168 BRIDGMAN. 



decreases numerically, an a, 0, or Cp to one side of the line indicates 

 that on that side of the line the compressibility or the thermal expan- 

 sion or the specific heat is the greater, and the crystalline systems have 

 been indicated in those cases where known by letters. The abbrevia- 

 tions used for the crystalline forms are: C, cubic; Q, tetragonal or 

 quadratic; R, rhombohedric or trigonal, including hexagonal; O, 

 orthorhombic, or rhombic; M, monoclinic; T, triclinic. 



The diagrams bring out the fact, which was also brought out by the 

 general survey, that mere chemical similarity is not sufficient to ensure 

 similarity of phase diagram. A polymorphic change cannot be re- 

 garded, except from certain restricted thermodynamic view points, 

 as a special case of a chemical reaction, but involves different and 

 undoubtedly more mechanism. Similarity of the phase diagram of 

 two substances involves a much more far-reaching correspondence of 

 mechanism than similarity of chemical behavior. There are, as a 

 matter of fact, only two groups of phase diagrams in the collection 

 above which are equivalent. They are RbNOs, CsNOa, TINO3, 

 with a remote possibility of KNO3 on the one hand, and NH4CI, 

 NH4Br, NH4I on the other. In these cases it seems to be a first pre- 

 requisite for similar phase diagrams that the corresponding phases 

 form complete series of mixed crystals in the range of temperature in 

 which corresponding modifications of both pure components are 

 stable. It would seem, however, that thorough going correspondence 

 of phase diagrams demands a higher order of identity than simply 

 ability to form a continuous series of mixed crystals at some one 

 temperature and pressure. A probable example of this is KHSO4 

 and NH4HSO4. It is very likely, although not definitely proved, that 

 the ordinary rhombic form of NH4HSO4 is identical with the form 

 of KHSO4 above 180°, and that at pressures and temperatures in the 

 region of stability of KHS04(I) the two salts form a continuous series 

 of mixed crystals, unless indeed it should chance that one of the melt- 

 ing points is too low. On the other hand, it is conceivable that two 

 substances should have corresponding phases with corresponding 

 diagrams, but, because of special relations of the transition tempera- 

 tures, no region of continuous mixed crystals. For instance, if the 

 entire phase diagram of TINO3 were lifted to higher temperatures so 

 that the transition from trigonal to rhombic takes place at say 160°, 

 there would be no region in which the trigonal forms of CsNOs and 

 TINO3 are completely miscible, but nevertheless the two phase 

 diagrams would be closely corresponding. Whether cases of such 

 large displacements of corresponding transition points occur in nature 

 is a matter for experiment. 



