636 Sir J. A. Ewing on Specific Heat of Saturated Vapour- 

 On differentiating this with respect to T, and multiplying 

 by T, the Clausius equation quoted above is at once 

 obtained : — 



dT ~ tlT + dT T ' 



^ iz ^ L L 



or K 6 — K„. -j- ^7jt — r j . 



Bat when it is practicable to draw the entropy diagram the* 

 changes of K s are made evident without recourse to this- 

 equation. 



To draw the entropy diagram one may proceed by first 

 calculating (p w and then adding L/T to find cp s . According 

 to the usual convention <p R . is taken as zero when the tem- 

 perature is 0° 0. Then at any scale-temperature I, (or 

 T-273°-l) 



where C p is the specific heat of the liquid at constant 

 .pressure, namely the pressure of saturation for the given 

 temperature. When an empirical formula connecting 0^ 

 with the temperature is available, this allows (j> u . to be readily 

 calculated. The values so found cannot be regarded as 

 more than approximate at temperatures beyond the experi- 

 mental range from which the formula for C p has been 

 deduced, and in general that range is rather narrow. But 

 the method gives a first approximation to the entropy- 

 temperature diagram, w-hich at least suffices to distinguish 

 what may be called normal cases, such as are represented by 

 fig. 1, from others which clearly resemble fig. 3. In these- 

 last there are unquestionable reversals of the sign of K s . 



I have drawn the diagrams in this way for a number of 

 fluids, using in most instances the values of L given by 

 Young in his paper on the vapour-pressures and heats of 

 vaporization of thirty pure substances *, along with such 

 data for C^ as I have been able to find. The following- 

 are representative examples. 



Alcohol. — From the experiments of Bose f, 0^ for ethyl 

 alcohol is taken as 



0-5396 + O001698 t, 

 from which 



^=0-1747 log 10 T + 0-001698 T-0'8892. 



* S. Young-, Scientific Proceedings of the Royal Dublin Society, 

 vol. xii. p. 3/4 (1910). 



t Bose, Gott. Nachr. 1906, p. 278 ; Zeit. fur phys. JChem. vol. Iviii. 

 p. 585 (1907). 



