76 BRIDGM.VN. 



that the ordinary modifications of KSCN and NH4SCN are not cor- 

 responding modifications. The low temperature form of NH4SCN 

 is monoclinic, and the high temperature form rhombic, whereas the 

 low temperature form of KSCN is rhombic, and the high temperature 

 form belongs to some other system of which all that can be said is that 

 it is doubly refracting. The rhombic forms of the two substances are 

 isomorphous, forming mixed crystals. If XH4SCN and KSCN are 

 really isotrimorphous we would expect another modification (not 

 yet discovered) 6i KSCN at low temperature. Failure to find an- 

 other form of NH4SCN at high pressures is evidently because the 

 lower transition is of the ice type. The absence of the upper transi- 

 tion line for NH4SCN may be explained by supposing that the rhom- 

 bic modification melts before the temperature can be raised to the 

 transition value. 



Potassium Sulfide. — This was Kahlbaum's purest, "zur Analyse," 

 not further purified. Two runs were made; the first at higher pres- 

 sures, and the second at 77 kgni. The quantity used was about 47 gm. 

 For both runs the salt was hammered into an open steel shell with 

 lateral perforations, and pressure was transmitted directly to it by 

 kerosene. K2S is exceedingly hygroscopic; it was necessary to work 

 very rapidly in transferring it to the shell, and it was almost impossible 

 to avoid the collection of some slight amount of moisture from the air. 



It does not seem to have been previously noted that KoS is dimorphic 

 at atmospheric pressure. This is doubtless to be explained by the 

 very slight change of Aolume (accompanied by a very small heat 

 effect), and the inconvenience of making observations at atmospheric 

 pressure because of the extreme hygroscopy. With the apparatus 

 and the quantity used the discontinuity of the piston displacement 

 was only 0.01 inches. Observation of the transition at low pressures 

 is further rendered difficult by the fact that the transition point is 

 fairly near the decomposition point at atmospheric pressure. At 77 

 kgm. efl^ects were observed only 15° above the transition temperature 

 which are probably to be explained by incipient decomposition. 



The transition line must be passed by about 200 kgm. in either 

 direction before the reaction runs. The change of volume is so slight 

 that the pressure does not return to the equilibrium value after the 

 reaction starts, but the reaction runs to completion with exhaustion of 

 one phase, before the pressure can return to the equilibrium. This 

 means that the equilibrium pressures are enclosed within considerably 

 wider limits than is usually the case. The p-t points are, therefore,. 



