HIGH PRESSURE BRIDGMAN. 193 



these corresponding- to the point 10 on the diagram, the water sub- 

 stance will be found to exist in the form of ice VI. Or, again, if 

 the pressure is 2,000 or 2X10 3 kgm. and the temperature -{-20° (point 

 on the diagram 2, 20), then the water substance is in the form of 

 ordinary liquid water; or, thirdly, if the pressure is 1,000 or 10 ; kgm., 

 and the temperature —20° (point on the diagram I, —20), then 

 the water substance is in the form of ice I, the form we are ordinarily 

 familiar with. 



On any of the boundary lines of the regions in figure 2 the two 

 adjacent forms of water substance are in equilibrium with each other, 

 but if the state of the mass be changed slightly so that it is repre- 

 sented by a point within either of the regions, the kind of ice in 

 that region prevails and the other disappears. Thus, let us suppose 

 that there is ordinary ice, ice I, at say — 10° and atmospheric pres- 

 sure, in the apparatus at the beginning of an experiment, then if 

 the pressure be increased (keeping the temperature constant at 

 — 10°) at about 1,000 or 10 3 kgm. ("point I, — 10), the ice melts to 

 water. But if now we continue to increase the pressure, at about 

 4,400 or 4.4X10 3 kgm. (point 44, — 10), the liquid water freezes 

 again to a new kind of ice, ice V, which is denser than water. If 

 we still further increase the pressure, at about 6,300 or 6.3 X10 3 kgm. 

 (point 6.3, — 10), the ice V suddenly changes to ice VI, the volume 

 again decreasing during the change. Or, if we commence at atmos- 

 pheric pressure and —30° (point 0, —30), and increase the pres- 

 sure, we first change ice I (ordinary ice) into ice III, then, on still 

 further increasing the pressure, ice III changes to ice II ; on further 

 increase, II changes to V, and finally V changes to VI. The high 

 temperature to which the curve between ice VI and the liquid runs 

 is of interest; by the application of 20,000 or 20X10 3 kgm. we may 

 freeze water, although it- is nearly boiling hot. 



The manner in which one ice changes into another is truly remark- 

 able. We know that water freezes slowly or that ice melts slowly, but 

 some of these kinds of ice will change into another kind so rapidly 

 that the reaction reminds one of an explosion. For instance, if ice I 

 is changed to ice III at —25°, the reaction takes place so suddenly 

 that it is impossible to follow the change of pressure which takes 

 place after the reaction. On several occasions I have heard a click 

 in the apparatus when the transformation took place, so rapid was it. 

 Still another remarkable thing is that the effect of temperature on 

 the velocity of the reaction is very great indeed. If ice I is cooled to 

 about —50°, the reaction occurs so slowly that it takes hours for its 

 completion. Similar behavior is found also on the curves III-V and 

 V-VI; the reaction from one solid form to another is very rapid 

 indeed at temperatures near the melting temperature, but as the tem- 

 perature is reduced the speed of the reaction becomes very much less. 



