28 



phosphorus lacks any connection with that of tiie violet and the 

 red niodificalion. Precisely the same holds for cyane and paracyane, 

 and what is most remarkable about it, is that what follows only 

 indirectly from experiment for phosphorus, may be easily realised 

 for the system cyane and vice versa. The critical point of liquid 

 white phosphorus below the melting point of the red modification 

 is not to be realised, but follows from the vapour tension curve 

 and from the surface tension ; the critical point of cyane on the 

 other hand is easy to observe, and lies at 126. °5 and under a 

 pressure of 58.2 atmospheres. 



On the other hand the melting-point under the vapour pressure 

 of the red phosphorus can be determined, though not easily, at 

 588.°5 and 42 atmospheres, whereas the melting-point of paracyane, 

 which represents the stable modification and is therefore comparable 

 to the red phosphorus above 460^, cannot be realised, because 

 paracyane is decomposed before this temperature is reached. It 

 follows from this that the systems phosphorus and cyane supplement 

 each other in the most convenient way. As I suspected this, I 

 charged Mr. Terwen with the examination of the system cyane hoping 

 to come to a decision in this way as to whether we have to do with 

 internal equilibria for the system cyane, just as for phosphorus. 



The tirst thing required was to have cyane of great purity at our 

 disposal, and it was particularly this part of the investigation that 

 presented ditficulties which were diflicult to surmount. In the prepa- 

 ration of cyane a part is always decomposed, and in consequence 

 of this the product always contains nitrogen. The removal of this 

 nitrogen presented great difficulties, and only after a long laborious 

 process, in which the solid cyane was subjected to repeated fraction- 

 ated sublimation did we finally succeed in obtaining cyane of so 

 great purity that the investigation about the complexity of cyane 

 could be undertaken. The result rewarded the pains. 



Just as for phosphorus the complexity was examined by the 

 determination of curves of solidification under perfectly the same 

 circumstances during the solidification. The difference in the successive 

 experiments consisted only in this that in one case the liquid had 

 been previously slowly cooled down, in the other case rapidly from 

 higher temperature. 



As was expected the result was that in case of slow previous 

 cooling a line of solidification with perfectly horizontal intermediate 

 portion was found, and that this was wanting when the cooling 

 took place rapidly. The initial point of solidification could be raised 

 0°,5 in the latter case. So the result was qualitatively the same as 



