1890.] Argon and Helium submitted to Electric Discharge. 259 

 The mean results are as follow : — 



Oxygen 23 mm. 



Air 33-0 „ 



Hydrogen 390 „ 



Argon 45 ■ 5 „ 



Helium probably 250 or 390 mm. 



The tube was not long enough to obtain measurements with helium. 

 But another tube with permanent electrodes, distant from each other 

 200 mm., gives, not sparks, but a ribbon of flame, due apparently to 

 the luminosity of the first spark remaining until the second has 

 begun its journey. This band is of a blue colour, and does not show 

 the yellow D 3 line very brightly. The other helium Hues are much 

 more brilliant. Indeed, the D 3 line is not a marked feature of the 

 helium spectrum at high pressure, with an unaided Ruhmkorff coil 

 discharge. 



As regards the purity of the samples of gas employed, it is sufficient 

 to mention the method of preparation. The oxygen was made by heat- 

 ing potassium permanganate in a small test-tube ; the air having been 

 expelled until the nitrogen spectrum had faded out, a large test-tube 

 was filled with the gas over mercury, and several times washed out. 

 The third or fourth filling was taken for experiment. During the 

 pumping empty of the experimental tube, it could be seen that at 

 low pressure, none of the characteristic nitrogen bands were visible. 

 The air was ordinary air, free from carbon dioxide, dried by passage 

 through phosphoric anhydride ; all the gases were dried thus on their 

 way into the experimental tube ; and the absence of the hydrogen lines 

 in the case of argon and helium showed water- vapour to be absent. 

 It will afterwards be shown what a small percentage of hydrogen is 

 recognisable. The hydrogen was prepared from charged palladium, 

 and gave a pure spectrum. The argon had been sparked down with 

 oxygen and soda, the excess of oxygen having been removed with hot 

 phosphorus. The helium had been circulated over red-hot magnesium 

 until it gave a pure spectrum. 



It will, we think, be acknowledged that the length of spark in 

 helium is a very remarkable phenomenon. To what is it to be attri- 

 buted ? Any attempt to answer this question must necessarily, in our 

 ignorance of what takes place during passage of discharge through 

 gases, be of a speculative character. And inasmuch as it is probably 

 connected with other properties of helium, we shall postpone our 

 attempt at an explanation until our further experiments have been 

 described. 



After these experiments had been made it was discovered that the 

 spark discharge changes to a ribbon discharge at some definite pres- 

 sure with all gases. The discovery was an accidental one. We wished 



