448 



that NH^I probably crystallizes pentagonikositelrahedricallj, we sliould 

 arrive by the same train of reasoning as above at the conclusion 

 that H- and /J-ainnionium cliloride ai'e both pentagonikositetrahedrical. 



An entirely ditterent indication for the existence of two modifi- 

 cations has been fonnd by Prof. Zeeman and Hoogenuoom '). In the 

 research of the birefringency of the ammoninin chloride ciond in 

 the electric field it appeared that this can have a different sign and 

 that also the reversal of sign of tiie refraction can be demonstrated. 

 These phenomena are explained l»y the assnmption that the refraction 

 of positive and negative sign mnst be due resp. to the two ammo- 

 nium chloride modifications. 



The above mentioned experiments prove that ammonium chloride 

 occurs in two modifications, but whether we have to do here with 

 enantiotropy or monotropy cannot be inferred from the above. 



Wallace''), however, has shown of late that NH^Cl and NH,Br 

 are enanliotropic. From cooling curves he found the points of transi- 

 tion resp. at 159° and 109°. By the aid of Lehmann's Heating mi- 

 croscope he could directly observe the conversion; besides, dilato- 

 metric determinations furnished a confirmation of these results. For 

 NHJ no transition could be observed. 



2. The question whether ammonium chloride shows allotropy 

 is of importance in connection with Johnson's well-known experiment"), 

 according to which drj' and somewhat moist ammonium chloride 

 have the same vapour pressure, though in the first case the partial 

 dissociation of the vapour in ammonia and hydrochloric acid does 

 not take place. Prof. Abegg, in whose laboratory these experiments 

 were carried out, considered this fact as in contradiction with our 

 views on chemical equilibrium phenomena''). In the discussion of 

 these experiments I proved before that Johnson's experiment leads 

 to the conclusion that the thermodynamic potentials of the solid 

 substance in dry and moist state are different^); I did not venture, 

 then, however, to give an explariation of this dilference in thermo- 

 dynamic potential; especially as the occurrence of allotropy for 

 NH^Cl was not known to me then, and even though NH,C1 were 

 allotropic, the connection with Johnson's experiment would require 

 a separate proof. Besides the possibility did not seem excluded 



1) Zeeman and Hoor.ENBOOM, These Proc. XIV, p. .558 and 1^^ XV, p. 178. 



^) Wallace 1. c. 



'} Johnson, Zeilschr. f physik. Chemie 61. 457. (1908) 



*) Abegg, Zeitschr f. physik. Chemie 61. 455 (1008). 



6) ScHEFFER, Dissertatie Amsterdam 1909. Zeitschr. f physik. Chem. 72. 451. (1910). 



