266 Dr. Collie and Prof. Ramsay. Behaviour of [Feb. 13, 



to 0*002, and its spectrum could not be detected. Hence some- 

 what less than 1 part of nitrogen in 10,000 of helium can be 

 detected. 



In endeavouring by the fourth set of experiments to establish how 

 small a proportion of helium could be detected in nitrogen, we 

 were unable to recognise tbe D 3 line on account of its coincidence 

 with one of the yellow nitrogen bands. But the helium green is 

 not interfered with, and it was therefore looked for. The results 

 were : — 



Helium, 50*0 p. c. at 1*67 mm. pressure. 



50-0 

 22-0 

 22-0 



22-0 

 12-5 

 125 

 12-5 



0- 28 



1- 28 

 0-21 



0- 03 



1- 16 

 0-19 

 0-03 



Green visible. 

 Easily ,, 

 Green just visible. 

 Yellow just visible ; 



green stronger. 

 Both intensified. 

 Green invisible. 

 Green j ust visible. 

 Yellow 



Hence it may be concluded that it is not easy to detect 10 per 

 cent, of helium in nitrogen, even at low pressures. 



The fifth set of experiments relates to the detection of argon in 

 helium. 



Argon, 50'0 p. c. at 1'67 mm. pressure. 



12-5 

 2-3 

 2-3 



0-76 



0-06 



1-16 

 1-28 

 021 



1-04 



1-04 



Argon bright ; helium faint. 



Argon bright. 



Argon still bright. 



Argon still bright ; the green 



group especially; the reds 



still visible. 

 Argon orange and greens 



faintly visible. 

 Argon still faintly visible. 



It is seen that 1 part of argon in 2,000 of helium is still de- 

 tectable. The orange line and the first green group of the argon 

 spectrum are very persistent. Introduction of a spark-gap here 

 causes the argon spectrum to change to blue, and a number of blue 

 lines become visible, without, however, much interfering with, the 

 brilliancy of the orange and green lines. 



The sixth, set of experiments was directed to ascertain how much 

 helium could be detected in argon. 



Helium, 50 p. c. at T67 mm. pressure. 

 33 „ 2-62 



Both easily visible. 

 Helium barely visible. 



On reducing pressure by pumping, the helium spectrum strength- 



