170 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1915. 



and physical qualities of an element in passing from one state to 

 another is a matter of common knowledge, but it is not for that 

 reason very easy of explanation. These changes are believed to be 

 connected with the average distance which separates one atom or 

 molecule from the other and their rapidity of motion. In the gas 

 or vapor form the molecules are, on an average, so far apart that 

 their mutual attractions are relatively unimportant. With lowering 

 of temperature, the distance and rapidity of motion of the molecules 

 diminish until, under certain conditions, the attraction of the mole- 

 cules for one another predominates, resulting in a much closer pack- 

 ing 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 mole- 

 cules 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 com- 

 pression or extension, it has been supposed that the force between 

 molecules is attractive at large distances, but repulsive at small dis- 

 tances. While we are able to offer a general explanation of the 

 passage of an element from one state to another, a complete expla- 

 nation of such phenomena will only be possible when we Iniow 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 

 unnecessary for the explanation to know anything of the structure 

 of the atom or molecule or of the forces between them. In some 

 investigations, in order to explain some of the more recondite prop- 

 erties 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 a satisfac- 

 tory method of estimating approximately the dimensions of mole- 

 cules 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 as- 

 sumes 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 



