246 The Spectrum of Hydrogen Positive Rays. 



5. Effect of Magnetic Field on the Discharges. 



A curious phenomenon was noticed with the discharge 

 tube illustrated in fig. 2. This tube gave only atomic rays 

 under ordinary conditions except at very low pressures, and 

 sometimes even then the molecule was barely visible. 

 When, however, the discharge was influenced by an electro- 

 magnet held near it in a certain position, the molecular 

 parabola w T ould suddenly appear. This only happened over 

 a certain range- of pressure, and at the pressure at which 

 PL IV. No. 1, was taken it had no effect. In addition to 

 bringing out the molecular parabola the magnet always 

 increased the deviation of the atomic parabola, and as this 

 depends chiefly on the cathode fall of potential, it presum- 

 ably made the tube go easier ; while the atomic parabola 

 was generally long and frequently beaded, indicating a wide 

 range of velocities, the molecular parabola produced by the 

 magnet was always short, being little more than a patch of 

 light at the place corresponding to the greater velocity. 

 The magnet had the effect of spreading out the negative 

 glow, which was originally concentrated near the mouth of 

 the anode tube, and bringing it forward towards the 

 cathode. In addition to this effect the magnet would 

 occasionally brighten the atom as well, but careful inspection 

 showed that this was always due to a better centring of the 

 beam of positive rays on to the fine tube, owing to slight 

 deviations of the rays themselves by the magnet. 



6. Intensity of Second Spectrum Lines. 



The brighter lines in the second spectrum generally occur 

 in groups of two or three, too close together to be separated 

 with the dispersion used. It may, however, be of interest to 

 note that lines {e.g., the group near 4680) which Merton* 

 found abnormally weak in a mixture of helium and hydrogen 

 occur quite strongly in the positive rays. Unfortunately, 

 the region examined does not contain any of the lines which 

 Merton found intensified under his conditions. 



* Proc. Ro^y. Soc. A, vol. xcvi. p. 382. 



