MOLECULES. 367 



the molecules of air in the hall, everyone 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 mole- 

 cules 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 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 everything 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 shewed that it takes place quite independently 

 of any chemical action between the inter-diffusing 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 can 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 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 undisturbed 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. 



