540 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Before discussing the shape of the p-v-t surface as given by the deter- 

 minations of compressibility, it may be well to review briefly the known 

 behavior of water, especiallj'' with regard to the differences it shows 

 "when compared with ordinary liquids. 



All ordinary liquids show a decreasing compressibility with rising 

 pressure, the compressibility decreasing faster than the volume, and 

 they also show an increasing compressibility with rising temperature. 

 The mathematical equivalent of this last statement is that the thermal 

 dilatation decreases wdth rising pressure. This normal behavior is 

 exactly as we would expect if we regard the liquid as composed of 

 nuclei of more or less invariable volume, separated by spaces which 

 may be altered in size by pressure and temperature. It is not 

 necessary for a qualitative understanding of the phenomena even to 

 inquire whether these nuclei are subatomic or atomic ; that is, whether 

 the major part of the compression is given by the change of volume of 

 the spaces between the atoms or by changes in volume of the atoms 

 themselves. 



For water the effects are anomalous. The compressibility decreases 

 with rising pressure, as it does for everything else, but with rising 

 temperature the compressibility at first becomes less, passes through a 

 minimum, and then becomes greater again. This minimum is situated 

 at about 50°. The position of the minimum is nearly independent of 

 pressure but the minimum itself becomes less and less pronounced 

 with rising pressure, and at 3000 kgm. has entirely disappeared. 

 Corresponding to this anomalous behavior, the dilatation shows anom- 

 alous behavior with rising pressure, becoming greater with greater pres- 

 sure at tempere.tures below 50°. It has been recognized by Amagat as 

 possible, however, that at temperatures below 50° the dilatation would 

 decrease with rising pressure at pressures sufficiently high. In the im- 

 mediate neighborhood of 0° and atmospheric pressure, there are special 

 anomalies connected with the maximum density point. In particular, 

 the temperature of maximum density, which at atmospheric pressure is 

 at about 4°, is depressed by rising pressure. This depression of the 

 maximum density point is nearly linear with the pressure, and is so 

 rapid that at 300 kgm. it has fallen below the freezing temperature at 

 that pressure. So much has been shown by Amagat, ^^ who worked up 

 to 3000 kgm. The results may be briefly summed up in the state- 

 ment that water, abnormal at low temperatures and pressures, tends 

 to become normal at high temperatures and pressures. 



Now consider the information given by the present data, examining 



" Amagat, loc. cit. 



