96 Transactions of the Royal Canadian Institute 



point is given by 



S = c logT/To+Lx/T 

 where c is the specific heat of the liquid (taken as constant), T is the 

 temperature at which evaporation occurs, To is the initial temperature 

 (0° C), L is the latent heat of vaporization, and x the dryness or fraction 

 of the mass which is vapour. If x be taken as unity the graph of the 

 equation S = c log T/T" + L/T is the boundary between saturated vapour 

 and unsaturated vapour. If we put x equal to any proper fraction, 

 we have in the same way a curve of constant x, or a curve of constant 

 dryness. These curves in Fig. 2 were drawn from calculations based 

 on the data of Regnault. The curves for water slope downward toward 

 the right for x greater than .5 and toward the left for smaller values. 

 The curves for ether all slope downward toward the left. The ordinates 

 represent temperature and the abscissae, entropy from 0**C. 



Mathias^ has shown that for a saturated vapour an adiabatic increase 

 in volume is always accompanied by a lowering of temperature so that 

 an adiabatic or isentropic expansion is represented by a vertical line on 

 the diagram and the state point descends as the substance expands. 

 The condition of the vapour, which is always in the upper part of the 

 tube, corresponds to a point well over toward the right of the diagram, 

 in fact it actually may be in the unsaturated portion owing to the effect 

 of gravity which separates liquid and vapour everywhere except at the 

 interface. So, when water vapour is expanded the state point moves 

 down and either cuts across the condensation line or meets successively 

 lines of increasing values of x so that condensation must result. 



In the case of the adiabatic expansion of ether vapour, at least 

 at the temperatures included in the diagram, the state point moving 

 down cuts the lines of constant x, so that the value of x is decreasing or 

 the vapour gets dryer, If there is liquid present, it evaporates. The 

 data for ether given by Regnault extend only to 230° C. so that the range 

 of phenomena interpreted though the diagram is limited. If, as stated 

 by Raveau, the value of the specific heat becomes negative as the critical 

 point is approached, it should be possible to get condensation with 

 expansion, at sufficiently high temperatures. This was found to be the 

 case and it is quite easy within a small range of temperature near the 

 critical temperature to bring a dense cloud of vapour with a slight 

 expansion. As the temperature is lowered it is not easy to accomplish 

 this and as the temperature is lowered still more no cloud appears on 

 expansion but upon compression there is evidence of condensation on 

 the walls of the tube, though we never see the dense cloud that we do 

 near the critical temperature. 



sMathias, Jour, de Physique, 4, 7, 618, 1908. 



