120 THE FITNESS OF THE ENVIRONMENT 



stantly recombining to form molecules and 

 the molecules constantly dissociating once 

 more to form ions. At the same time, noth- 

 ing hinders the union of sodium ions with 

 bromine ions, or of any other pair of positive 

 and negative ions. Accordingly, the solu- 

 tion at once contains not only the three origi- 

 nal salts and the six different varieties of 

 ions, but also the following new molecules : — 

 sodium bromide, NaBr, sodium iodide, Nal, 

 potassium chloride, KC1, potassium iodide, KI, 

 lithium chloride, LiCl, and lithium bromide, 

 LiBr. All nine varieties of molecules and 

 all six of ions are concerned in a complicated 

 system of chemical reactions which are now 

 well understood, the state of equilibrium 

 depending upon known conditions. For in- 

 stance, if the solution be a moderately dilute 

 one and the original substances be present in 

 chemically equivalent quantities, about 90 

 per cent of the material will be in the ionic 

 state, each variety of ions making up about 15 

 per cent of the total, and about 10 per cent 

 will be in the form of molecules, each variety 

 constituting about 1.1 per cent. 



There can be no doubt that ionization 

 plays a great part in determining the char- 

 acteristics of solutions of acids, bases, and 

 salts, and in bringing about the reactions which 



