288 Mr. W. Crookes. [Jan. 31, 



tube for some time when platinum terminals are sealed in. This is 

 not the only way of purifying the argon. By the kindness of Pro- 

 fessor Ramsay, I was allowed to take some vacuum tubes to his 

 laboratory and there exhaust and fill them with some of his purest 

 argon. On this occasion I simultaneously filled, exhausted, and 

 sealed off two Pliicker tubes, one having platinum and the other 

 aluminium terminals. On testing the gas immediately after they 

 were sealed off, each tube showed the argon spectrum, contaminated 

 by a trace of nitrogen bands. The next day the tube with platinum 

 terminals was unchanged, but that having aluminium terminals 

 showed the pure spectrum of argon, the faint nitrogen bands having 

 entirely disappeared during the night. After an hour's sparking the 

 tube with platinum terminals likewise gave a pure argon spectrum. 



A vacuum tube was filled with pure argon and kept on the pump 

 while observations were made on the spectrum of the gas as exhaus- 

 tion proceeded. The large coil was used with a current of 8'84 amperes 

 and 11 volts, no jar was interposed. At a pressure of 3 mm. the 

 spectrum was that of the pure red glow of argon. This persisted as 

 the exhaustion rose, until, at a pressure of about half a millimeter, 

 flashes of blue light made their appearance. At a quarter of a milli- 

 meter the colour of the ignited gas was pure blue, and the spectrum 

 showed no trace of the red glow. 



An experiment was now made to see if the small quantity of argon 

 normally present in the atmosphere could be detected without previous 

 concentration. Nitrogen was prepared from the atmosphere by burn- 

 ing phosphorus, and was purified in the usual manner. This gas, well 

 dried over phosphoric anhydride, was passed into a vacuum tube, the 

 air washed out by two fillings and exhaustions, and the tube was 

 finally sealed off at a pressure of 52 mm. It was used for photo- 

 graphing the band spectrum of nitrogen on several occasions, and 

 altogether it was exposed to the induction current from the large coil 

 for eight hours before any change was noticed. The last time when 

 photographing its spectrum difficulty was experienced in getting the 

 spark to pass, so I increased the current and intercalated a small 

 jar. The colour immediately changed from the reddish-yellow of 

 nitrogen to the blue of argon, and on applying the spectroscope the 

 lines of argon shone out with scarcely any admixture of nitrogen 

 bands. With great difficulty, and by employing a very small jar, I was 

 able to take one photograph of its spectrum and compare it with the 

 spectrum of argon from Professor Ramsay, both being taken on the 

 same plate. The argon spectra were the same in each. After this it 

 became difficult to illuminate the tube, except by employing a danger- 

 ously large current. Whenever a flash passed it was of a deep blue 

 colour. Assuming that the atmosphere contains 1 per cent, of argon, 

 the 3 mm. of nitrogen originally in the tube would contain 0'03 mm. 



