1886.] Passage of an Electric Discharge througli Nitrogen, 333 



surface. Then a series of sparks from six very large Leyden jars 

 charged with a Holtz machine were sent through the tube ; at first 

 when the light produced by the sparks was examined by the spec- 

 troscope the hydrogen lines were seen to be very bright, the hydrogen 

 presumably coming out of the electrodes ; as the sparking and pump- 

 ing continued the hydrogen lines diminished in brightness, and we 

 went on sparking alternately with the Holtz and the induction coil 

 until they had disappeared. We may mention in passing that the 

 relative brightness of the hydrogen and nitrogen lines in a mixture 

 of these gases is to a very large extent a question of pressure ; we 

 found that after we had gone on sparking until there were no 

 hydrogen lines visible at a pressure of 8 mm. of mercury, if we 

 pumped out the gas until the pressure was reduced to 2 mm., the 

 hydrogen lines immediately reappeared, and it required a great deal 

 more sparking to get rid of them at this pressure. The lower the pres- 

 sure the more prominent were the hydrogen lines. We went on spark- 

 ing until there were no hydrogen lines visible at a pressure of 8 mm., 

 when the sparks, were produced either by the Holtz or the induction 

 coil, and until there were no hydrogen lines visible at a pressure of 

 2 mm., when the sparks were produced by the induction coil. We 

 never, however, were able to satisfy ourselves that the hydrogen lines 

 were absent when the large sparks from the Holtz were sent through 

 the tube at this pressure, though if they were there they were only 

 very faint. When we had reached this stage the hydrogen lines 

 showed no tendency to reappear when fresh nitrogen was introduced 

 into the tube, showing that the hydrogen came from the electrodes 

 and not from damp in the nitrogen. We found more difficulty in 

 getting the hydrogen out of aluminium terminals than out of platinum 

 ones. When the tube had reached this stage fresh nitrogen was let 

 in and pumped out until the pressure in the tube was the required 

 value, generally 8 mm. of mercury ; the tubes g and h, fig. 3, were then 

 fused off, the gauges being watched all the time to see that there was 

 no influx of air during this operation. When the tube had cooled the 

 difference of level of the fluid in the LM UDe was rea( i by the cathe- 

 tometer. It was then generally left to stand over night, and another 

 reading taken the next day ; except in the few cases when the tube 

 had cracked, the readings were always found to be the same as those 

 taken on the previous evening. The tube was then ready to be 

 experimented on. 



Preparation of the Nitrogen. 



The nitrogen was prepared by passing air over red-hot copper. 

 A porcelain tube about 70 cm. long was placed on a gas furnace, it 

 was filled with copper turnings and copper gauze ; during one-half of 



