( 363 ) 



Chemistry. ''The behaviour of the halogens towards each other ' . 

 By Prof. H. W. Bakhuis Roozeboom. 



If the phase-doctrine in its first period was concerned mainly with 

 the question whether two or more substances in the solid condition 

 give rise to chemical compounds, or mixed crystals, or remain un- 

 changed in the presence of each other, lately it has commenced to. 

 draw conclusions from the form of the melting point lines of the 

 solid mixtures, both for the nature of those solid mixtures and of 

 the liquid mixtures into which they pass, namely whether, and to 

 what extent, compounds occur therein: 



Likewise, the same questions may be answered in regard to liquid 

 and vapour from the equilibrium lines for those two phases, namely 

 boiling point lines or vapour pressure lines. 



The three systems of the best known halogens having now been 

 investigated their mutual behaviour may be surveyed. 



As regards chlorine and iodine, Stortenbeker had proved in 1888 

 that no other compounds occur in the solid condition but ICI3 and 

 ICl. He also showed that it is probable that ICl, on melting, liquefies 

 to a very large extent without dissociation, whilst on the other 

 hand ICl, is almost entirely dissociated into ICl -(- Cl^. 



Miss Karsten has now added to this research by the determination 

 of the boiling point lines. This showed that the liquid and the vapour 

 line approach each other so closely in the vicinity of the composition 

 ICl ^), that the conclusion must be drawn that the dissociation of 

 ICl is also exceedingly small in the vapour, it being already known 

 that it is very large in the case of ICl,. 



From the investigation of Meerum Terwogt ') it has been shown 

 that Br and I form only one compound BrI which in the solid state 

 forms mixed crystals both with Br and I and which on account of 

 the form of the vapour pressure and boiling point lines is largely 

 dissociated in the liquid and gaseous states. 



Finally it now appears from an investigation by Miss Karsten 

 that Chlorine and Bromine only give mixed crystals on cooling and 

 that in a connected series, whilst, in agreement with this no indication 

 for the existence of the compound in the liquid or vapour could be 

 deduced from the form of the boiling point line. 



We, therefore come to the conclusion that ICl, is a feeble and 

 ICl a strong compound. IBr is also a feeble compound and no com- 

 pound exists between CI and Br. The combining power is, therefore, 



1) Still closer than represented in Fig. 7, p. 540. These proceedings [VIII] 1904. 

 *) These proceedings VI, p. 331. 



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Proceedings Royal Acad. Amsterdam. Vol. IX. 



