WHAT THIS WORLD IS MADE OF 



chlorine must have been split in two. They must, 

 therefore, have contained two atoms. If the aver- 

 age diameter of a molecule is about two-tenths of a 

 micro-micron, then the atom cannot be more than 

 half as big. It is likely much smaller. This, 

 though, is hardly more than guess-work. The very 

 existence of an atom is simply an inference, a de- 

 duction. The molecule we may regard as some- 

 thing tangible and real ; the atom is a kind of work- 

 ing hypothesis. 



But even of the molecules, we know, as yet, very 

 little ; and when we come to picture them, there is 

 difficulty enough. For example, water is practi- 

 cally incompressible. We might infer, then, that its 

 molecules touch each other. Evidently, though, 

 they do not occupy all the space, for you may 

 fill a glass brimming full, and then empty into 

 it a considerable quantity of salt or other dis- 

 solvable substance without making the water run 

 over. Did the salt slip in between, or inside the 

 molecules ? We have no mortal idea. 



Moreover, if there are two atoms in the molecule, 

 one would be tempted to think of the molecule as 

 longer one way than another. And there is some 

 reason to think that this is true. Again, it is hard 

 to understand why water freezing in a pitcher over- 

 night should burst it. Most things contract with 

 the cold; water changing to ice expands. There 



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