The Constitution of Matter 77 



experiments, thereby widening our general knowledge, 

 and, should new facts be discovered by this means, 

 our belief in the stability of the hypothesis becomes 

 more assured. 



The hypothesis framed to account for such pheno- 

 mena as the diffusion of fluid bodies is termed the 

 " Kinetic Molecular Hypothesis ". In this it is assumed 

 that each form of matter consists of extremely minute 

 particles called " molecules " (little masses). In solids, 

 these are supposed to be comparatively closely packed, 

 yet still to be separated from one another by distances 

 which are large when compared with the space occupied 

 by the matter in the molecule ; in liquids, the packing 

 is usually less close ; while in gases the molecules are 

 very widely separated. Further, each molecule is 

 supposed to be in a state of constant movement. 



The movements of the molecules of a solid substance 

 are of a comparatively restricted character. They are 

 supposed to travel in limited paths around certain 

 definite points in the solid and they are never far 

 distant from these points. A mutual attractive force- 

 termed cohesion exists between the molecules and 

 prevents them from separating from one another 

 except under the action of a considerable external 

 force. A solid body consequently offers considerable 

 resistance to change of shape. 



In a liquid substance the molecules have partially 

 lost their cohesive power, although this is still quite 

 appreciable and prevents the volume of a liquid body 

 from being readily affected by the application of 

 external force. The freedom of movement of the 

 molecules is, however, more pronounced than is the 

 case with those of solids. They interchange positions 

 without difficulty, sliding over and around one another, 



