Foundations of the Universe 249 



one molecule with another is the normal condition of affairs; not 

 one of them is at rest. The reason for this has been worked out, 

 and it is now known that these particles move about because they 

 are being incessantly bombarded by the molecules of the liquid. 

 The molecules cannot, of course, be seen, but the fact of their 

 incessant movement is revealed to the eye by the behaviour of the 

 visible suspended particles. This incessant movement in the 

 world of molecules is called the Brownian movement, and is a 

 striking proof of the reality of molecular motions. 



2 

 The Wonder-World of Atoms 



The exploration of this wonder-world of atoms and molecules 

 by the physicists and chemists of to-day is one of the most impres- 

 sive triumphs of modern science. Quite apart from radium and 

 electrons and other sensational discoveries of recent years, the 

 study of ordinary matter is hardly inferior, either in interest or 

 audacity, to the work of the astronomer. And there is the same 

 foundation in both cases marvellous apparatus, and trains of 

 mathematical reasoning that would have astonished Euclid or 

 Archimedes. Extraordinary, therefore, as are some of the facts 

 and figures we are now going to give in connection with the 

 minuteness of atoms and molecules, let us bear in mind that we 

 owe them to the most solid and severe processes of human thought. 



Yet the principle can in most cases be made so clear that the 

 reader will not be asked to take much on trust. It is, for instance, 

 a matter of common knowledge that gold is soft enough to be 

 beaten into gold leaf. It is a matter of common sense, one hopes, 

 that if you beat a measured cube of gold into a leaf six inches 

 square, the mathematician can tell the thickness of that leaf with- 

 out measuring it. As a matter of fact, a single grain of gold has 

 been beaten into a leaf seventy-five inches square. Now the 

 mathematician can easily find that when a single grain of gold is 

 beaten out to that size, the leaf must be ^sAinr of an inch thick, 



