BRIDGMAN. — WATER UNDER PRESSURE. 531 



the liquid. No good reason for this has ever been given, but no excep- 

 tion has ever been found, and it is coming to be regarded as a law of 

 nature. One would think that if there were ever a chance to find an 

 exception it would be here, where the materials are made viscous by the 

 high pressure, and where the reaction must run with increase of volume 

 against the pressure, doing considerable external work. 



On the other hand, it is the easiest possible matter to subcool the 

 liquid with respect to any one of the four solid phases bordering on 

 the domain of the liquid. In fact, it is often a matter of some diffi- 

 culty to start the reaction liquid-solid, superpressures of 1500 or 2000 

 kgm. being sometimes necessary. The amount of superpressure or 

 subcooling necessary to start the teaction is again a matter of caprice. 

 On the VI-L curve, however, where the greatest range was open to 

 observation, there seemed to be in general a tendency for the subcool- 

 ing to increase at high pressures. 



In consequence of the possibility of carr3dng one phase into the 

 region of another, it is possible to prolong certain of the equilibrium 

 lines beyond the triple point into the region of instability beyond, 

 thus realizing equilibrium points between two unstable phases. The 

 equilibrium lines which have thus been extended are : I-L into the 

 region of II, III-L into the region of I (by Tammann, not in the pres- 

 ent work), III-L into the region of V, I-III into the region of II, III-V 

 into the region of II, and VI-L into the region of V. It may also be 

 possible to extend II-III into the region of I, although this was not 

 tried. The only curves which it was found experimentally impossible 

 to extend were II-III into V, II-V into III, and V-L into VI. There 

 seems to be no obvious generalization about the possibility of extend- 

 ing these curves; everything seems to depend on the particular 

 character of the substances in question. There was one plausible 

 generalization from only two instances, which was missed by a very 

 narrow margin, namely that it is impossible to extend a melting curve 

 at the upper end into the region of another solid. The attempt always 

 failed on the V-L curve, and until the very last day failed on the III-L 

 curve. On this very last day, when the change of volume points II- 

 III-V were being found, it was desired to pass from III to V. The 

 greatest difficulty was experienced in doing this. Pressure was in- 

 creased on III at about — 20° to 4500, and the temperature then raised 

 until III began to melt, thus spoiling the generalization. It was finally 

 necessary to lower the temperature to — 40° before V appeared. It is 

 therefore conceivable that there are circumstances when V-L might be 

 extended into VI, or II raised into the region of III or V. 



It may be mentioned that at every triple point there is at least one 



