152 THE ROYAL SOCIETY OF CANADA 



Iodine 



Before giving the results obtained with hydrogen it is desirable 

 to refer briefly to those obtained with iodine. When a projecting 

 stem of the bulb was immersed in a mixture of ice and water, it was 

 found that two distinct types of ring discharge could be obtained. 

 With a spark gap of the order of 1 mm. the ring had a pale yellow 

 appearance, probably the same colour as the chamois yellow which 

 Wood^ describes. Examination with a spectroscope of small dis- 

 persion showed a continuous band extending from red to green, then 

 a wide absorption band, followed by a second apparently continuous 

 band in the blue-violet. As the spark gap was increasedin length the 

 appearance changed abruptly to a pale green ring with an inner border 

 of pink. The spectrum of this second type of discharge showed 

 numerous bright lines, with faint continuous background in the red 

 region. The abrupt change in the appearance of the discharge is 

 doubtless the result of dissociation, the pink border probably corre- 

 sponding to the lesser degree of dissociation one might expect in the 

 weaker electric field nearer the centre of the bulb. Observation with 

 a small direct vision spectroscope directed successively at the green 

 an^l at the pink portions of the ring showed that the lines in the pink 

 were much feebler than in the green. This is what one would expect 

 from a smaller degree of dissociation. 



Hydrogen 



Every precaution was taken to use pure, dry hydrogen. The 

 gas, prepared electrolytically, was passed over hot copper, and for 

 several days was in contact with phosphorus pentoxide before being 

 admitted to the observation bulb. U-tubes, one on either side of the 

 bulb, were immersed in liquid air, while the bulb itself was heated 

 with the flame of a large Meker burner before the admission of hy- 

 drogen. In this way mercury, as well as water vapour, was excluded, 

 although it should be stated that, on two of the three days on which 

 observations were made, traces of mercury X 5461 could be seen. 



At pressures ranging from 0.3 to 0.1 mm. Hg. one readily 

 obtained ring discharges, which, at any given pressure, changed with 

 changing spark gap, in the following manner. Below a minimum 

 gap, which was less, the lower the pressure, but of the order 1 .3 mm. 

 to 1 .8 mm., no ring discharge was obtained. With increasing gap a 

 whitish ring appeared, whose spectrum showed the secondary, feebly 



»R. W. Wood, Researches in Physical Optics, Part II, p. 53, 1919. 



