BRIDGMAN. — WATER UNDER PRESSURE. 533 



obtain initially. On the V-L curve a succession of A V points was ob- 

 tained without difficulty, V always putting in an appearance when de- 

 sired, separating directly from the liquid with very little subcooling, 

 although V was never obtained directly from the liquid the first time, 

 bat only by way of VI after considerable subcooling. This ability of 



V to separate directly from the water could be retained for several 

 hours at points removed 1000 or 2000 kgm. from the equilibrium 

 curve. The ability was once retained over night, pressure not being 

 far removed from the equilibrium curve. The disposition to react de- 

 pends both on the element of time and on the amount by which pres- 

 sure has been changed from the equilibrium value. This predisposition 

 to react is lost if a third modification has intervened. Thus, on the 

 occasion mentioned, when III was melted at 4500 kgm. in the endeavor 

 to obtain V, only a half hour previously the reaction II-V had been 

 running in either direction with the greatest facility. The subsequent 

 conversion of II into III resulted in the complete loss of disposition of 



V to appear. 



In explanation, there must be some structure in both the liquid and 

 the solid not ordinarily accounted for, some nucleation or aggregation of 

 the molecules left as a heritage from the previous modification, which 

 is particularly adapted to fall back again into the old position. The 

 possibility of such nuclei in the solid must show that the molecules in 

 a crystal are not arranged in the absolutely symmetrical way usually 

 thought of. The fact that these nuclei may persist for some time in 

 the solid does not seem so surprising as the fact that they exist at all. 

 For the liquid, the reverse is the case. The existence of nuclei might 

 be expected ; they are known to exist even in a gas, but that these 

 nuclei may persist for some hours in an assemblage of molecules sup- 

 posed to be in constant interchange with each other might not be ex- 

 pected at first. The disappearance of these nuclei is a matter of 

 extraordinary slowness, considering the usual times involved in the 

 motion of the molecules as a liquid. Doubtless the formation of these 

 nuclei is intimately connected with the freezing of a liquid for the first 

 time. The freezing can start only from one of these nuclei. The for- 

 mation of one of these nuclei in an unimpregnated liquid is a matter 

 of chance, and until the molecules do happen to fall together into the 

 right position the freezing cannot start. 



Reaction Velocity. 



Here again no accurate measurements were made. The velocity 

 depends on too many things to allow quantitative results of value 



