ASSOCIATION AND DISSOCIATION. 



IT is now almost universally granted that the molecular 

 weights of volatile compounds can be directly deter- 

 mined by measuring their vapour densities. It is only by 

 acceptance of Avogadro's law that chemists bring into re- 

 conciliation with the atomic theory the known facts with 

 reference to combining proportions by weight and by 

 volume, and physicists deduce the same law in simple 

 manner from the kinetic theory of gases. 



Until recently, direct determinations of molecular 

 weights were only possible in the case of volatile com- 

 pounds, and our knowledge of molecular weights was 

 therefore in the main derived from the study of vapour 

 densities. And even now this method is the only one that 

 can be regarded as of general application. For although 

 the researches of Raoult and the generalisation of Van't 

 Hoff with reference to dilute solutions, lead to the view 

 that the dissolved substance in such solutions is in a 

 condition comparable with that of the same substance when 

 vaporised, the consequent application of Avogadro's law 

 to dilute solutions has not been invariably attended with 

 success. Exceptions of a very baffling character are too 

 frequently noticed which still await satisfactory explana- 

 tion, as, for example, that the molecular weights of certain 

 metals, determined by Ramsay from the reduction which 

 they effect in the vapour pressure of mercury, are found 

 to be only about half their usually accepted atomic weights. 

 While this is the case we are therefore compelled to fall 

 back on vapour densities as forming the final criteria in 

 molecular weight determinations. 



But as long as our knowledge of molecular weights 

 is mainly confined to what may be learnt from the study 

 of vapour densities it is obvious that it can only apply 

 in all strictness to matter in the gaseous state. Of the 

 molecules of liquids and solids we could learn little in 

 this way, unless it were possible to show that a change 



