370 MOLECULES. 



Henoe in a liquid the diffusion of motion from one molecule to another 

 take* 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 

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

 a solution of copper sulphate, while the upper part contains pure water. It 

 has been standing here since Friday, and you see how little progress the blue 

 liquid has made in diffusing itself through the water above. The rate of 

 diffusion of a solution of sugar has been carefully observed by Voit. Comparing 

 hia results with those of Loschmidt on gases, we find that about as much 

 diffusion takes 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 wonderful 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. In the space between, the water is perfectly calm ; and yet t\vu 

 opposite currents of oxygen and of hydrogen must be passing through it. The 

 physical theory of this process has been studied by Clausius, who has given 

 reasons for asserting that in ordinary water the molecules are not only moving, 

 hut every now and then striking each other with such violence that the ox 

 and hydrogen of the molecules part company, and dance about through tlu> 

 crowd, seeking partners which have become dissociated in the same way. In 

 ordinary water these exchanges produce, on the whole, no observable effect; l>ut 

 no sooner does the electromotive force begin to act than it exerts its guiding 

 influence on the unattached molecules, and bends the course of each toward its 

 proper electrode, till the moment when, meeting with an unappropriated molecule 



