POLYMORPHISM 557 



A few years ago Smits put forward a new theory as to the 

 nature of polymorphic modifications. He assumes that all 

 substances which exist in two polymorphic forms contain when 

 in the liquid state two kinds of chemical molecules which we may 

 call x and y in equilibrium . When the liquid crystallises, mixed 

 crystals are formed still containing the two kinds of molecules, 

 but in proportions differing from those previously existing in 

 the liquid. On lowering the temperature the proportions alter 

 both in the crystals and in the liquid until we arrive at a point — 

 the transition point — at which the crystal structure will not 

 allow of any further increase of x at the expense of y ; another 

 kind of crystal structure therefore makes its appearance which 

 will allow of a further proportion of x. 



Polymorphic modifications are therefore space lattices 

 composed of the two different kinds of molecules, each inter- 

 penetrated by the other, the proportion of the one not forming 

 the space lattice varying with the temperature. 



The temperature of transition is that at which the proportion 

 of the one not forming the space lattice takes upon itself the 

 formative function and produces the space lattice, the molecules 

 of the old space lattice now interpenetrating the new one. 



The theory, however, is not supported by much experi- 

 mental evidence and has not received any wide acceptance. 

 Attempts have been made from time to time to lay down 

 rules for distinguishing between polymorphic and isomeric 

 substances. They have generally been empiric and far from 

 satisfactory. Sidgwick has recently suggested a method free 

 from objection depending on the solubility. 



If to a saturated solution of the more soluble of two poly- 

 morphic modifications some of the other modification is added, 

 the concentration of the solution will not increase, since the two 

 forms give the same molecule in solution ; on the contrary it 

 will diminish to a greater or less extent, depending on the 

 difference in solubility of the two forms, although this difference 

 may be too small to measure. If, on the other hand, the two 

 forms are isomeric or tautomeric, the addition of the less 

 soluble form will cause an increase in the concentration of the 

 solution, for chemically different molecules do not seriously 

 influence each other's solubility. As a measure of the con- 

 centration of the solution, Sidgwick determines the cryohydric 

 point, using an ordinary Beckmann apparatus. The cryohydric 



