io8 THE POPULAR SCIENCE MONTHLY 



that their mutual attractions are relatively unimportant. With lower- 

 ing of temperature, the distance and rapidity of motion of the mole- 

 cules diminish until under certain conditions, the attraction of the 

 molecules for one another predominates, resulting in a much closer 

 packing, and the appearance of the liquid form. The molecules, how- 

 ever, still retain a certain freedom of motion, but this is diminished 

 with lowering of the temperature until at a certain stage the molecules 

 form a tighter grouping, corresponding to the solid state where the 

 freedom of motion of the individual molecules is much restricted. In 

 order to account for the resistance of solids to compression or exten- 

 sion, it has been supposed that the force between molecules is attractive 

 at large distances but repulsive at small distances. While we are able 

 to offer a general explanation of the passage of an element from one 

 state to another, a complete explanation of such phenomena will only 

 be possible when we know the detailed structure of the atoms and the 

 nature and magnitude of the forces between them. 



While the kinetic theory of gases has proved very successful in ex- 

 plaining the fundamental properties of gases, its strength, and at the 

 same time its weakness, lies in the fact that in most cases it is un- 

 necessary for the explanation to know anything of the structure of the 

 atom or molecule, or of the forces between them. In some investiga- 

 tions, in order to explain some of the more recondite properties of gases, 

 assumptions have been made of definite laws of force between the 

 molecules, but no very definite or certain results have so far been 

 achieved in this direction. It should, however, be pointed out that the 

 kinetic theory afforded us for the first time with a satisfactory method 

 of estimating approximately the dimensions of molecules and the actual 

 number in a given weight of matter. As the recent development of 

 science has provided us with more certain methods of estimation of 

 these important quantities, we shall not enter further into the question 

 at present. 



Crystals 



There is another very striking form that matter sometimes assumes, 

 which has always attracted much attention and which has recently 

 emerged into much prominence. It is well known that the majority 

 of substances under suitable conditions form crystals of definite geo- 

 metrical form, which is characteristic of the particular atoms or groups 

 of atoms. The great variety of crystal forms that are known have 

 all been classified as belonging to one or more of the 230 forms of point 

 symmetry which are theoretically possible. While considerations of 

 symmetry are a sufficient guide to the classification of crystals, they 

 offer no explanation of the definite architecture of the crystal nor of 

 the nature of the forces that cause the atoms or molecules to arrange 

 themselves in such definite geometric patterns. We are inevitably led 



