BRIDGMAN. — WATER UNDER PRESSURE. 535 



The explanation does not suggest itself. The mere fact of such 

 rapid reactions between solids is itself sufficiently surprising. It does 

 not seem as if the mechanism could be the same as that of an ordinary 

 chemical reaction. It is as if the molecules changed from one crystal- 

 line arrangement to another by snapping round on their axes, like the 

 supposed molecular magnets of a piece of iron in a magnetic field. 

 The high temperature effect is difficult to account for. Certainly no 

 known viscosity effect has so high a temperature coefficient. 



Some connection seems likely between this high velocity at the 

 triple point and the impossibility of superheating a solid. There is no 

 doubt but that at this point the molecules of the solid have a perfectly 

 astounding possibility of motion. The passage to the liquid may still 

 depend on the chance formation of nuclei in the solid, but the freedom 

 of motion of the molecules in the solid may be so great as to secure 

 the practically instantaneous formation of the proper grouping. This 

 recalls the question proposed a few pages back as to the possibility of 

 predicting the presence of a new phase from the behavior of a single 

 other phase. Here we have a hint as to the possibility of predicting a 

 third phase by an enormous reaction velocity between two others. 



The Compressibility of the Different Forms of Ice. 



These compressibilities have not, with one exception, been directly 

 measured, but it is possible, nevertheless, to obtain some idea as to this 

 magnitude for those varieties of ice which are anywhere in equilibrium 

 with the liquid. This includes all the varieties except II. The com- 

 pressibility of VI has been directly measured above 0°, and the initial 

 compressibility of ice I has been computed. It has been already stated 

 that the data do not possess the requisite accuracy to permit a direct 

 determination of the compressibility of the other forms of ice. The 

 approximate determination comes by finding the actual volume of the 

 different kinds of ice along the equilibrium lines. The volume of 

 water is known along these lines, and also the change of volume when 

 the liquid passes into the solid, so that the actual volume of the solid 

 may be obtained. This volume will change with temperature and 

 pressure along the equilibrium line. The approximation to the com- 

 pressibility is made by assuming that the change of volume due to 

 changes in temperature along an equilibrium line is negligible com- 

 pared with the change due to pressure. The error so introduced may 

 be found at 0° for ice I at atmospheric pressure, where it is 3 per cent. 

 Furthermore, at any point except in the immediate neighborhood of 

 the origin, the thermal expansion of the ice is very probably less than 



