VAN'T HOFF'S HYPOTHESIS 37 



energy so liberated appear as kinetic energy without the inter- 

 mediation of heat, electricity, etc.? 



When a solid goes into solution it at once loses the properties 

 characteristic of the solid state. Its particles become mobile, 

 and all the properties dependent on regular molecular arrange- 

 ment disappear. Thus the solid may be optically active 

 or doubly refracting, and the solution quite void of these 

 properties. 



The passage of the substance into solution bears some resem- 

 blance to its passage into the liquid state. A doubly-refracting 

 crystal almost invariably loses its double refraction when it melts ; 

 and most substances which are optically active in the solid state 

 are inactive when fused. 



The substance might conceivably have passed into the gaseous 

 state. Physical chemists are agreed that this is the most prob- 

 able course. They find that for dilute solutions, at any rate, 

 the simple gas laws hold good. 



In order to explain and correlate these gas laws and the phe- 

 nomena of solution, evaporation, 'etc., the Kinetic Theory of the 

 structure of matter has been formulated. The views that have 

 been held regarding the constitution of solutions have been very 

 varied, and since Thermodynamics is too general in its method 

 of treatment to yield a complete answer to the problem, hypotheses, 

 guided and tested by experiment, are accepted. The following 

 theory was first propounded by van't Hoff in 1885, and it has 

 been improved by later physicists. It allows the Second Law of 

 Energetics to be applied with conspicuous ease and clearness 

 to the theoretical investigation of the quantitative relations 

 between the properties of solutions. Matter is regarded as an 

 aggregation of particles (molecules), each of which is perfectly 

 elastic and structurally independent. Between them there exist 

 spaces. 



Two opposite forces are at work on molecules. 



(1) A Cohesive Force. Newton's Law states that every portion 

 of matter attracts every other portion of matter. The stress 

 between them depends on the mass of the particles and the 



m 1 xra 2 

 distance between them. Stress = ^- where m and w 2 are 



the masses of the particles and d the distance between them. 



(2) A Repellent Force (Real Kinetic Energy =|rau 2 ). Every 

 molecule is free to vibrate in a straight line within the limits of 

 the intermolecular spaces. In a solid these spaces are small, 



