Relations of Pressure, Temperature, and Volume. 299 



Some of Dr. Alder "Wright's recent work is thus open to cri- 

 ticism on account of his theoretical conclusions on the results 

 of the comparison of his values with those found in the calori- 

 meter. Another desideratum in his recent papers is the state- 

 ment of the effects of temperature on the E.M.F. of his cells. 

 Such measurements should always be made, especially since 

 the publication of Prof. Helmholtz's mathematical researches 

 already referred to. 



This paper is only put forward as a preliminary investigation 

 of the electromotive forces developed during the combination 

 of zinc and iodine, and the difficulty of dealing with zinc 

 iodide as a salt in solution must be my excuse for not produ- 

 cing final results on going over the ground for the first time. 



XLII. The Relations of Pressure, Temperature, and Volume 

 in Saturated Vapours. By W. C. Unwin, M. Inst. C.E.* 



IN the interesting and laborious paper of Messrs. Eamsay 

 and Young, there are given four relations of a simple 

 kind between the pressures, temperatures, and volumes of 

 saturated vapours and the heat of vaporization of a number of 

 substances. These relations are not given as exact relations, 

 but as suggestive of laws approximately satisfied by very dif- 

 ferent substances over a very wide range of conditions. 



Now long ago Hirn pointed out that, as for saturated vapours 

 there exists only a single value of p and t corresponding to 

 one specific volume v, there must be a direct relation between 

 t and v, and between p and v, independent of p or t. But 

 hitherto no exact relation has been found, and we remain still 

 in the condition of being obliged in every thermodynamical 

 calculation to fall back at every step on data either interpo- 

 lated from tables of experimental values, or determined by 

 the aid of purely empirical formulas. Nor is this all : the 

 formulas are themselves cumbrous and are not interrelated. 



Regnault's formula, giving p in terms of t for instance, 

 cannot be inverted to get t in terms of p, though in practical 

 calculations the pressure is almost always the given datum. 



Further, if differentiated to obtain -£ an expression is obtained 



far too cumbrous for ordinary use. Eankine's formula is 

 nearly as accurate, and is both simpler and gives an inverse 



function ; but the expression for -£ is cumbrous. The ex- 

 pression for latent heat is necessarily independent, but then 

 * Communicated by the Physical Society ; read February 27, 1886. 



