MOLECULAR AGGREGATION 259 



Hydrogen and nitrogen, on the other hand, do 

 not show any perceptible absorption ; this may be 

 due, as we have said, to the small number of 

 luminous molecules, but if combined with ammonia 

 gas, their absorption and emission become con- 

 siderable. 



All these facts, therefore, are in agreement, that 

 luminosity results from the formation of more or 

 less complex molecular groups. 



The absorption and phosphorescent spectra of 

 oxygen are worthy of special consideration in their 

 intimate bearing upon the question at issue with 

 reference to the formation of molecular groups. 



The absorption spectrum of oxygen has been 

 attributed by Angstrom to ozone or to oxygen 

 in the state in which it becomes phosphorescent, 

 as he failed to discover any other emission spectrum 

 resembling these absorption bands. They appear to 

 be identical with the bands which Schuster has ob- 

 served in the negative glow of oxygen which have 

 been identified by Newall with those of the phos- 

 phorescent glow in certain circumstances. 



The absorption bands exhibit a most remarkable 

 property observed by Jansen. 



The absorption for the A and B lines varies as 

 the thickness and as the density ; but for the rest 

 of the spectrum as the thickness and the square 

 of the density ; so that it is also proportional to 

 the number of molecular collisions, these being 

 proportional to the square of the density. 



Ordinary collisions between molecules, however, 



s 2 



