively affected by hydrogen, the 

 number of iron lines affected by nitrogen is small, as has been found to 

 be the case with other metals. Many of the lines that are affected by 

 nitrogen are impurity lines. Of fifty iron lines between A 3660 and A 4060 

 that are markedly enhanced by hydrogen, six are distinctly reduced by 

 nitrogen. 



On examination of the region between A 3600 and A 4600 I found 

 twenty-six lines that are particularly affected by nitrogen. A few others 



