376 PROCEEDINGS OP THE AMERICAN ACADEMY. 



Magnesium. 



The ordinary arc lines of magnesium seem to be almost wholly 

 unaffected by substituting nitrogen for air. The heavy spark line at 

 A 4481, which appears also in the spectrum of the rotating arc, is reduced 

 by nitrogen to about one-fifth its intensity in air. Naturally, the mag- 

 nesium oxide fluting at A 5007 is practically blotted out in nitrogen and 

 greatly intensified in oxygen. On the other hand, no new lines make 

 their appearance in nitrogen. 



The intensity and reversal of the characteristic line at A 2852, which 

 are so strongly affected by hydrogen * and ammonia, are unaffected by 

 nitrogen, while the sharp line at A 4571 is slightly reduced by hydrogen 

 and ammonia, although not changed by nitrogen. 



The magnesium-hydrogen fluting beginning at A 52 10, discovered by 

 Liveing and Dewar,f appears in ammonia, as does also the F line of 

 hydrogen. Fairly intense hazy lines at approximately 4580, 4434, 4430, 

 and 4390 also appear in ammonia. These lines are apparently intensi- 

 fied by oxygen also. By nitrogen they are unaffected. A very faint trace 

 of these lines can be seen in air. I have not yet succeeded in identifying 

 them. 



Tin. 



Of the four metals studied, tin is the one whose spectrum is most 

 modified by changes of atmosphere. In air, nitrogen, and oxygen, the 

 tin arc works well. But in hydrogen and ammonia the arc is very short 

 and is maintained with difficulty. 



The intensity of the tin arc in nitrogen is estimated at one-third its 

 intensity in air ; while in ammonia its intensity is not more than one- 

 twentieth its iutensity in air ; and in pure hydrogen it is even less. 



Not only is the average intensity of the tin spectrum strongly affected, 

 but also the character and relative intensity of its individual lines. In 

 Kayser and Runge's table of wave-lengths of the tin arc more than 

 forty lines are described as reversed. Many of these reversals show very 

 clearly on my plates. When the arc is surrounded with nitrogen, how- 

 ever, some of these lines appear to be doubly reversed, the rest not 

 reversed at all. On the other hand many of the lines that are reversed 

 in air appear to have their reversal widened by ammonia. These 

 reversals are not affected by an atmosphere of oxygen. 



* Astroph. Jour., 12, 171 (1900). Liveing and Dewar, Proc. Roy. Soc, 32, 192. 

 t Proc. Roy. Soc, 27, 494-496 ; 30, 93-99. 



