738 



JOHN JOHNSTON 



which shows the fields of stability of the several forms and the 

 several melting and transformation curves. From the diagram it 

 is evident that if we keep the temperature constant at — 20°, and 

 increase the pressure gradually, the following sequence of events 

 will occur: at 2,000 atm. ice I will melt, but the liquid will freeze 

 again to ice III at 2,500 atm. ; this in turn will pass over into ice V 

 at 3,500 atm. which at 6,300 atm. will pass, over into ice VI. At 

 any temperature above 0°, no solid would appear until the pressure 

 exceeded 6,400 atm. when ice VI would appear at a pressure depend- 

 ing on the temperature, e.g., 20,000 atm. at +73°. 



TABLE II 



Numerical Data Pertaining to the Equilibrium Diagram for Water — 

 Liquid and Five Solid Forms* (after Bridgman) 



Pressure kg. per sq. cm. 



Corresponding 

 Temperature 



Phases in Equilibrium 



I 

 1,000 

 2,000 



2,115 



2,170 



3,510 

 3,530 

 4,500 

 5,500 

 6,380 



8,000 

 12,000 

 16,000 

 20,000 



0.0 



— 8.8 

 -20.15 



— 22.0 

 -34-7 

 -24-3 



— 17.0 



— 10.2 



— 4.2 

 + 0.16 

 + 12.8 



+37-9 

 +57-2 

 + 73-6 



Freezing-point: ice I-liquid 



Triple point {A): ice I-ice III — liquid 



" (5): " « —ice II 



" (C): icelll-iceV— icell 



" " {D): " " —liquid 



Freezing-point: ice V-liquid 



« a u u 



Triple point (E) : ice V-ice VI — liquid 

 Freezing-point: ice Vl-liquid 



* The notation ice IV was not used, as it had already been assigned by Tammann to another reputed 

 form, which differs little from ordinary ice (I): the existence of this form, which was not encountered by 

 Bridgman, is very doubtful. 



Nor must it be supposed that the foregoing example is excep- 

 tional; indeed, it is now certain that polymorphism — i.e., the 

 ability of a single chemical substance to appear in more than one 

 crystalline form — is of frequent occurrence, even without the inter- 

 vention of change of pressure. For example, sulphur exists in at 

 least four forms, silica in at least six forms, etc. Now the form 

 stable under high pressure may persist at ordinary pressure, just 

 as high-temperature forms may persist at ordinary temperature; 

 we may expect, therefore, that extended pressure investigations 



