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LXIII. On Formula for the Maximum Pressure and Latent Heat 

 of Vapours. By W. J. Macquorn Rankine, Civil Engineer, 

 RR.SS. Lond. and Edinb. ^c * 



1. TT is natural to regard the pressure which a liquid or solid 

 -i- and its vapour maintain when in contact with each other 

 and in equilibrio, as the result of an expansive elasticity in the 

 vapour, balanced by an attractive force which tends to condense 

 it on the surface of the liquid or soHd, and which is veiy intense 

 at that surface, but inappreciable at all sensible distances from 

 it. According to this view, every solid or liquid substance is 

 enveloped by an atmosphere of its own vapour, whose density 

 close to the surface is very great, and diminishes at first very 

 rapidly in receding from the surface ; but at appreciable distances 

 from the surface is sensibly uniform, being a function of the 

 temperature and of the attractive force in question. 



2. Many years since I investigated mathematically the con- 

 sequences of this supposition, and arrived at the conclusion, that 

 although it is impossible to deduce from it, in the existing con- 

 dition of our knowledge of the laws of molecular forces, the exact 

 nature of the relation between the temperature and the maximum 

 pressure of a vapour, yet that if the hypothesis be true, it is 

 probable that an approximate formula for the logarithm of that 

 pressure for any substance will be found, by subtracting from a 

 constant quantity, a converging series in terms of the powers of 

 the reciprocal of the absolute temperature, the constant and the 

 coefficients of the series being determined for each substance from 

 experimental data. Such a formula is represented by 



logP==A-2^^-.&c., 



where P denotes the pressure, r the absolute temperature, that 

 is, the temperature as measured from the absolute zero of a per- 

 fect gas thermometer, A the constant term, and B, C, &c. the 

 coefficients of the converging series. 



3. On applying this formula to M. Regnault's experiments on 

 the pressure of steam, it was found that the first three terms 

 were sufficient to represent the results of these experiments with 

 minute accuracy throughout their whole extent ; that is to say, 

 between the temperatures of 



—30° and ■+■ 230° Centigrade 

 = -22° and 446° Fahrenheit, 



and between the pressures of ^^^jjj^dih of an atmosphere, and 82 

 atmospheres. 



* Communicated by the Author ; having been read to the British Asso- 

 ciation for the Advancement of Science, Section A, at Liverpool, Sept. 1864. 



