Properties of Pure Substances : Nitrogen. 25 



the discharge ; and in subsequent experiments only variable 

 results were met with. The tube in question was finally sealed 

 off at 9*2 millim., and sparked for six hours without effect. 

 The next day the sparks were made stronger, and sent through 

 the tube for four hours ; still no effect was produced. The 

 tube then cracked on the next attempt to spark it. 



Tube 3. — April 25. Pressure 9*4 millim. In preparing 

 the tube I noticed what I always noticed afterwards, viz., that 

 at about 2-3 millim. the discharge begins by being pink, and 

 then in a minute or two becomes golden, and keeps so. On 

 turning off the spark and reducing the pressure to about 1 

 millim., the same effect is observed. I have noticed this effect 

 in all the tubes prepared since my attention was first directed 

 to it. The sparking of this tube was as follows : — 



April 25. 4 hours with the effective sparks described 

 in the former paper. No effect. 



April 27. 6 hours ditto. No effect. 



April 28. 4 hoars. Sparks of various intensity. No 

 effect. 



April 29. 4 hours. Ditto. Ditto. 



This tube was sound throughout, and the result was con- 

 vincing. It then occurred to me that possibly the mercury 

 used in our gauges might account for the results obtained by 

 Professor Thomson and myself in 1886. At that time we 

 used a gauge fused directly on to the discharge-tube, with 

 perhaps 10 centim. of 2-millim. tubing between the mercurj^ 

 surface and the tube, and with globules of condensed mer- 

 cury hanging to the walls of the connecting- tube almost up 

 to the discharge-tube itself. I remember that the glow often 

 extended down to the mercury. 



Tube 4. — Made with a side tube containing some very pure 

 and dry mercury, so as to imitate our older form of tube. 

 Pressure 8'5 millim. 



June 11. Sparked 3 hours with effective sparks. No 

 effect. 



June 12. Sparked 4 hours with effective sparks. Exa- 

 mined on 15th, when -it showed a just perceptible diminution 

 of pressure. 



June 15. Sparked 8 hours. Effect increased to about 1"2 

 millim. In order to try whether large sparks would reduce 

 the effect, as was formerly found to be the case, I increased 

 the intensity of the discharge, but did not succeed in producing 

 any increase of pressure during half an hour (except that due 

 to rise of temperature). The tube was then carefully exa- 

 mined, and it was found by holding a sheet of white paper 



