446 



with £1^ is therefore separated thrice, and as tiiis section must 



be counted once more as part of the section with A', </»" has 



degenerated into the snrface *' fonnd above and in I lie four times 

 counted surface ii\ 



§ 12. On each of the straiglit Hues P(2 considered in § 1 lies 

 an involution of associated points, of whicli the double points are 

 situated on L\ If these are associated to each other an involution 

 on h" is obtained. It has been deduced in a (bfterent waj bj Stukm 

 (Die Lehre von den geometrischen Verwandtscliaften, Vol. Ill, p. 409). 

 He proves among others that in this way to each plane section c* 

 of A* a twisted curve o' of order siv and rank sixteen is associated. 



Chemistry. — "On the allolropt/ of the mnmoaium hdliden /." 

 By Dr. F. E. C. Scheffer. (Communicated bv Prof. A. F. 



HOLLEMAN). 



(Communicated in the meeting of .June 26, 1915). 



1. Introdnclion. In tiie literature, in particular in the crystallogra- 

 phical literature, there are a number of papers to be found which lead 

 us to the conclusion that ammonium chloi-ide and ammonium bromide 

 can occur in two different crystalline forms. Thus St.\s ') found that 

 the transparent crystalline mass which deposits from the vapour of 

 subliming ammonium chloride, comes off from the wall when cooled, 

 and becomes opaque ; he also states that the specific weight of the 

 transparent and the opaque ammonium chloride are different. Though 

 Stas does not enter into further details about these phenomena, these 

 experiments would already be sufficient to suggest dimoi-|ihy here. 

 It is remarkable that Stas has evidently succeeded in cooling the 

 transparent ammonium chloride, which accoi'ding to the above is 

 metastable at the ordinary temperature, to room temperature without 

 the conversion taking place, the more so because in the papers that 

 have appeared later no indications are to be found for this possi- 

 bility. GossNER ''), who repeated Stas' sublimation experiment, says 

 that generally conversion sets in already during the sublimation, and 

 the clear crystals can only be |)reserved for a short time. 



Lehmann ') was the first to conclude to dimorphy ; he tried 



') Stas Untersuchungen fiber die Geselze der chemischen Proportionen u. s. w. 

 iibersetzt von Aronstein. S. 55 (1867 . 



2) GossNER, Zeitschr. f. Kryst. 38 110 (1903). 



3) Lehmann, Zeitschr. f. Kryst. 10 321 (1885). 



