EVOLUTION OF COLLOIDS 133 



discovered that the molecules have united 

 exactly in the ratio of two molecules of water 

 to one molecule of bromide or iodide, respec- 

 tively. 



But this is precisely the proof upon which 

 the whole atomic theory rests, by means of 

 which Dalton proved that in atomic reactions, 

 union occurred always proportionately to 

 atomic weights, or their multiples. If the 

 argument holds for atoms, it is difficult 

 to see why it is to be refused for molecules. 



In this reaction both salt and water are 

 completely saturated compounds, and they 

 unite without any atoms being displaced or 

 set free. Specific molecular affinity is shown 

 by its absence in the case of the sodium 

 chloride, and its presence in the other two 

 cases. Molecular valency is also illustrated 

 by the exact relationship of two molecules to 

 one. The two salts possess an equal molecular 

 valency which is double that of the water 

 molecule. 



The facts as to true chemical union are also 

 demonstrated by the large amount of heat 

 set free when the molecular union between the 

 salts and water takes place. 



The quantitative difference in the amount 

 of molecular chemical affinity is easily shown 

 by measuring the amounts of heat evolved in 



