218 ESSAYS BIOGRAPHICAL AND CHEMICAL 



freely into one another when left in contact; so that a 

 heavy gas will mix readily with a much lighter one, even 

 though the heavy one may be below and the lighter one 

 above. This diffusion results from the motion of the 

 molecules of gases ; and as the rate of motion depends on 

 the temperature of the gas, those molecules which happen 

 to have a high temperature move much more rapidly than 

 those with a lower. When two gases mix, however, or 

 when a hot gas mixes with a cold one, the more rapidly 

 moving, and therefore hot, molecules very rapidly com- 

 municate their motion to the colder gas, raising its 

 temperature until the temperatures of both sets of mole- 

 cules are equalised. This is due to the enormous number 

 of encounters which take place between the molecules, 

 partly on account of the minute size of each molecule, 

 and the consequent number in even a very small volume ; 

 and partly to the great rate at which they are moving. 

 For example, it can be calculated that in all probability 

 there are 50,000,000,000,000,000 or 50 quadrillion mole- 

 cules of hydrogen in a cubic millimetre of that gas (about 

 the volume of the head of a large pin) and that the 

 average velocity of each molecule is at the rate of 4J miles 

 per second. No wonder, therefore, that the exchange 

 between molecules of different temperature is almost 

 instantaneous. It must nevertheless be borne in mind 

 that hot gases diffuse much more rapidly than cold ones. 

 The densities of the gases constituting the atmosphere 

 are as follows, the standard being that of oxygen taken as 

 16:- 



Water Vapour, 9 0-333 Neon, 10 0-316 



Nitrogen, 14 0'287 Argon, 20 0'224 



Oxygen, 16 0'250 Krypton, 41 0156 



Carbon Dioxide, 22 0*213 Xenon, 64 0125 



Helium, 2 0'707 - 



