556 



Professor Bewar 



[Jan. 22, 



liquid absorption known as A and B of Frauenhoffer appear now as 

 bands with sharp edges on the less refrangible side, fading away 

 gradually towards the more refrangible, which is just the opposite 

 character to that of the gaseous absorption of the same groups. The 

 change from the gaseous to the liquid state has not caused any material 

 alteration in the general character of the absorption from what it 

 was under high gaseous compression. The question may therefore 

 naturally be put, does Janssen's law expressing the relation of absorp- 

 tion and density in the gaseous state extend to the liquid condition ? 

 This may be answered by calculating what thickness of the liquid at 

 its boiling point, taken as being 800 times denser than the gas at 

 ordinary temperatures, would be required (provided the same law held) 

 to render visible the absorption band in the yellow. The resulting 

 number is about 3 * 4 mm., and this is confirmed by laboratory experi- 

 ments which show that between 3 and 4 mm. thickness of liquid oxygen 

 at ■- 183° is sufficient to cause the appearance of this band. Thus it 

 appears Janssen's law extends to the liquid condition, the square of 

 the density still defining the intensity of the absorption. It is pro- 

 bable that the band spectrum has its origin either in complex mole- 

 cules generated by condensation, or it may originate from encounters 

 between molecules of the ordinary mass which become more frequent 

 when the free path is diminished. The following table gives the 

 results of observations (made with my colleague Prof. Liveing) in 

 order to find the gaseous pressure required to originate definite 

 absorption bands together with some data of liquid absorption. 



The gaseous oxygen in the 1 • 65-metre tube under 85 atmospheres 

 compression appears to be very transparent for violet and ultra-violet 

 up to the wave-length 2745, or about the limit of the magnesium 

 spark spectrum. When the pressure was increased to 140 atmo- 

 spheres the ultra-violet absorption was complete beyond wave-length 

 2704. In the 18-metre tube with the oxygen under 90 atmospheres 

 pressure, a faint absorption band appeared about L of the solar 

 spectrum, a strong one between 3640 and 3600 wave-length, and a 



