ATOMS AND MOLECULES 59 



tain different kinds of molecules, therefore different kinds of 

 substance; but the atoms are not changed in the operation, 

 and we know no method of changing them. 



You may well ask, how can we get hold of molecules in 

 order to study them? It is impossible to catch a single mole- 

 cule and examine it, or even a hundred molecules; but we have 

 every reason to believe that in gases the molecules behave 

 more independently than they do in liquids or in solids. 

 Hence when we wish to study molecules we begin by studying 

 gases. Imagine a big globe filled with inconceivably small 

 pellets that shoot to and fro in straight lines and never come 

 to rest, and you have a notion of our conception of a vessel 

 filled with a gas. Whenever one of these molecules hits the 

 side of the vessel it pushes against it and tries to push it out- 

 wards; the sum total of countless impulses of this kind is 

 supposed to constitute the pressure of a gas upon its confining 

 walls. If I stand on a street corner on a windy day, the mole- 

 cules of air strike my face, and I feel this as a direct pressure 

 which seems to me continuous because there are so many 

 billions of molecules striking my face in succession. The 

 more molecules there are in a given space of gas, the more 

 frequently they must hit the walls, and the greater becomes 

 the pressure. The hotter the gas is, the livelier the molecules 

 become; they travel faster; thus we explain the fact that 

 heating increases the pressure of the gas upon its confining 

 walls. 



In the liquid or solid state, the molecules no longer travel 

 around freely in all directions, but hang together, and occupy 

 a limited portion of the vessel into which they are put, so that 

 it is much harder to know what they are really doing. In the 

 gas each molecule is supposed to be acting for itself, in the 

 liquid or solid they must all act together. 



Interesting evidence of the fact that molecules are little 



