MORSE. — SPECTRA OP WEHNELT. 537 



The Wehnelt seems to lie about half-way between the arc and the 

 spark in relation of intensity of lines. X 3278.0 is plainly visible, though 

 of lowest order of intensity in the spark and invisible in the arc spec- 

 trum. The lines at X 3650.0, X 3655.0, and A 3663.0 have about the 

 same intensities as in the spark. X 3984.0 is very feeble, of intensity 

 4 in the table, as against 40 for the next line, X 4046.0, and 6 for X 4078.0. 

 The arc line at X 4916.5 is plainly visible, while the line at X 3681.0, also 

 belonging to the arc spectrum, is not to be seen. The spark lines at 

 X 5426.0 and X 5678.0 are not present. There is a strong line at X 3381.0 

 in the spectrum of the Wehnelt which has not been placed satisfactorily 

 to the credit of any impurity. No traces of a banded spectrum are to 

 be seen. 



Tin. 



The points of especial interest in the table are : — 



1. Marked differences in intensities in spark, arc, and Wehnelt spectra. 

 X 3283.5 is much weaker in the last than in the spark, and A 3330.0, 

 X 3655.5, A 3801.0, X 4525.5, A 5632.0 are much stronger. The spectrum 

 of the Wehnelt is in many parts more like that of the arc. 



2. The presence of an underlying band spectrum, containing many 

 flutings, and not present either in the spark or the arc spectrum of the 

 metal. The wave-lengths of these maxima have been placed in the 

 table without certainty as to whether the maximum in question is a 

 sharp line or the head of a fluting. So far as the author knows this 

 spectrum of tin has not been observed under any other circumstances. 



3. The appearance of certain lines. Of striking interest in this con- 

 nection is the pair of lines at A 5562.0 and A 5588.0. These lines are 

 strong in the spark, absent from the arc, and of small intensity in the 

 Wehnelt spectrum as compared with the spark. They are also the most 

 striking lines in the spark spectrum on account of their marked breadth 

 and diffuseness. The lines at X 3283.5 and X 3352.0 have the same 

 peculiarities in a much less degree, and they also follow the same course 

 in their varying intensities in the three spectra. 



A similar change in the latter pair of lines has been observed in the 

 arc between tin electrodes in an atmosphere of nitrogen, these lines 

 being much weaker than in the spark in air.* In ammonia gas and 

 hydrogen they are greatly enhanced. The possibility of the formation 

 of metal-hydrogen compounds in the Wehnelt and of a similar inlluenee 



* Vid. Porter, Astropliys. J., 15, 274 (1902). 



