530 PROCEEDINGS OF THE AMERICAN ACADEMY. 



tion, the two quantities determining the characteristic equation. And 

 on an equilibrium line, the neighborhood of a triple point does not 

 cast its shadow before, either by a change in the direction of the equi- 

 librium line, or of the change of volume line, or of the curves of latent 

 heat or internal energy. So far as the data presented in this paper 

 are concerned, there seems to be no way of making the prediction. 

 Nevertheless the conviction remains that if one had a complete de- 

 scription of the internal mechanism, it would be possible to find some 

 criterion as to the possible stability of other configurations. What 

 additional data are needed to give a sufficient knowledge of the 

 jnechanism? The question is an interesting one for investigation. 



The Possibility of Subcooling or Superheating. 



The facts regarding this subject have nearly all been mentioned in- 

 cidentally in the course of the detailed description of the experiments. 

 Although no observations of the possibility of subcooling or superheat- 

 ing were made separately for their own sake, yet the observations 

 collected incidentally on this subject are nearly as numerous as all the 

 other observations together- The effect was of course observed during 

 every measurement of a change of volume, and frequently on other 

 occasions. It was likely to be a very troublesome effect, preventing 

 the appearance of the modification desired, so that at least sufficient 

 familiarity had to be obtained with the slight regularities shown by 

 the apparently confused mass of facts so that the modification of ice 

 desired could be forced to appear. 



First with regard to the solid phases. It has been found possible to 

 go across nearly every one of the boundary lines into the region of 

 instability on the other side. I has been found in the region of III 

 and II ; II in that of I (at low temperatures) ; III in that of I, II, and 

 V ; V in that of III, II, and VI ; and VI in that of V. The only ex- 

 cursion of this kind which has not been found possible is that of II into 

 the region of III or V. This latter reaction always ran immediately 

 on passing the slightest amount into the neighboring country. For 

 the other reactions no fixed limits could be set to the extent by which 

 it was possible to overstep the boundary. The amount depends on the 

 size and shape of the vessel, on the materials in contact with the ice, 

 on the element of time, and on caprice. In general, however, the 

 limits became narrower at high temperatures, as cme would expect. 



With regard to the passing from the solid to the liquid, the experi- 

 ence here is but a verification of the experience of everybody else ; 

 that it is impossible to superheat a crystalline phase with respect to 



