102 PHYSICAL SCIENCE 



molecules in each case from each other ; any 

 interaction between solvent and solute would not 

 affect the result, and the result therefore cannot be 

 used as evidence for or against such interaction. 



The similarity in pressure-volume laws, then, 

 cannot be regarded as determining the question 

 whether solution is, in its essential nature, 

 chemical or physical. To settle such a problem 

 other evidence must be sought. Very little such 

 evidence is yet available ; what little there is 

 seems rather to favour the chemical view, which 

 regards a solution, say of salt and water, as 

 in some way a chemical compound of these 

 components ; a compound in which the relative 

 proportion between the components can vary 

 continuously between certain wide limits. 



The results in this case are characteristic of 

 the methods of thermodynamic theory as applied 

 in physical science. Thermodynamics is not 

 concerned with the physical 7nodus operandi of 

 the phenomena. It does not involve molecular 

 hypotheses ; it is free from any doubt which 

 accompanies such hypotheses, though it gives 

 less insight into the intimate processes of 

 the phenomena than do successful molecular 

 conceptions. 



In the development of several branches of 

 physics and chemistry two stages can be traced. 

 It has sometimes happened that the earliest 

 theoretical account of a subject has been given 

 from the mechanical or molecular standpoint. In 

 this way a definite working hypothesis has arisen, 

 on the lines of which much investigation has been 

 undertaken. Gradually, however, this preliminary 

 scaffolding has been found to be unnecessary, and 



