Foundations of the Universe 247 



of a chemical element. No one has ever seen an atom. Even the 

 wonderful new microscope which has just been invented cannot 

 possibly show us particles of matter which are a million times 

 smaller than the breadth of a hair; for that is the size of atoms. 

 We can weigh them and measure them, though they are invisible, 

 and we know that all matter is composed of them. It is a new 

 discovery that atoms are not indivisible. They consist themselves 

 of still smaller particles, as we shall see. But the atoms exist all 

 the same, and we may still say that they are the bricks of which 

 the material universe is built. 



But if we had some magical glass by means of which we could 

 see into the structure of material things, we should not see the 

 atoms put evenly together as bricks are in a wall. As a rule, two 

 or more atoms first come together to form a larger particle, which 

 we call a "molecule." Single atoms do not, as a rule, exist apart 

 from other atoms ; if a molecule is broken up, the individual atoms 

 seek to unite with other atoms of another kind or amongst them- 

 selves. For example, three atoms of oxygen form what we call 

 ozone; two^atoms of hydrogen uniting with one atom of oxygen 

 form water. It is molecules that form the mass of matter ; a mole- 

 cule, as it has been expressed, is a little building of which atoms 

 are the bricks. 



In this way we get a useful first view of the material things 

 we handle. In a liquid the molecules of the liquid cling together 

 loosely. They remain together as a body, but they roll over and 

 away from each other. There is "cohesion" between them, but it 

 is less powerful than in a solid. Put some water in a kettle over 

 the lighted gas, and presently the tiny molecules of water will 

 rush through the spout in a cloud of steam and scatter over the 

 kitchen. The heat has broken their bond of association and 

 turned the water into something like a gas; though we know that 

 the particles will come together again, as they cool, and form once 

 more drops of water. 



In a gas the molecules have full individual liberty. They 



