1896.] Argon and Helium submitted to Electric Discharge. 2G7 



ened, and then faded out, while the argon spectrum persisted after 

 all helium had disappeared. 



Helium, 25 p. c. at 3*49 mm. pressure. Helium invisible. 



„ ,, „ 0"58 „ Still invisible. 



,, „ „ 0*09 „ D 3 very faint 



,, ,, ,, 0*015 „ Stronger. 



It therefore requires a low, but not a too low, pressure, to render 

 25 per cent, of helium visible in argon ; for on still further reducing 

 pressure the helium disappeared, leaving the argon spectrum visible, 

 though faint. 



The seventh set of experiments deals with argon and nitrogen. 



Nitrogen, 14*3 p. c. at 1"02 mm. pressure. Both easily visible. 



„ 2-6 „ 1*04 ,, Nitrogen strong. 



2-6 „ 0-17 ., Nitrogen faint. 



0-42 „ 1-05 „ Nitrogen visible. 



„ 0'42" ,, 0-17 „ Nitrogen gone. 



„ 0'08 „ 1'05 „ Nitrogen just visible. 



0'08 „ 018 „ Nitrogen gone. 



One part of nitrogeu is therefore visible in 1,250 parts of argon at 

 a pressure of 1 mm. ; but, on reducing pressure, its spectrum fades. 



The eighth set of experiments were directed to ascertain how little 

 argon is visible in nitrogen. With 37 per cent, of argon, the reds at 

 the least refrangible end of the spectrum were still visible but very 

 faint; the green group was absent; and the conspicuous orange 

 could not be distinguished on account of the nitrogen bands. After 

 exhausting by several strokes of the pump, the reds became more 

 distinct, but the other lines were not seen, on account of the inter- 

 ference of the brilliant nitrogen spectrum. It is doubtful whether 

 argon would be seen if a lower percentage were present. 



The ninth set of experiments were undertaken to see whether the 

 presence of oxygen affected the spectrum of argon. With 2*3 per 

 cent, of argon, at a pressure of 1'04 mm., the argon reds were 

 invisible, and the brilliant green group appeared as a luminosity, but 

 could not be distinguished as separate lines. The effect of oxygen is 

 greatly to decrease the luminosity of the argou spectrum, so that it 

 is difficult to tell whether it is visible or not. Even a very small 

 quantity of oxygen has this effect. The reduction of pressure has no 

 effect in intensifying the spectrum. It gradually grows fainter, and 

 disappears. 



The reduction of pressure alters the spectrum of pure helium in a 

 very remarkable way. The yellow line becomes less brilliant in 

 comparison with the green. This is not produced, as stated by Pro- 

 fessor Runge at Ipswich, by diffusion of one constituent of helium 



t 2 



