ATOMS AND MOLECULES 57 



light through a prism we have been able to weigh 

 the star from which the light has come ? " 



Some people find it difficult to understand how 

 matter can be particulate and yet continuous — 

 how it can be in motion and yet appear at rest. 

 There is no real difficulty in the conception. Each 

 molecule moves so fast that it is a host in itself 

 and seems ubiquitous. We have a recollection of 

 a small army that put a large army to flight by 

 scattering itself, and by making such a tumult that 

 it seemed a mighty host. And that is what 

 happens in the case of the molecules ; their activity 

 is so intense that they seem to be everywhere at 

 once, and give the impression of solidity and 

 stolidity. In just the same way, the air driven 

 before an avalanche may wipe a village away as 

 we might blow a fly off our hand. In just the 

 same way, falling stars are smashed on the atmos- 

 phere as on an avil. In just the same way, a 

 column of water " only 2 centimetres in diameter, 

 falling through a tube of the height of 500 metres, 

 cannot be broken into by a violent blow from a 

 sabre. The arm is stopped as if by a wall when 

 it arrives at the surface of the liquid. Professor 

 Bernard Brunhes, who witnessed this experiment, 

 is persuaded that if the velocity of the liquid 

 column were sufficient, a cannon-ball would not go 

 through it. A layer of water a few centimetres 



