458 Prof. J. C. Maxwell on Molecules. 



a minute ; and the only wind which approaches this velocity is 

 that which proceeds from the mouth of a cannon. How, then, 

 are you and I able to stand here ? Only because the molecules 

 happen to be flying in different directions, so that those which 

 strike against our backs enable us to support the storm which is 

 beating against our faces. Indeed, if this molecular bombard- 

 ment were to cease even for an instant, our veins would swell, 

 our breath would leave us, and we should literally expire. 



Bat it is not only against us or against the walls of the hall 

 that the molecules are striking. Consider the immense number 

 of them, and the fact that they are flying in every possible di- 

 rection, and you will see that they cannot avoid striking each 

 other. Every time that two molecules come into collision the 

 paths of both are changed and they go off in new directions. 

 * Thus each molecule is continually getting its course altered ; so 

 that, in spite of its great velocity, it may be a long time before it 

 reaches any great distance from the point at which it set out. 



I have here a bottle containing ammonia. Ammonia is a gas 

 which you can recognize by its smell. Its molecules have a 

 velocity of six hundred metres per second ; so that if their course 

 had not been interrupted by striking against the molecules of air 

 in the hall, every one in the most distant gallery would have 

 smelt ammonia before I was able to pronounce the name of the 

 gas. But instead of this, each molecule of ammonia is so jostled 

 about by the molecules of air that it is sometimes going one 

 way and sometimes another, and, like a hare which is always 

 doubling, though it goes a great pace, it makes very little pro- 

 gress. Nevertheless the smell of ammonia is now beginning to 

 be perceptible at some distance from the bottle. The gas does 

 diffuse itself through the air, though the process is a slow one \ 

 and if we could close up every opening of this hall so as to make 

 it air-tight, and leave every thing to itself for some weeks, the 

 ammonia would become uniformly mixed through every part of 

 the air in the hall. 



This property of gases, that they diffuse through each other, 

 was first remarked by Priestley. Dalton showed that it takes 

 place quite independently of any chemical action between the 

 interdiffusing gases. Graham, whose researches were especially 

 directed towards those phenomena which seem to throw light 

 on molecular motions, made a careful study of diffusion, and 

 obtained the first results from which the rate of diffusion could 

 be calculated. 



Still more recently the rates of diffusion of gases into each 

 other have been measured with great precision by Professor 

 Loschmidt, of Vienna. 



He placed the two gases in two similar vertical tubes, the 



