38 COHESION. 



together in the form of an elliptical bridge, so 

 do the different particles of iron which constitute 

 this nail hold together and make it one. And 

 here is a bar of iron ; why, it is only because 

 the different parts of this iron are so wrought 

 as to keep close together by the attraction be- 

 tween the particles that it is held together in 

 one mass. It is kept together, in fact, merely 

 by the attraction of one particle to another, and 

 that is the point I want now to illustrate. If I 

 take a piece of flint and strike it with a hammer 

 and break it thus [breaking off a piece of the 

 flint], I have done nothing more than separate 

 the particles which compose these two pieces so 

 far apart, that their attraction is too weak to 

 cause them to hold together, and it is only for 

 that reason that there are now two pieces in the 

 place of one. I will show you an experiment 

 to prove that this attraction does still exist in 

 those particles, for here is a piece of glass (for 

 what was true cf the flint and the bar of iron is 

 true of the piece of glass, and is true of every 

 other solid, they are all held together in the lump 

 by the attraction between their parts), and I can 

 show you the attraction between its separate 

 particles, for if I take these portions of glass 



