442 PROCEEDINGS OF THE AMERICAN ACADE^^5r. 



pressure is to wipe out this abnormality. The manner in which the 

 abnormality disappears with increasing pressure is interesting. Be- 

 yond the disappearance of abnormality, the pressure has been pushed 

 far enough to suggest in one or more particulars what the behavior of 

 any liquid must be under very high pressures. The results with mer- 

 cury did not suggest this so strongly, because the effect of pressure is 

 much less on the properties of mercury than on those of water, so that 

 to produce the effects peculiar to high pressure will require a much 

 higher pressure for mercury than for water. Corresponding to the 

 abnormal behavior of the liquid at low pressures, and probably con- 

 nected with it, the solid also shows abnormal behavior at low pressures, 

 appearing in no less than five allotropic forms. In addition to these 

 five forms of ice with known regions of stability, there may possibly be 

 two others with no domain of stability whatever. All of these forms, 

 except ordinary ice, are more dense than water. With higher pres- 

 sures, accompanying the return of the liquid to normality, the tendency 

 of the solid to take new forms apparently disappears, the modification 

 of ice stable at high pressures giving indications of being the last form, 

 corresponding to the completely normal liquid. Here again the last 

 form of ice has been studied over so wide a range as to suggest what 

 may be the effects peculiar to high pressure for the equilibrium between 

 any normal liquid and its solid. 



The experimental study of the allotropic forms of ice involves the 

 mapping of their regions of stability, by locating the transition curves, 

 whether to the liquid or to some other solid form, and the measure- 

 ment on these transition lines of the change of volume and the latent 

 heat of transformation. Five of the six possible stable triple points 

 have been found, and ten of the eleven possible stable transition lines 

 of equilibrium have been followed. The sixth triple point and the 

 eleventh equilibrium line lie at temperatures so low and at pressures 

 so high that the slowness of the reaction makes them practically impos- 

 sible to determine. 



The methods used gave evidence on other points of interest, such as 

 the variation in the reaction velocity with changes of temperature and 

 pressure, the possibility of subcooling or superheating, and the com- 

 pressibility and dilatation of the solid under pressure. 



The data on these different curves are so numerous and bewil- 

 dering in variety that there is considerable difficulty in choosing the 

 order of presentation. This difficulty is increased by the fact that the 

 paper itself was not planned from the beginning, but has been a growth. 

 The first intention was to measure the isothermal compressibility of 

 water at 0° and room temperature. The existence of a new modifica- 



