4 DIFFUSION AND OSMOTIC PRESSURE 



and free atoms or new groups of atoms, with new properties, 

 will result, the original compound having been destroyed by 

 the process of separation. Atoms may also unite in such a 

 way that their mutual attractive forces are but partially 

 neutralized, thus forming incomplete parts of molecules, 

 called ions. Under certain special conditions molecules 

 may split into two or more ions, and some of these cases of 

 ionization or dissociation, as the process of splitting is called, 

 have proved very important in the development of the sub- 

 ject of osmotic pressure. In some cases an ion may consist 

 of a single atom which has split off from some molecule. 

 Briefly, then, acording to the atomic theory as now made 

 use of, the nature of any mass is dependent upon that of its 

 component particles, these particles being atoms, molecules, 

 or ions. The same mass may contain, at the same time, all 

 three kinds of particles. 



b) The kinetic theory of matter. According to the 

 kinetic theory, the particles composing any mass, whatever 

 their nature may be, are in constant motion. This necessi- 

 tates their being considered, not as packed closely one 

 against another to make up the mass, but as separated 

 from one another by continuously varying spaces. The 

 continuous motion of the particles is probably for the most 

 part a vibratory motion. They are supposed to move in 

 straight lines and in the same direction until a collision 

 occurs, when they rebound according to the principle of the 

 reflection of moving bodies. It thus becomes necessary to 

 consider, for comparison, the average distance apart of these 

 particles, or their average or mean free path. This has been 

 demonstrated to be much greater than the diameter of a 

 single particle. 



A rough conception of the state of affairs within a mass 

 of matter may be obtained by comparing the mass to a cage 

 of angry bees. The insects in such a cage fly in straight 



