18 97.] Projperiies of Liquid Oxygen, 665 



WEEKLY EVENING MEETING, 



Friday, January 22, 1897. 



Sir Frederick Bramwell, Bart. D.C.L. LL.D. F.E.S. 



Honorary Secretary and Vice-President, 

 in tlie Chair. 



Professor Dewar, M.A. LL.D. F.R.S. M.B.I. 



Fullerian Professor of Chemistry E.I. 



Properties of Liquid Oxygen. 



Gaseous Absorption. — During recent years a great deal of research 

 has been directed to the study of what may be called the low tempera- 

 ture absorption spectrum of gaseous and liquid oxygen. It has been 

 shown that gaseous oxygen gives two types of absorption spectrum, one 

 composed of a number of well-defined groups of lines of exquisite sym- 

 metry, like the great groups A and B of the solar spectrum, the other 

 of bands relatively broad and more or less black. The band spectrum is 

 especially marked in gaseous oxygen under high pressure, and Janssen 

 has shown that the intensity of absorption in different columns of 

 gas under different pressure is identical when the length multiplied 

 into the square of the density is the same in each case. The band 

 that is most easily seen is one in the yellow, and, in order just to see 

 it, 18 metres of oxygen under 11 atmospheres pressure (or 11 times 

 the density under ordinary pressure) must be traversed by white light 

 before it enters the spectroscope. From this result and Janssen's law 

 just given, it follows that in order to detect the same band in a 

 column of gaseous oxygen at atmospheric pressure, it would require to 

 be 2178 metres long or about 1^ miles. The question arises what 

 would be the length of an oxygen tube at atmospheric pressure, equi- 

 valent to the absorption of a beam passing vertically through the 

 earth's atmosphere. This problem has been answered by Janssen, 

 who has shown that an oxygen column 172 metres long would have a 

 similar action. It follows at once from this result that the band in 

 the yellow cannot be seen in the spectrum of the midday sun, as 

 it would require a column of oxygen at least twelve times longer in 

 order to make it visible ; but that it ought to be seen provided the 

 sun was observed near the horizon. When the sun is 4° above the 

 horizon, the depth of atmosphere the rays have to penetrate is about 

 twelve times that of the zenithal thickness. This theoretical result 

 Janssen has confirmed by a series of observations made at sunrise 

 in the dry air of the Desert of Sahara. 



Liquid Absorption. — Both types of spectra are well marked in the 

 spectrum of liquid oxygen, the only marked difference being that the 



