262 ILLINOIS STATE ACADEMY OF SCIENCE 



Binary systems of similarly constituted molecules may 

 be expected to form mixed crystals. Examples are : 



para chlor nitro benzol — para brom nitro benzol, 

 para brom toluene — para iodo toluene. 

 It is interesting to note that substances having similar 

 structures occupy approximately the same place in the 

 table, thus : — 



Acetone, benzophenone, acetophenone, benzil. 



Pyridin, pyrrol, carbazol. 



Ethers anhydrides, esters. 



SimjDle amines. 



Simple nitro derivitives. 



Simple halogen derivitives. 

 In describing the above method of calculating solubil- 

 ity we have made the assumption that the log N vs. 1/T 

 curves having the ideal slope is known or may be deter- 

 mined. We shall now consider some of the methods of 

 obtaining this ideal slope in cases where it is not known. 



METHODS OF FINDING LATENT HEAT OF FUSION AND 

 IDEAL SLOPE 



The method wdiich has been found to give the most con- 

 sistent results is the following. There is first obtained 

 a complete freezing point curve of the substance in ques- 

 tion, for convenience called A, with some other substance, 

 B, the ideal slope for which is known and the position of 

 which in the chart of relative internal pressures has been 

 determined. In choosing the exact binary system to be 

 used it is necessary that there shall be no molecular com- 

 plexes formed in the solution, and it is convenient to 

 choose as the second component a substance having about 

 the same melting point as that of the substance to be in- 

 vestigated. The factor to be used between solute B and 

 solvent A is first determined by dividing the value of the 

 ideal slope for B into the value experimentally obtained 

 when the substance A, of unknown internal pressure, is 

 used as solvent for B. Having determined this factor, 

 the position of the substance A in the chart may be found. 

 If now the substance A be used as solute B, or any other 

 substance whose position in the chart is known, be used 



