193 



that in ivality in the case of the ammonium-salt a <//M ontinuous 

 (h.iiiLM may be present between 124 and 135 ('.. -<> that a 

 polymorphic change seems to follow immediately tin- tir-t on.-. 



From this it appears that tin- plu noninion ot /Wv.svwn/ 

 must be distinguished from tin case of true polymorphism '); and 

 for tin- explanation of the latter, Mallard's theory cannot then-fore 

 It must however be remarked that pseudosymmetrica] sub- 

 stances can often be changed at higher temperature into new, true 

 polymorphic modifications 2 ): potassium-sulphate is a well-known 

 example of this, it being suddenly changed at about 650 C. into 

 a new, really hexagonal modification. 



Also combinations of polysymmetrical and real polymorphic 

 changes may occur with the same substance *). 



es of this are: propylamine-stanni-chloride*) : (C 3 H 7 . NH^.,SnSl e ; 

 dicthylamine-platini-chloride : [(C 2 H 5 ).,NHo] 2 PtCl 6 ; diethylaminc- 

 stdiuii-chhride: [(CM^).,NH 2 ],,SnCl 6 , which even in it- external 

 habit completely simulates a cubic crystal; tripropylaminc-platini- 

 chloridc: [(C 3 # 7 ) 3 A / 7/] 2 P/C7] ; tetra-ethyl-ammonium-stanni-chloride: 

 [N(C z H & ) 4 ] z SnCl K , where the lamellar structure of the monoclinic, 

 but pseudocubic crystals is very distinctly recognisable ; tetrapropylam- 

 inoiiium-platini-chloride: [(C 3 H 7 ) { N] z PtCl 6 ; and tetramethylammoninm- 

 platini -chloride; [N(CH 3 ) t ] z PtCl^, where however a rather similar case 

 perhaps occurs to that of the isopropylamine-platini-chloride pre- 

 viously mentioned. While in the case of real polymorphism, metastable 

 -tales may occasionally occur under the influence of retardative 

 circumstances, it must be clear from what has been said about poly- 

 symmetrical changes in general, that no such metastable conditions 

 can be here in question. 



12. From these and many other researches it has gradually 

 become clear that even if a crystal be apparently a homogeneous 

 individual, only in rare cases it may be considered as a really homo- 

 geneous thing. According to Mallard's views and those of a number 

 of other investigators, the molecular arrangements which are charac- 



J) J. Beckenkamp, Zeits. f. Kryst. 34. 633. (1901). The author suggests 

 to adopt Mallard's theory in all cases of so-called polymorphism, when- U>th 

 modifications have the same or only unappreciably different sjH-cific t-nivitii'v 



) P. Groth, Einleitung in die Chemische Krystallographie (1904), p. 6. 



) P. Groth, loco cit. p. 7. 



<) A. Ries, Zeits f Kryst. 39. 50, 55, 56, 66, 69, 73. (1904); M. 360. (1902); 

 49. 513. (1911). 



'3 



