312 



HEAT. 



This has been verified by Ramsay and Young (" On the Influence 

 of Change from the Liquid to the Solid State on Vapour-Pressure," 

 Phtt. Trans., Part II., 1884, p. 461). They evaporated water and 

 ice in separate vessels at the same pressure below that of the triple 

 point, and found that the water when in equilibrium was colder than 

 the ice. 



While it is possible to supercool water in contact with its vapour, i.e. 

 to have it to the left of the line TO, Fig. 174, so that TB' has an actual 

 existence, no method of superheating ice has yet been devised. The pro- 

 longation of BT beyond T, therefore, represents nothing physical. But 

 ice can be obtained in a condition represented by points slightly to the 

 right of TO. For instance, if a block of ice at the triple point is subjected 

 to pressure, it is raised in temperature slightly (from the formula of p. 286 

 about 0'002 C. per atmosphere if we take its coefficient of expansion as 

 Q'00015 and its specific heat as 0'5), while it only melts at its surfaces. 



N N 



enlropy 

 FlG. 176. Equal Pressure Lines on the Entropy-Temperature Diagram. 



Thus it will be represented on Fig. 1 74 by a line from T nearly vertical 

 but sloping slightly outwards as it rises. And no doubt it would be pos- 

 sible to obtain ice under tension in a condition represented by the pro- 

 longation of this line downwards into the vapour region. In the case of 

 water substance there are only three phases and one triple point. But 

 there are substances, such as sulphur, with four phases, i.e. two solid forms 

 as well as the liquid and gaseous, and for these substances there are three 

 triple points. For an account of these triple points and for the general 

 phase rule of Willard Gibbs, giving the condition of equilibrium of 

 phases, we refer the Dreader to Whetham's Theory of Solution, chap, ii., 

 or to Findlay's The Phase Rule. 



The Second Latent Heat Equation. Another equation connect- 

 ing latent heat and its change with the specific heats of the two phases 

 may be readily obtained from the entropy temperature diagram. Let 

 ABOD, A'B'C'D' (Fig. 176) be equal pressure lines, the horizontal 

 portions representing the mixture undergoing change of state with the 

 receipt of latent heat at and QdO respectively. 



Taking the substance round the cycle BCC'B'B, the heat given along 

 BC is L. That given along CO' is - G z d6 where C 2 is the specific heat 



