( 614 ) 



Although there are many instances where a similar transformation, 

 as with phosphorus at a low temperature, proceeds exceedingly slowly, 

 the velocity in this case is certainly strikingly small. Even at 200°, 

 when the metastable substance possesses a considerable vapour tension, 

 it is still immeasurably small even though red phosphorus may be 

 present. *) This extraordinary slowness, notwithstanding the considerable 

 heat quantities liberated during the transformation, and the complete 

 alteration of properties caused thereby, have a long time since esta- 

 blished the conviction that the two moditicatious of phosphorus are 

 each other's polymers and that the red one has a much more com- 

 plex molecule than the yellow one, but the real cause of that slowness 

 is not elucidated thereby. 



As regards the question how this condensation takes place, 

 Schenck (I.e.) was the tirst to endeavour to answer this experimentally. 

 On boiling yellow phosphorus with an excess of PBr 8 , he succeeded 

 in changing it to the red modification at 172° with measurable 

 velocity; and from his tirst investigations he concluded that the 

 order of this reaction was a bimolecular one : 



2P 4 ^P 8 . 



This was meant to represent the tirst phase, for Schenck pointed 

 out thai red phosphorus had no doubt a higher molecular weight 

 than P 8 , which subsequent condensation should then take place with 

 great velocity; in other words he arrived at the rather improbable 

 result that the condensation of P 8 to P„ would take place much 

 more rapidly than that of the simple P 4 molecules to P 8 . 



At a repetition of these measurements with one of his pupils 

 (E. Buck), they came indeed to the conclusion that the reaction is 

 monomolecular (B. Oh. G. 1903 p. 5208). He remarks "Daraus 

 geht mit Sicherheit hervor, dass die Reaction der Umwandlung des 

 weissen Phosphors in rothen monomolekular verlauft." 



He, however, adds "Daraus könnte man den Schluss ziehen, dass die 

 Molekular-gewichte des weissen und rothen Phosphors identisch sind." 



It strikes me that Schenck arrives here at a less happy conclusion. 

 From the occurrence of a mono-molecular reaction we need not 

 necessarily come to the conclusion that the entire process proceeds 

 in this manner. 



l ) Roozeboom (I.e.) compares this to the retardation of the crystallisation of 

 strongly undercooled fusions as 200° is more than 400° below the melting point 

 of red phosphorus: I am, however, of opinion that this view is untenable on 

 account of the relatively high temperature, and particularly the very great mobility 

 of the yellow phosphorus (Roozeboom I.e. p. 89). The cause of the phenomena 

 must be looked for elsewhere. 



