( 698 ) 



follows from the motlei. In fig. 2 PI. IV, let AA' and BB' be a 



j)air of coexisting phases. A higher temperature belongs to AA' than 



to BB', while the pressure at AA' is greater than that at BB'. If 



we extend the fusion line of ice in the direction of falling pressure, 



dp 

 it is probable that for a given negative value of j), -y would change 



its sign and become positive. Those phases for which ^0 would 



dt 



be determined bv DD', EE' and FF'. The critical point water-ice I 



woidd be found at G and would therefore present a negative value. 



PoYNTiNG ^) came to the same conclusion in a different wav. A second 



critical point at positive pressure, which is deduced by Poynting 



and also by Planck ") by linear extrapolation of the variation of 



the latent heat of fusion, which is also given by us, becomes 



impossible by the appearing of the other ice varieties, which we will 



describe now. If we assume, returning to AA, that by rolling the 



common tangent plane to CC in the direction towards BB on the 



water and ice / ridge, we should also touch the ridge ice III 



dp 

 at H'. Then -- would be positive for water and ice ///, in agreement 

 dt 



with Tam:\iann's measurements. If now Ave suppose that ice III is 



non-existent, we may prolong the binodal line AC A! 6" to a little 



over CC' which gives us continually lower temperatures and higher 



pressures. For a given position the tangent plane will now also touch the 



ice II ridge. Hence we obtain a lower temperature T^ for this triple 



point than for the triple point water —ice I— ice III, while the pressure 



is higher for T^ than for T'j. This is also in agreement with Tammann's 



results. In the same wav for water —ice III and water— ice II is -- > 0. 



dt 



According to our model the fusion curve of ice II has a termination 



at higher pressures and temperatures and therefore we have assumed 



that a critical point water — ice II exists. 



Now we consider further the transformation line ice I to ice III. 



According to Tammaxn the heat of transformation from ice I to ice III 



is positive in the neighbourhood of the critical point T= 251° and 



at lower temperatures negative. In order to be in agreement with 



this, the ice I ridge has been given a strong curvature and the ice III 



ridge a weak (see fig. 3 PI. IV where the elevation of the ridges 



from the side of the i]s plane is shown). Hence the course of the 



1) Poynting Phil. Mag. (5). 12. 1881. 



2) Planck. Wied. Ann. Bd. 15 p. 460. 1882. 



