THE THEORY OF MOLECULES. 281 



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 inter- 

 rupted 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. It is like a 

 hare which is always doubling, and, though it goes a great pace, it 

 makes very little progress. 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 am- 

 monia 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 inter-diffusing gases. 

 Graham, whose researches were especially directed toward those phe- 

 nomena which seem to throw light on molecular motions, made a care- 

 ful study of diffusion, and obtained the first results from which the rate 

 of diffusion can be calculated. 



Still more recently, the rates of diffusion of gases into each other 

 have been measured with great precision by Prof. Loschmidt, of 

 Vienna. 



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

 gas being placed above the heavier, so as to avoid the formation of 

 currents. He then opened a sliding-valve, so as to make the two tubes 

 into one, and, after leaving the gases to themselves for an hour or so, 

 he shut the valve, and determined how much of each gas had diffused 

 into the other. 



As most gases are invisible, I shall exhibit gaseous diffusion to you 

 by means of two gases, ammonia and hydrochloric acid, which, when 

 they meet, form a solid product. The ammonia, being the lighter gas, 

 is placed above the hydrochloric acid, with a stratum of air between, 

 but you will soon see that the gases can diffuse through this stratum 

 of air, and produce a cloud of white smoke when they meet. During 

 the whole of this process, no currents or any other visible motion can 

 be detected. Every part of the vessel appears as calm as a jar of un- 

 disturbed air. 



But, according to our theory, the same kind of motion is going on 

 in calm air as in the inter-diffusing gases, the only difference being 

 that we can trace the molecules from one place to another more easily 

 when they are of a different nature from those through which they are 

 diffusing. 



