191 



feection a little bit. The effect will depend on the accidental choice 

 >t tin- place where the pencil of Rontgen-rays pierces the crystal- 

 lite, as was in fact stated in some cases 1 ). As stress or ten 

 >n>ilur<-il iii the plate by slightly compressing it, has no appreciable 

 . as long as the crystal be not internally dislocated by Un- 

 applied, the phenomenon mentioned here can only be caused 

 >v .1 local disarrangement of some of the component lamellae- In 

 tin opinion of the present writer, therefore, there is scarcely room 

 for doubt that the explanation given by Mallard will prove to 

 u> Id in the larger number of cases 2 ). 



11. At present however the explanation given by Mallard 

 >r the phenomenon of dimorphism can scarcely be maintained. 



The study of polymorphism in recent times has proved beyond 

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



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

 leterogeneous equilibrium between two different phases, which 

 mder any given pressure is determined by a definite temperature, 

 generally called the transition-temperature. Above this temperature 

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

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

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

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

 limorphism, such an abrupt change, accompanied by an appreciable 

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

 )rm were nothing but a mimetic aggregate of submicroscopical, 

 jpeatedly twinned lamellae of lower symmetry, that higher sym- 

 ictrical form would, from a thermodynamical standpoint, represent 

 fact the same phase as the lower symmetrical modification by 

 rtiich it is composed. Therefore one would expect that the change 

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

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

 *cause the component larnellae, according to Mallard's view, 

 *et gradually finer and finer with increase of temperature. In practice 

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



1) H. Haga und F. M. Jaeger, loco cit.; e.g. in the case of d-sodium- 

 ammonium-tartrate, parallel to (010). 



2) G. W. Wulff, Zeits. f. Kryst. 17. 592. (1890); G. Wyrouboff, Bull, de la 

 Soc. Miner. 8. 78, 398. (1885); 18. 213, 277. (1890); 14. 215, 233. (1891); Ann. de 

 Chim. et Phys. (6). 8. 340. (1886); etc.; F. Wall6rant, Bull, de la Soc. Miner. 

 24. 155. (1901); 27. 184. (1904). 



