Absorption-specti t um of Oxygen. 289 



It is remarkable that the compounds of oxygen do not show 

 any similar absorptions. Angstrom thought it improbable 

 that oxygen should have a spectrum of such a character, since 

 he failed to obtain an emission spectrum resembling it ; and 

 suggested that the absorptions might be due to carbonic-acid 

 gas or to ozone, or possibly to oxygen in the state in which 

 it becomes fluorescent (Sped. Norm, p. 41). Neither carbonic- 

 acid gas nor nitrous oxide, at a pressure of 50 atmospheres in 

 our tube, show any sensible absorption in the visible spec- 

 trum ; and the absorption of the ultra-violet rays by the latter 

 gas begins at a higher point, namely about X 2450, than that 

 of uncombined oxygen. In fact we see the anomalies of the 

 selective absorption by compounds as compared with that of 

 their elements when we take the case of water, which has a 

 remarkable transparency for those ultra-violet rays for which 

 oxygen is opaque. 



These observations show that all stellar spectra observed in 

 our atmosphere, irrespective of the specific ultra-violet radia- 

 tion of each star, must be limited to wave-lengths not less 

 than X 2700, unless we can devise means to eliminate the atmo- 

 spheric absorption by observations at exceedingly high altitudes. 



Postscript. — Since the foregoing paper was written we have 

 extended our observations to much longer columns of oxygen. 

 A steel tube 18 metres long was fitted with the same quartz 

 ends as had been used with the shorter tube, and with two 

 quartz lenses symmetrically placed inside the tube, one near 

 each end, so that when an arc-lamp was placed about 14 cen- 

 tim. from one end of the tube the image of it was formed on 

 the slit of the spectroscope at the same distance from the 

 other end. 



When the tube was filled with air only at ordinary pressure 

 nc absorptions could be detected, but when the air was replaced 

 by oxygen at the pressure of the atmosphere the absorption of 

 A was just visible, though neither B nor any other absorption- 

 band could be traced. As the pressure of the oxygen was 

 increased A became much darker and more distinct, and B 

 came out sharply defined. The absorption-band about X5785 

 was next seen, and the dark bands about X 6300 and X4770 

 were just visible when the pressure reached 20 atmospheres. 



At a pressure of 30 atmospheres A was very black, B also 

 strong and sharply defined, and the forementioned bands were 

 all quite strong and had the same general characters as when 

 seen through the shorter tube ; the band about X5350 also 

 could be seen, but there was only a bare trace of that in the 

 indigo about X 4470. At 60 atmospheres these last two absorp- 

 tions could be well seen, all the other bands were very strong, 



Phil. Mag. S. 5. Vol. 26. No. 160. Sept. 1888. U 



