Prof. J. C. Maxwell on Molecules. 461 



dilated the number of collisions which each must undergo in a 

 second. They are given in the Table, and are reckoned by 

 thousands of millions. No wonder that the travelling power of 

 the swiftest molecule is but small when its course is completely 

 changed thousands of millions of times in a second. 



The three kinds of diffusion also take place in liquids ; but 

 the relation between the rates at which they take place is not so 

 simple as in the case of gases. The dynamical theory of liquids 

 is not so well understood as that of gases ; but the principal dif- 

 ference between a gas and a liquid seems to be that in a gas 

 each molecule spends the greater part of its time in describing 

 its free path, and is for a very small portion of its time engaged 

 in encounters with other molecules ; whereas in a liquid the 

 molecule has hardly any free path, and is always in a state of 

 close encounter with other molecules. 



Hence in a liquid the diffusion of motion from one molecule 

 to another takes place much more rapidly than the diffusion of 

 the molecules themselves, for the same reason that it is more 

 expeditious in a dense crowd to pass on a letter from hand to 

 hand than to give it to a special messenger to work his way 

 through the crowd. I have here a jar, the lower part of which 

 contains a solution of copper sulphate, while the upper part con- 

 tains pure water. It has been standing here since Friday, and 

 you see how little progress the blue liquid has made in diffu- 

 sing itself through the water above. The rate of diffusion of 

 a solution of sugar has been carefully observed by Voit. Com- 

 paring his results with those of Loschmidt on gases, we find 

 that about as much diffusion take's place in a second in gases 

 as requires a day in liquids. 



The rate of diffusion of momentum is also slower in liquids 

 than in gases, but by no means in the same proportion. The 

 same amount of motion takes about ten times as long to subside 

 in water as in air, as you will see by what takes place when I 

 stir these two jars, one containing water and the other air. 

 There is still less difference between the rates at which a rise of 

 temperature is propagated through a liquid and through a gas. 



In solids the molecules are still in motion, but their motions 

 are confined within very narrow limits. Hence the diffusion 

 of matter does not take place in solid bodies, though that of 

 motion and heat takes place very freely. Nevertheless certain 

 liquids can diffuse through colloid solids, such as jelly and gum ; 

 and hydrogen can make its way through iron and palladium. 



We have no time to do more than mention that most won- 

 derful molecular motion which is called electrolysis. Here is an 

 electric current passing through acidulated water, and causing 

 oxygen to appear at one electrode and hydrogen at the other. 



