PAPERS OX CHEMISTRY AND PHYSICS 261 



ing solvents. In this connection the question may be 

 asked, What mixtures of substances will form immiscible 

 liquids? It has been observed that when the difference 

 in internal pressures of the substances is so great that 

 the factor connecting them in the above chart has a 

 greater value than about 4.0 — 4,5 partially miscible liq- 

 uids may be expected to occur. In general the higher 

 the melting point of the components the greater must be 

 the difference in internal pressure to cause a separation 

 into two liquid phases. "When the factor between two 

 components is greater than 5.0 — 5.5 the substances may 

 be considered as practically immiscible. Mixtures of 

 two compounds containing enolic or ketonic oxygen mav 

 not follow this iiile due to the foi*mation of oxouium 

 compounds. 



In making the calculations involved in constructing 

 this table some generalizations have been obser^'ed 

 which, although probably well known, will be valuable in 

 placing other substances in the table and also in showing 

 the uses and limitations of tiiis method of calculating 

 solubility. 



It has been found that structural isomers very seldom 

 form mixed crystals or chemical compounds: pheuan- 

 threne and anthrecene, however, do form an unbroken, 

 series of mixed crystals. 



Structural isomers generally have very nearly the 

 same internal pressures ; hence the freezing point curves 

 between isomers are useful in determining the ''ideal 

 slope". Eesorcin and hydrochinone are apparently ex- 

 ceptions to this rule. 



It has been observed that when substances containing 

 hydroxyl groups enter into systems containing enolic or 

 ketonic oxygen, positive deviations from Eaoult's law 

 may be expected to occur, i. e., the solubility will be 

 greater than that calculated from the principles outlined 

 above. 



A typical exception to this rule is foimd in the sys- 

 tem Eesorcin- Water, the log X vs. 1/T curve for which 

 is almost a straight line. 



