BRIDGMAN. — WATER UNDER PRESSURE. 529 



probable that the five discrepant points found on the lower end of the 

 VI-L curve may be due to the presence of some other form of ice than 

 VI. These points were obtained on two separate occasions more than 

 two months apart. Every one of the equilibrium points which have 

 ever been obtained has been plotted in the diagrams, except a few 

 where the temperature control was defective, and which have been 

 mentioned. There are nowhere any points lying so far off the curve 

 as these five. It seems hardly probable that the five worst points 

 should have all been bunched in the same place, all lying consistently 

 on a new curve. The probability of there being a new kind of ice is 

 strengthened by the change of volume measurements made at these 

 same points. These points are shown on Figure 33 for the change of 

 volume VI-L. They lie far below the smooth curve, further than even 

 the wildest of the discarded points, and they also lie consistently on 

 another curve. The probability seems very strong for another modi- 

 fication of ice. This modification, if it does exist, is unstable in the 

 locality found, V being a more stable form. Whether this new form 

 has any region of stability at all is open to question ; there is no neces- 

 sity for it. 



Whether there are still other forms stable at the highest pressures is 

 of course a matter of pure conjecture. No inconsistencies were ever 

 found suggesting in any way the existence of another form in the 

 region studied. The domain of stability of ice VI as found is already 

 five times more extensive than that of any of the other modifications. 

 Furthermore, the course of the freezing curve and of the change of 

 volume curve is such that both of these curves could be extended 

 without difficulty to infinite pressures and temperatures. VI seems at 

 any rate suited to be a final modification. 



In this connection, some comment seems called for as to the possi- 

 bility of predicting new forms. Of course it is well known that there 

 is no such possibility from the equations of thermodynamics alone ; the 

 domain of existence of any form may be extended indefinitely in either 

 direction without running into thermodynamic inconsistencies. But 

 every substance, besides satisfying the identical relations of thermody- 

 namics, also satisfies its own particular characteristic equation. This 

 characteristic equation is determined by the special internal mechanism 

 of the substance in question. It seems a jwiori possible that the 

 approach of a new form should be heralded by some change in the 

 mechanism, which should have its effect on the characteristic equation. 

 But no such effect as this has been noticed. One substance may be 

 extended across the boundary line into the unstable region beyond, 

 with no appreciable change in either the compressibility or the dilata- 



VOL. XLVII. — 34 



