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



away from another ; but as lie himself has acknowledged in a private 

 letter, by the canse which we ascribe. From the experiments detailed 

 above, the relative strengthening of the green line may, however, 

 well be due to the persistence of that constituent of the gas which 

 shows the green line ; although at present we hold it as unproved 

 that what is termed helium is a mixture. It may, however, be taken 

 as confirmatory of that view that, for instance, the spectrum of 

 nitrogen is invisible in argon, if 0'42 per cent, of the former is pre- 

 sent at a pressure of 0'17 mm. ; and yet at a pressure of 1'05 mm. as 

 little as 0*08 per cent, of nitrogen is visible. In a mixture of nitrogen 

 and argon, increase of pressure increases the brilliancy of the nitrogen 

 spectrum; and it may be argued that increase of pressure brings out 

 the yellow constituent of helium, leaving the luminosity of the green 

 unchanged. 



The results of these experiments are summed up in the short table 

 which follows. 



Amount of Gas Detectable in a Mixture. 



(1) Helium in hydrogen. 330 p. c. of helium invisible at 



2'61 mm. 



„ „ 1O0 p. c. of helium barely visible at 



lowest pressure. 

 0*001 p. c. of hydrogen visible at all 



pressures. 

 O'Ol p. c. of nitrogen almost invisible. 

 10*0 p. c. of helium difficult to detect. 

 0"06 p. c. still visible at all pressures. 

 33 p. c. invisible at 2"62 mm. pres- 

 sure. 



25*0 p. c. invisible at 0'58 ram. pres- 

 sure. 



0'42 p. c. invisible at 0'17 mm. pres- 

 sure. 



c. invisible at 0*18 mm. pres- 

 sure, though just visible at 

 T05 mm. 



c. barely visible at any pres- 



(2) Hydrogen in helium. 



(3) Nitrogen in helium. 



(4) Helium in nitrogen. 



(5) Argon in helium. 



(6) Helium in argon. 



(7) Nitrogen in argon. 



0-08 



(8) Argon in nitrogen. 37"0 p, 



(9) Argon in oxygen. 2*3 p. c. difficult to distinguish at 



1*04 mm. pressure. 



Broadly stated, these experiments show that while relatively large 

 amounts of helium and argon require to be present in hydrogen and 

 nitrogen before their spectra become visible, an exceedingly minute 

 amount of hydrogen or nitrogen is visible in helium or argon ; and 

 that a rise of pressure increases the visibility of nitrogen, both in 



