1897.] on Properties of Liquid Oxygen. 557 



diffuse band about the solar line O with complete absorption beyond 

 P. The intensity of the absorption in the latter case was, following 

 Janssen, 4J times what it was under the highest pressure in the 

 short tube. From this we should infer that in the liquid state 

 medium thicknesses like a centimetre or two would be transparent to 

 the ultra-violet, but depths of 10 to 20 cm. would become more 

 and more opaque. Actual experiments confirm this suggestion. 



Thus the passage of light through a layer of liquid 3 to 4 mm. 

 thick is sufficient to cause visible absorption in the yellow, while it 

 requires more than five hundred thousand times this thickness of 

 oxygen gas at atmospheric pressure to do the same thing. Provided 

 the density of tbe oxygen gas is much below that corresponding to 

 the atmosphere, then an enormous thickness of gaseous oxygen would 

 be required to cause any visible absorption. This may explain why 

 such a spectrum is not shown in sunlight, quite independently of the 

 earth's atmosphere, provided we assume that any oxygen in the solar 

 atmosphere must have a relatively small density. 



Absorption of Liquid Air. — If the surface of the earth was cooled 

 to below — 200° C. then the atmosphere would liquefy, and the ocean 

 of liquid air would form a depth of about 80 to 35 feet. The actual 

 proportionate depth can be experimentally observed by taking a tube 

 about 52 feet long, or about 3^^th part of the height of the homo- 

 geneous atmosphere, and cooling one end to — 210°, when about 

 f inch of liquid is obtained. Of this liquid air layer, about 6 to 

 7 feet may be taken as the equivalent of the oxygen portion. A 

 question of considerable interest arises as to the effect of the presence 

 of liquid nitrogen on the oxygen absorption ; although nitrogen is 

 colourless yet the dilution of the liquid oxygen in a neutral solvent 

 has altered the concentration of the colour-absorbing medium. In 

 order to examine into this matter Professor Liveing and the author 

 compared the absorption of 1*9 cm. of liquid air with 0*4 cm. of 

 liquid oxygen, or the proportionate thickness of oxygen which the 

 layer of 1 * 9 cm. of liquid air contains. The light which had passed 

 through the latter was, by means of a reflecting prism, brought into 

 the field of view of the spectroscope at the same time with that 

 which had passed through the liquid air. The positions of the lamps 

 were then adjusted so that the brightness of the spectra of those 

 parts where there were no absorption bands was equal in the two 

 spectra. Under these circumstances it was seen that the absorption 

 bands were very much more strongly developed by ' 4 cm. of liquid 

 oxygen than by five times that thickness of liquid air. 



Another sample of liquid air was rapidly mixed with an equal 

 volume of liquid oxygen, and the absorption of this liquid compared 

 as before with that of liquid oxygen. It was seen that the absorption 

 of 2 • 4 cm. of the mixture was much greater than that of • 4 cm. of 

 liquid oxygen. The density of the liquid oxygen in the mixture 

 was, in fact, three times that in pure liquid air, and by an extension 

 of Janssen's law to liquid mixtures the absorption should have been 



