Spectrum of Hydrogen Positive Bays. 245 



in the rays corresponded with increasing brightness of the 

 second spectrum. 



Since hydrogen in the normal state does not absorb the 

 second spectrum, the latter cannot be due to collisions or 

 neutral molecules which do not result in ionization ; but 

 beyond this there is at present no sufficient evidence to 

 determine the state of charge of the molecules before or 

 after the collisions which lead to its emission. 



4. The "Resting'' Spectrum. 



It is clear from an inspection of PL IV. No. 1, that when 

 atoms only are present in the positive rays, the second spec- 

 trum occurs only very faintly, it at all. It is probable, however, 

 that even in this case it is not entirely absent (a very faint 

 blackening can be detected on the negative of No. 1 between 

 Hp and H v , though there is no trace of any definite lines), 

 and is presumably due to the rays exciting the spectrum of 

 the gas through which they pass (the " resting " spectrum 

 of Stark). When molecules are present the second spec- 

 trum occurs in considerable quantities, and one would 

 naturally expect it to show the Doppler effect if examined 

 along the direction of motion of the rays. Stark and Wilsar 

 (loc. cit.) have found that the second spectrum shows no 

 such effect. To account for this, one of two suppositions 

 must be made. Either, which seems rather improbable, 

 the second spectrum is only emitted as the result of colli- 

 sions between molecules moving comparatively slowly with 

 respect to each other — i. e., rays near the end of their path ; 

 or Stark and Wilsar never got molecules in their positive 

 rays at all. The second hypothesis seems the most likely. 

 They used a narrow tube which is not favourable to the 

 development of molecules, and in Stark's account of his 

 experiments with the second spectrum the potential differ- 

 ence used in the discharge tube never seems to have 

 exceeded 7500 volts. In all the work done at Cambridge 

 on positive rays fairly powerful induction coils have been 

 used. 



When a discharge is produced by these means in a tube 

 at the low pressures found favourable for the production of 

 molecular rays, the potential difference is much greater than 

 7000 volts. Stark used a wide range of pressures, but it 

 seems not improbable that his comparatively low-potential 

 batteries are less effective in producing molecular rays than 

 the coil discharges used in the Cambridge experiments. 



