Spectra of Helium, Sfc. in the Ultra- Violet. 303 



surprising if our results do not check exactly with his. 

 These observations also support the hypothesis stated above, 

 that the absorption of radiation facilitates impact ionization. 



In order to keep mercury vapour out of the hydrogen it 

 was necessary to keep the trap adjacent to the apparatus 

 immersed in liquid air throughout the experiments. In 

 consequence, the glowing filament cut down the pressure of 

 the gas steadily and very rapidly during the course of the 

 observations. The resulting changes in pressure are shown 

 on fig. 7. The character of the effect of this on the shape 

 of the curves is seen by comparing curves A and B. The 

 observations on A were taken in the usual way, going from 

 low values of the field to high values — those on B were 

 taken in the reverse sense. 



After the hydrogen had been thoroughly cleaned out of 

 the apparatus, a short glass tube containing mercury was 

 sealed on near M and an attempt was made to examine the 

 mercury spectrum. The vapour was produced by heating 

 the short tube in a bath. It is not possible under these 

 conditions to get a steady or definitely measurable pressure 

 of vapour. In order to do satisfactory work with mercury 

 the apparatus will need to be redesigned in certain parti- 

 culars. The results obtained strongly indicate that the 

 mercury spectrum ceases at somewhat longer wave-lengths 

 than does that of hydrogen. The curves shown in fig. 8 are 

 sufficiently definite on this point, but uncertainty arises 

 from the suspicion that the shortening of the spectrum may 

 result from an insufficient pressure of vapour in the discharge 

 chamber. Pressure A was in the neighbourhood of # 001 mm. 

 If the limit shown (1 ampere) is the proper one, the limit of 

 radiation is indicated to lie between A, 1000 and X1200, 

 which checks either with Bohr's calculated ionization 

 potential (10*5 volts) which gives radiation at X1180, or 

 with the limiting value calculated from the eV = hv relation 

 and the observed secondary ionization potential (10*25 volts). 

 It is proposed to repeat these measurements under more 

 satisfactory conditions. The extension to 1*2 amperes in 

 curves C and D may possibly be due either to the mercury 

 pressure being higher than the admissible value or to the 

 presence of some hydrogen contamination. 



It has already been stated that the reflexion disturbances 

 arising from too high gas pressures cause the spectrum to 

 spread out. The effect then becomes a function of the 

 driving potential on the electrons. It is easy to tell when 

 one enters the region of too high pressure, since the readings 



Y2 



