220 ESSAYS BIOGRAPHICAL AND CHEMICAL 



The air, then, as it ascends from equatorial latitudes 

 cools itself in the process ; and from what has been said, 

 it would seem that gases of the argon group would cool 

 more rapidly than the other atmospheric gases, oxygen 

 and nitrogen. Pour prdciser les idtes, as the French say, 

 let us confine our attention to the northern hemisphere, 

 and let us suppose that a vertical partition has been set 

 up in the neighbourhood of the equator, quite permeable 

 to gases, and surrounding the earth much as the wooden 

 frame of a terrestrial globe surrounds the globe. Indeed, 

 if we conceive of that frame as a double one, and the 

 ascending current rising between the walls, we shall 

 realise what is intended. As the upward current gains in 

 height, it falls in temperature ; and during the whole of 

 the ascent the nitrogen and oxygen are passing through 

 the porous diaphragm at a rate greater than that of the 

 argon gases. With increasing height the density of all 

 gases decreases ; their molecules are more widely separated 

 from each other, and interchange of velocity or, what is 

 equivalent, interchange of temperature becomes less rapid ; 

 hence the separation should be a more perfect one the 

 greater the altitude. But, at the same time, the argon 

 gases are not wholly left in the upward current; many 

 molecules will pass the barrier; why should they not 

 return in as great number as they pass ? Because after 

 passing the partition they no longer move upwards with 

 the same velocity as before; the farther they progress 

 towards the north the less inducement to rise, for the 

 temperature of the earth is lower. 



This reasoning is equally applicable if we regard the 

 barrier as removed ; we may mentally surround the earth 

 with an infinity of such barrier rings, parallel to the plane 

 of the equator ; and it will still remain true that the 

 warmer gases will tend to escape in the lower regions 

 of the atmosphere, leaving the cooler gases to ascend. 



