197 



11. At present, however, the explanation given by Mallard 

 for the phenomenon of dimorphism can scarcely be maintained. 



The study of polymorphism in recent times has proved beyond 

 all doubt, that in the case of reversibility of this phenomenon, i.e. if 

 true enantiotropy be present, we have in reality to deal with a true 

 heterogeneous equilibrium between two different phases, which under 

 any given pressure is determined by a definite temperature, generally 

 called the transition-, or inversion-temperature. Above this temperature 

 the one modification is the stabler one, below it the other form; 

 and if no retardation-phenomena occur, the transformation of the 

 one form into the other occurs abruptly, with a specific heat-effect 

 and a change of specific volume. Now in Mallard's explanation of 

 dimorphism, such an abrupt change, accompanied by an appreciable 

 heat-effect, would be hardly conceivable. For if the higher symmetrical 

 form were nothing but a mimetic aggregate of submicroscopical, 

 repeatedly twinned lamellae of lower symmetry, that higher sym- 

 metrical form would, from a thermodynamical standpoint, represent 

 in fact the same phase as the lower symmetrical modification of 

 which it is composed. Therefore, one would expect that the change 

 would neither be accompanied by a considerable heat-effect, nor 

 by an abrupt transition, but rather by a gradual transformation, 

 because the component lamellae, according to Mallard's view, 

 get gradually finer and finer with increase of temperature. In practice 

 this traject may be larger or smaller, and the change may occa- 

 sionally even give the impression of occurring suddenly. Indeed, as 

 far as experience goes, the change of true pseudosymmetrical sub- 

 stances into the higher symmetrical forms, even when it seems to 

 take place instantaneously, is never accompanied by an appreciable 

 heat-effect, nor by a measurable change of specific volume. 



A study of these phenomena from these points of view has been 

 made in several very convincing cases. Thus, the temperature at 

 which the monoclinic, pseudo trigonal uranyl-magnesium-sodium- 

 acetate 1 ): NaMg(U0 2 ) 3 (C 2 H 3 O z ) 9 + 9# 2 is changed into an 

 real trigonal crystal, was determined by Steinmetz to be 28 C., 

 who stated at the same time that the change observed is accom- 

 panied neither by an appreciable dilatometrical, nor by a thermal 

 effect. 



!) G. Wyrouboff, Bull, de la Soc. Miner., 24, 93, (1901); Zeits. f. Kryst., 37, 

 192, (1903). 



