271 



more subtle differences than Hiose, which are assumed between the 

 ordinary isomers. Nevertheless when the idea of "isomery" is taken 

 in a wider sense, also these different kinds of molecules may be 

 classed under this category. 



We cannot say as yet what kinds of molecules occur in the diffe- 

 rent phases of the pure SOj. The molecular size in the vapour 

 phase agrees about with SO,, but it is very well possible that there 

 occur isomer molecules of SO3 at the same time, and it is also 

 possible that there is also a polymer kind of molecules present in 

 small concentration. The kinds of molecules tiiat occur in the gas 

 phase, will also be present in the liquid phase, hence according to 

 the theory of allotropy also in the solid phase, though in a different 

 proportion, when the idea molecular of conception is taken in a 

 wide sense ')• up to now we have been completely in the dark as 

 far as the internal slate of solid SO, is concerned. The measurements 

 of the surface tension can, indeed, extend our knowledge concerning 

 the complexity of the liquid phase somewhat, but we still lack means 

 to decide whether a unary solid phase is a mixed crystal in internal 

 equilibrium or not. 



Contrary to Le Bi.anc's opinion it is not possible to conclude to 

 the molecular size of a substance in the solid state in a solvent 

 from the found mol. weight of this substance. ') 



With a view to supplementing our methods of research with those 

 that make use of Röntgen rays in the hope of learning something 

 more in the end about the more delicate inner state of equili- 

 brium in the solid |)hase, I instituted a department for the Röntgen 

 investigation of the solid substance in my laboratory some years 

 ago. Though the way which I had decided to follow, leads to the 

 typical allotropic substances, it seemed desirable first to examine 

 some simple, but nevertheless very interesting, substances, in which 

 results were to be expected which might be of great importance 

 for getting a clearer insight into the nature of the chemical bond. 

 Accordingly Messrs J. M. Bijvokt and A. Karssen have studied 

 Li, LiH, NaClO,, NaBrO,, in which it was possible to determine 

 the structure and the binding of the particles on definite suppo- 

 sitions. ') Now the investigation of Hgl, has been taken in hand, 

 though we know that by means of this investigation we shall not 

 be able to decide whether the solid phase in a mixed crystal. 



1) Gf. "The Theory of Allotropy" p. 220. 

 ') Loc. cit. 



3) Partly published in These Proc. 23, 644, 1365 (1921); 25, 27 (1922); Zeit- 

 schr. f. Physik. 14, 291 (1923). 



