and Dissociation of Hydrogen Molecules, 



439 



slit S. Each constituent of the rays is drawn out into a 

 parabolic curve, and, by increasing the magnetic field, the 

 various parabolas may successively be brought to fall on the 

 slit. When this occurs the charged particles pass through 

 S and give up their charges to the Faraday chamber F. The 



s + 





T 



F<* 



advantages obtained by using a Wehnelt cathode are, that 

 any desired potential may be used, in particular low potentials, 

 and that the pressure of the gas may be made as low as 

 desired and may be varied without changing the potential. 

 The change that occurs in the constituents of the rays as the 

 pressure is decreased is described in this paper. 



With 800-volt rays and hydrogen at a pressure of about 

 •01 mm. the curve in fig. 2 was obtained. The abscissae 

 represent the strength of the deflecting magnetic field, the 

 zero being to the left of the origin, and the ordinates give 

 the charge obtained in the Faraday chamber as the field was 

 increased so as to bring the three lightest constituents of the 

 rays over the parabolic slit. There are present hydrogen 

 atoms, hydrogen molecules, and very much of the constituent 

 with atomic weight 3. The curve obtained with a pressure 

 of *0017 mm. is given in fig. 3 ; there is a noticeable decrease 

 in the relative amount of both Hj and H 3 . Fig. 4 gives the 

 curve when a charcoal bulb with liquid air was used. Here 

 the pressure was less than *0005 mm., and H^ and H 3 have 

 practically disappeared. That this change was due to de- 

 creasing pressure, and not to the removal of some constituent 

 of the gas by the charcoal, was shown by the fact that when 

 hydrogen was admitted, while the charcoal and liquid air 

 were on, H^ and H 3 regained their original relative 

 intensities. 



