447 



to prove tliis by crystallisation experiments of a mixtnre of amnio- 

 iiiinn fliloride, bromide, and iodide from aqiieons solution. Witii a 

 snilahle choice of the concentrations he succeeded on cooling in 

 obtaining a cubic kind of crystal, which is tiansformed on further 

 cooling to the well-known skeletons, which the chloride and bromide 

 of ammoninm exhibit at the ordinary temperature. It is evident that 

 only the appearance of a transfurinittinn can prove the dimorphy of 

 the halogen salts ; for ammonium chloride ajid bromide have the 

 skeleton form at the oidinary tenijjerature, whereas amnioniiim iodide 

 crystallizes into cubi. From a solution which contains a mixture of 

 the salts, both cubi and skeletons can deposit. According to Lehmann 

 the transformation must be explained in this way that mixed crystals 

 of the skeleton type are converted to cubic mixed crystals, in which 

 then at the same time interchange of substance with the solution 

 will take place. That in mixtures of the three salts two kinds of 

 mixed crystals occur, becomes also probable because of the very 

 close crystallographic re.semblance of NH^Cl and NH,Br, and from 

 the limited miscihility of NH^Cl and NH J, which Gossnek ') observed. 

 According to Krickmeykr ^) NH^Cl and KCl show limited misei- 

 bility. Groth ') expressed the supposition in virtue of this isodimorphy 

 that the crystalline form of the ammonium chloride, which can form 

 at higher tempeiature, would be isomorphous with KCl. Wallace") 

 points out in his treatise that if this supposition is correct, this would 

 lead to a very remarkable conclusion. As KCl belongs to the penta- 

 gonikositetrahedrical class of symmetry, this would also have to be 

 the case for the form of the ammonium chloride, which is meta- 

 stable at the ordinary temperature, and which I shall call the jJ-form 

 in what follows. As, however, «-ammonium chloride also belongs to 

 the same class of symmetry, we should have two modifications with 

 the same crystallographical symmetry. Then we should be obliged 

 in my opinion to seek the difference between the two modifications 

 in a different structure of the molecule. Ammonium chloride and bro- 

 mide would tlien be very suitable examples for a test of Prof. Smits' 

 theory of the allotropy ; according to this theory the phenomenon 

 of allotropy is namely generally explained by the assumption of 

 different kinds of molecules. If we consider that the above mentioned 

 experiments of Lehmann render it probable that NH^Cl and NH,Br 

 can dissolve in NH^I with formation of cubic mixed crystals, and 



1) GossNER, Zeitschr. f. Kryst. 40. 70 (1905). 



2) Krickmeyer, Zeitschr. f. physik. (Jliemie. 21. 72 (1896). 



3) Groth, Ghcm. Kristall. 1. 167. 



<) Wallace, Gentralblalt fur Mineralogie u. s. w. 1910 S. 33, 



