THE AURORA BOREALIS 221 



At the poles the process is reversed. The argon gases 

 are more heated during their descent than the oxygen and 

 nitrogen, and will escape into neighbouring layers of 

 atmosphere at a greater altitude, on the average, than the 

 latter. The relative rates of diffusion of these gases, too, 

 may play an even more important part in effecting the 

 separation ; the heaviest argon, carbon dioxide, krypton, 

 and xenon will remain in the ascending layer in larger 

 relative proportion than the oxygen, nitrogen, and other 

 lighter gases, and will, therefore, be carried to the upper 

 regions of the atmosphere by the ascending equatorial 

 current ; but in the descending polar current the process 

 would be reversed, for the heavier gases would be carried 

 down in the current with less escape than the lighter 

 ones. The effect of diffusion alone, neglecting the heating 

 or cooling of the gases, would, therefore, be neutralised 

 during each complete circulation. 



But the process of separation which depends on the 

 difference of temperature of the gases, were there only one 

 circulation, would in all probability be productive of very 

 small result; it is to be observed, however, that the effect 

 is cumulative, that the process of concentration of the 

 argon gases in the higher regions of the atmosphere goes 

 on from age to age, and that there may now be an 

 apparently stationary state, when separation of argon 

 gases from oxygen and nitrogen balances mixture by 

 diffusion and the mixing which inevitably accompanies 

 winds. It may be suggested that the greater frequency 

 of auroras during years when the sunspots are large and 

 easily visible may be connected with the higher tempera- 

 ture of such years, as well as with the magnetic disturb- 

 ances which invariably accompany sunspots. 



To sum up : 1. The gases of the argon group are more 

 easily heated and cooled than are oxygen and nitrogen, 

 2. They are cooled more than the latter during their 





