Foundations of the Universe 251 



than the TITT. 7^.^17 of an inch in diameter. In a single cubic 

 centimetre of air a globule about the size of a small marble 

 there are thirty million trillion molecules. And since the molecule 

 is, as we saw, a group or cluster of atoms, the atom itself is smaller. 

 Atoms, for reasons which we shall see later, differ very greatly 

 from each other in size and weight. It is enough to say that some 

 of them are so small that it would take 400,000,000 of them, in 

 a line, to cover an inch of space ; and that it takes at least a quin- 

 tillion atoms of gold to weigh a single gramme. Five million 

 atoms of helium could be placed in a line across the diameter of a 

 full stop. 



The Energy of Atoms 



And this is only the beginning of the wonders that were done 

 with "ordinary matter," quite apart from radium and its revela- 

 tions, to which we will come presently. Most people have heard 

 of "atomic energy," and the extraordinary things that might be 

 accomplished if we could harness this energy and turn it to human 

 use. A deeper and more wonderful source of this energy has been 

 discovered in the last twenty years, but it is well to realise that 

 the atoms themselves have stupendous energy. The atoms of 

 matter are vibrating or gyrating with extraordinary vigour. The 

 piece of cold iron you hold in your hand, the bit of brick you pick 

 up, or the penny you take from your pocket is a colossal reservoir 

 of energy, since it consists of trillions of moving atoms. To realise 

 the total energy, of course, we should have to witness a transfor- 

 mation such as we do in atoms of radio-active elements, about 

 which we shall have something to say presently. 



If we put a grain of indigo in a glass of water, or a grain 

 of musk in a perfectly still room, we soon realise that molecules 

 travel. Similarly, the fact that gases spread until they fill every 

 ''empty" available space shows definitely that they consist of 

 small particles travelling at great speed. The physicist brings his 

 refined methods to bear oh these things, and he measures the 



