DISSOCIATION 61 



the energy content of the reacting substances. Yet, out of soki- 

 tion, molecules can only be resolved into their atoms or dry gases 

 ionised, by the application of considerable external force. The 

 latter phenomenon has been much studied of late years, especially 

 in connection with the passage of X-rays and ultra-violet rays, and 

 it has been found to depend on the frequency of the incident 

 radiation. The former rays knock electrons off the molecules of 

 the oxygen and nitrogen of air 1,000 times more efficiently than the 

 latter rays because their frequency is 1,000 times as great. That is, 

 the energy of escape of electrons from gases is an accurately linear 

 function of the frequency of the incident radiation provided the fre- 

 quency exceeds a certain limit. 



Without this tremendous display of energy, by merely putting a 

 substance in solution electrons are freed. The necessary energy 

 might come from all or any of the heat liberating actions that take 

 place during the process of solution {e.g., heats of hydration and 

 dilution, heat of combination of the anion with an extra valence 

 electron), and the process might be aided by the heat of hydration of 

 the ions as they are set free. The whole subject is bristling with 

 difficulties, and so far explanations can only be regarded as reasoned 

 guesses. 



Water. 



The solutions dealt with above have all been aqueous. Solu- 

 tions with water as the solvent were early recognised as the most 

 important. According to the old Greek philosophers water was 

 " the beginning of all things," Thales said, " All things have their 

 origin in water and return unto the same." Aqueous solutions are 

 fundamental for all biological phenomena. The physical properties 

 of water are in general extreme — their numerical expressions are 

 cither extremely large or extremely small, and usually the former. 

 Its specific heat and its dielectric constant are the highest of any 

 of the more common liquids. Therefore, water should have a very 

 high ionising power as a solvent. 



One has been accustomed to look upon water as a simple inert 

 substance, of the chemical formula H . OH and with a molecular 

 weight of 18. Physical chemists have proved that this conception 

 does not account for all the properties of water. Lewis and also 

 Langmuir, from thermodynamical principles and also from the 

 study of the colligative properties (see p. -il ) of water, have 

 constructed diagrams of the molecide of water. Discussion of 

 this work is somewhat without the bounds of this book. 



In recent years it has been amply demonstrated that a triatomic 

 molecule could not possess the properties of water. For instance, 



