of States of Aggregation. 207 



rated by three regions of transition (figs. 9, 10, 11). Each of 

 these three regions of transition has the form of a cylindrical 

 surface whose generating lines are parallel to the volume-axis ; 

 viewed, therefore, in this direction, it appears a line (fig. 10). 



As regards the water-edge JW and the steam-edge L D, 

 we may assume that they meet at a certain height (near the 

 isothermal for 410° C, according to Cagniard cle la Tour), or 

 subside into the surface ; for it is exceedingly likely that at 

 very high temperatures there is no difference between the 

 liquid and gaseous states of aggregation*. 



To each point of the temperature-surface correspond defi- 

 nite values of the coordinates _p, v, T ; and by these three 

 values the state of the whole mass is in general unambigu- 

 ously defined. An important exception, however, occurs in 

 the case of those points that lie on the straight segment JKL 

 of the isothermal for 0°'00744 C. or of the isobar for 0*006 

 atmosphere. This segment (which is represented in fig. 10 by 

 the point J) corresponds to those values of the pressure and 

 temperature at which water can simultaneously exist in all 

 three states of aggregation f. If, then, the pressure, tempe- 

 rature, and volume of the whole mass be given by any point 

 on this segment, the internal condition of the mass is not suffi- 

 ciently defined, since a knowledge of the total volume is not 

 sufficient to determine the proportions in which steam, water, 

 and ice are mixed together. As, further, the intrinsic energy 

 (die innere Warme) of steam is considerably greater than 

 that of water, and this latter than that of ice, the intrinsic 

 energy of the mixture is by no means defined by the position 

 of the point, an infinite number of values of the intrinsic 

 energy corresponding indeed to each single point of this 

 segment. 



Thus, for example, the point K of this segment may repre- 

 sent the state of 1 kilogram of water which has increased 9 per 

 cent, in volume by freezing throughout ; or it may represent 

 the state of this mass after a 9-per-cent. increase in volume by 

 partial vaporization. In the latter state, however, the mass 

 would possess about 80 calories more of intrinsic energy than 

 in the former state. 



This straight isothermal and isobaric segment JKL (figs. 9 

 and 11) forms an edge of the temperature-surface along its 

 whole length ; and as this edge is distinguished from all the 

 other lines and edges of the temperature-surface by the above- 

 mentioned remarkable properties, we shall call it in future the 

 a principal edge of the temperature-surface?' (Following J. 



* Andrews, Pogg\ Ann. Erganzbd. v. p. 64. 



t The point J was on this account named the triple point by J. Thomson. 



