288 Profs. Liveing and Dewar on the 



us a little above D was resolved into fine lines when the sun 

 was high, but appeared as a continuous band when the sun 

 was near the horizon. 



On letting down the pressure the bands were all weakened ; 



A, though weaker, became more sharply defined at the more 

 refrangible edge. The faint band in the indigo \4470 

 remained just visible until the pressure fell below 110 atmo- 

 spheres. At 90 atmospheres A and B were still well seen and 

 sharp, but all the other bands weaker. B remained visible 

 until the pressure fell to 40 atmospheres. A was then still 

 well seen, the band just above D very faint, and the others 

 almost gone. At 30 atmospheres A was still easily seen, and 

 there was a trace of the band above D. At 25 atmospheres this 

 band had gone, but A remained visible until the pressure fell 

 to less than 20 atmospheres. Hence an amount of oxygen not 

 greater than that contained in a column of air 150 metres long 

 at ordinary pressure is sufficient to produce a visible absorp- 

 tion at A. The quantity of oxygen in the tube at the 

 highest pressure we used falls, however, far short of the quan- 

 tity traversed by the solar rays in passing through the 

 atmosphere when the sun is vertical. 



It will be noted that the bands, if we except the faint two 

 in the green and indigo respectively, appear to be identical 

 with those terrestrial bands in the solar spectrum which 

 Angstrom found to be as strong when the air was dried by 

 intense frost as at other times. At least the positions of the 

 maxima agree closely, and that near D shows the same pecu- 

 liarity in having its maximum near the less refrangible end. 

 We did not, however, observe a, which would be fainter than 



B, and if, like A and B, unresolvable, would be lost in the 

 diffuse band which covers that region. The bands above 

 numbered 3, 4, 5, 6 agree also with those observed by 

 Olszewski (Wied. Ann. xxxiii. p. 570) to be produced by a 

 layer of liquid oxygen 7 millim. thick. The point also at 

 which the absorption of the ultra-violet rays begins agrees 

 with that at which the absorption by ozone begins, as ob- 

 served by Hartley (Journ. Chem. Soc. xxxix. p. 57); but the 

 oxygen, as we used it, did not appear to transmit the more 

 refrangible rays beyond 2320 which seem to pass through 

 ozone. Egoroff (Comptes Rendus, ci. p. 1144) found that A 

 remained visible when he looked through 80 metres of atmo- 

 sphere, but 3 kilog. of atmosphere failed to produce a. 



When the pressure in our tube was reduced, a cloud was 

 always formed which rendered the contents of the tube nearly 

 opaque ; the faint light which was then transmitted had 

 always a green tinge. 



