272 



Tlie investigation \)\ means of Röntgen rajs is by no means so 

 powerful as it is often supposed to be. Thanks to the researclies of 

 Bakhuis Roozeboom and his pupils we have got to nnderstand the 

 behaviour of the mixed crystal phases in binary systems to a great 

 extent, but what does the Röntgen investigation teach us about tliese 

 mixed crystals? 



Let us e.g. take the simple system KCl.KBr, a system of which 

 we know that the solid components are homogeneously mixable in 

 all proportions, and let us now 8up[)08e an arbitrary mixed crystal 

 from this continuous series to be given to a Röntgen analyst. If this 

 investigator is under the impression that he has to do with a solid 

 phase of a simple substance, he will interpret the intensities found 

 in the usual way, and will (ind them in very good agreement with 

 the image of the system that was supposed l)y him to i)e niono- 

 componeulial. For the intensities can only serve as a test of an 

 already assumed model, and as there are still so many factors that 

 are not sufficiently accurately known in the interpretation of these 

 intensities, and because besides there are nearly always some para- 

 meters that have to be chosen so as to suit, a good agreement 

 can be found, even when the supposition is erroneous. 



Partly in consequence of these circumstances, partly in conse- 

 quence of the impossibility to give already now a sharp image of 

 the complexity, as this has also been assumed by me for the solid 

 phase, the Röntgen invesrigation, in its present stage of development, 

 cannot serve as yet for a further elaboration of the theory of 

 allotropy, and it will, no doubt, be still some years before the 

 Röntgen research will be able to throw new light on the inner 

 equilibria, which have already been found in the solid state. 



All the same we have started the Röntgen study of the interesting 

 Hgl,, because we wished in any case to ascertain if any changes occur 

 in the Röntgen spectrum of these compounds in the temperature 

 interval of 130—255°, and, if so, what changes, hoping that some 

 conclusions may be drawn from this with some probability. 



I have thought it necessary to publish the above discussion, because 

 a great many mistaken ideas still prevail in this region. 



When we now return to Le Blanc's investigation, I will remark 

 that he found, among other things, that on cooling of the supercooled 

 liquid below 13.9° solidification suddenly sets in, ' on which the 

 vapour tension ap|)eared to have risen, also after the temporary rise 

 of temperature had disappeared. Hence at the same temperature 

 the solid phase formed presented a higher pressure than the super- 

 cooled liquid, and Le Blanc thought this phenomenon comparable 



