BRIDGMAN. — WATER UNDER PRESSURE. 557 



maximum melting temperature and a maximum melting pressure are 

 both predicted. That is, in Figure 43, van Laar has the same maxi- 

 mum and the same right-hand vertical tangent as Tammann, but the 

 results differ from Tammann 's in that the minimum and the left-hand 

 vertical tangent cannot exist, or at any rate if they do, they must 

 always lie at temperatures below the absolute zero and at negative 

 pressures. 



These results of van Laar were obtained with the specific assump- 

 tion that the actual volume of the molecules, and so the change of 

 volume when a double molecule passes into two single molecules, is 

 independent of temperature and pressure. This is almost certainly not 

 the case. The value of the compressibility of water at high pressures, 

 the way in which the abnormalities are smoothed out in the neighbor- 

 hood of 0°, and the point of inflection in the A F curve for VI above 0°, 

 all suggest most strongly that the assumption is not true, and further- 

 more that it is not approximately enough true to enable even the gen- 

 eral character of the melting curve to be predicted at high pressures. 



The conclusion of the whole matter seems to be that at high pres- 

 sures, over 10,000 kgm. for water, we have a new effect appearing, 

 probably connected with the compressibility of the atoms. This means 

 that at high pressures the compressibility of the liquid and solid are 

 going to become more and more nearly equal, which will have as a con- 

 sequence that the equilibrium curve will continue rising indefinitely. 



Besides the data just discussed for the liquid-solid curves, we have 

 the corresponding data for the solid-solid curves. There is no theory 

 at present of the equilibrium solid-solid, and the data here only bear 

 out a remark of Roozeboom 22 that different allotropic solids would be 

 expected to show every conceivable relation to each other. Two triple 

 points between three solid phases had been found, l-II-III, and II- 

 III- V. The first of these is of a type already known, but the second is 

 of a type of which, according to Roozeboom, no examples have yet been 

 discovered. This is Roozeboom's sixth type.23 The equilibrium lines 

 are for the most part straight, but this merely indicates that the com- 

 pressibility and thermal dilatation of the solids are nearly constant 

 over the range of temperature and pressure in question, as one might 

 expect. The only curved equilibrium lines are I-III and II-III. In 

 both of these III is involved. But it was to be expected a priori that 

 III is a form of ice with more variable properties than the others, be- 



*2 Roozeboom, Die Heterogenen Gleichwichte, vol. 1, p. 206. (Vieweg, 

 Braunchweig, 1901). 



^' Roozeboom, loc. cit. p. 202. 



