no 



between the two modifications of' the substance cyane, and that 

 therefore strictly speaking it is not correct to speak of monotropy 

 here. Now, however, we come to another conclusion. It is true the 

 lines for the internal equilibrium in the different phases of cyane 

 and paracyane lack any connection, and yet we know that all 

 kinds of jjlienomena are known which very certainly do point to a 

 connection between cyane and paracyane ; thus e.g. the vapour of 

 paracyane yields cyane on cooling, and from the vapour of liquid 

 or solid cyane, paracyane may be obtained when it is heated to a 

 high temperature. So there is, indeed, a connection, but this connection 

 could not be demonstrated formerly, and this was also the case with 

 phosphorus. The theory of allotropy guides us here again. The 

 systems phosphorus and ci/ane are, as experiment has shown, pseudo- 

 systems, an J for the present it is sufficient to assume that each of 

 these systems is built up of two kinds of molec(des. Now it has 

 been pointed out before that the pseudo-system phosphorus may be 

 expected to belong to the type ether-anthraquinone, and the same 

 thing applies to the pseudo-system cyane. 



If we first discnss the system j)liosphorus it is easy to show that 

 the existence of two critical endpoints in the pseudo-system ctP — /?P 

 enables us to explain the peculiar behaviour of phosphorus in an 

 exceedingly simple way. In Fig. 1 in the first place the P7 -projection 



Fig. 1. 

 of the pseudo-binary system aP — i3P is indicated. The points p and 

 and q denote the critical endpoints and the only thing that makes 

 this 7^, Z'-figure somewhat less simple than that of ether-anthraquinone 

 is this that the three-phase line <]o, which refers to the equilibrium 

 between red mixed crystals, liquid, and vapour, at o passes into 



