206 M. A. Bitter's Contributions to the Study 



just like the isobar, it has four angular points and two straight 

 segments, the meaning of which is obvious when we consider the 

 gradual transformation under an isothermal compression of a 

 mass of water out of the state of superheated vapour, first into 

 the solid and then into- the liquid state of aggregation. The an- 

 gular point M corresponds to the passage of the vapour from 

 the superheated to the saturated state ; the straight segment 

 M X exhibits the gradual passage from the vapour to the solid 

 state of aggregation (snow-formation). The point L marks 

 the beginning of melting that only starts under higher pres- 

 sure ; and the straight segment L K represents the gradual pas- 

 sage from the solid into the liquid state of aggregation. 



The isothermals of the ice-region are therefore distinguished 

 from the isothermals for higher temperatures (that are repre- 

 sented in fig. 7 by the dotted line) by having three angular 

 points N, L, K, instead of one, J, as in the latter. We pass 

 from the one group to the other at the isothermal which cor- 

 responds to a temperature higher than 0° C. by 0*00744°; and 

 this isothermal must therefore be counted among those of the 

 ice-region. This limiting isothermal corresponds to the tem- 

 perature (0°*00744 C.) at which water freezes or ice melts 

 under a pressure equal to its vapour-tension (comp. § 5)*. 



Though isothermals may in general be likened to hori- 

 zontal paths on a mountain-face, this comparison is unsuitable 

 in the case of the isothermals of the ice-region as they stretch 

 along the under surface of an overhanging cliff. 



To the temperature 0°*00744 C. corresponds a vapour-ten- 

 sion of 0'006 atmosphere. If we draw the successive isobars 

 (as in fig. 6) for continually smaller pressures, we shall find, as 

 shown later, that for the pressure of 0"006 atmosphere the two 

 straight segments coincide, since each will coincide with the 

 straight segment of the isothermal for o, 00744 C. For still 

 smaller pressures the isobar takes the form in fig. 8. There 

 is but one straight horizontal segment in this line ; and it cor- 

 responds to the direct passage from the vapour into the solid 

 state of aggregation. 



§ 4. Edges of the Temperature-surface. 



Consider the successive isobars drawn as in fig. 6, or the 

 successive isothermals as in fig. 7; the angular points of these 

 lines make up edges of the temperature-surface. These edges 

 bound the three regions on the surface which correspond to 

 Hie three different states of aggregation. These three regions, 

 however, do not directly border on each other, but are sepa- 



* [This peculiarity was first pointed out by Professor PI ticker in the 

 Proceedings of the Royal Society for 1874, p. 457.-— Tn.] 



