EVOLUTION AXD THEISM. 389 



tate, the conclusion, that the ultimate particles of all kinds of 

 gaseous matter arc constantly darting about in all directions, \\ith 

 enormous rapidity, and impinging not only against each other, 

 but against the walls of any space in which any portion of gas 

 may be enclosed ; the rates of movement of the particles of 

 different gases, and the number of their impacts against each 

 other, being very diverse, though constant for the particles of 

 each gas so long as its conditions remain the same. Thus the 

 particles of hydrogen are moving at the rate of something like 

 seventy miles in a minute, and every particle has an average 

 number of 17,700 millions of collisions with other particles, by 

 each of which its course is changed ; whilst in atmospheric air 

 (in which the mixture of oxygen and nitrogen has become so 

 complete that it behaves itself in this respect like a single gas), 

 the particles have an average velocity of only one-fourth of that 

 of hydrogen, and the number of collisions for each particle is only 

 half as great. Hut though the hypothetical assumption of these 

 molecular movements in the gaseous particles, is said to &quot;explain &quot; 

 all their sensible actions such as their escape from the vessels 

 in which they are imprisoned, and the uniform difTusion of one 

 gas through another it really does nothing more than carry us 

 a step higher in generalization. For supposing we accept this 

 hypothesis as a fundamental fact in physics, the question remains 

 as to the source of the movements, and the nature of the force 

 by which they are sustained. And it does not help us in the 

 least to attribute them to an inherent activity of matter ; seeing 

 that our only conception of that activity is based on observation 

 either of the movements or of the phenomena from which those 

 movements arc inferred ; just as the old notion that &quot; nature 

 abhors a vacuum,&quot; merely expresses the general fact that air or 

 water will rush in to fill a void space, without giving us any 

 understanding of why it does so. 



It is not a little instructive to find that two such masters in 

 the philosophy of science as Clerk Maxwell and Sir John Hcrschel, 

 agreed in the view they took as to the ultima ratio of any attempt 

 to explain the constitution of the universe by the &quot; properties&quot; of 

 its component atoms. For any such attempt as Sir John Her- 

 schel long since pointed out lands us in the conception of a very 



