462 Prof. J. C. Maxwell on Molecules. 



In the space between the water is perfectly calm ; and yet 

 two opposite currents of oxygen and of hydrogen must be pass- 

 ing 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, but every now 

 and then striking each other with such violence that the oxygen 

 and hydrogen of the molecules part company and dance about 

 through the crowd, seeking partners which have become disso- 

 ciated in the same way. In ordinary water these exchanges 

 produce, on the whole, no observable effect ; but no sooner does 

 the electromotive force begin to act than it exerts its guiding in- 

 fluence on the unattached molecules, and bends the course of 

 each toward its proper electrode till the moment when, meeting 

 with an unappropriated molecule of the opposite kind, it enters 

 again into a more or less permanent union with it till it is again 

 dissociated by another shock. Electrolysis, therefore, is a kind 

 of diffusion assisted by electromotive force. 



Another branch of molecular science is that which relates 

 to the exchange of molecules between a liquid and a gas. It 

 includes the theory of evaporation and condensation, in which 

 the gas in question is the vapour of the liquid, and also the 

 theory of the absorption of a gas by a liquid of a different sub- 

 stance. The researches of Dr. Andrews on the relations be- 

 tween the liquid and the gaseous state have shown us that 

 though the statements in our elementary text-books may be so 

 neatly expressed as to appear almost self-evident, their true 

 interpretation may involve some principle so profound that, 

 till the right man has laid hold of it, no one ever suspects 

 that any thing is left to be discovered. 



These, then, are some of the fields from which the data of 

 molecular science are gathered. We may divide the ultimate 

 results into three ranks, according to the completeness of our 

 knowledge of them. To the first rank belong the relative masses 

 of the molecules of different gases, and their velocities in metres 

 per second. These data are obtained from experiments on the 

 pressure and density of gases, and are known to a high degree 

 of precision. 



In the second rank we must place the relative size of the mo- 

 lecules of different gases, the length of their mean paths, and 

 the number of collisions in a second. These quantities are de- 

 duced from experiments on the three kinds of diffusion. Their 

 received values must be regarded as rough approximations till 

 the methods of experimenting are greatly improved. 



There is another set of quantities, which we must place in the 

 third rank, because our knowledge of them is neither precise, as 

 in the first rank, nor approximate, as in the second, but is only 



