[ 199 ] 



XXX. On Saturated Vapours. By W. J. Macquorn Rankine, 

 C.E., LL.D., F.R.SS.L.fyE. $c* 



AS this paper consists almost wholly of formulae, calculations, 

 and tables, it is not suited for being read to a meeting, 

 and therefore the following short abstract of its contents is alone 

 offered for the purpose of being read aloud. 



In the Edinburgh Philosophical Journal for July 1849, the 

 Author proposed the following formula for the pressure of satu- 

 rated vapour corresponding to a given boiling-point, 



1 A B C 



where t is the absolute temperature reckoned from the absolute 

 zero, and A, B, and C are three specific constants, to be deter- 

 mined from at least three experiments on each substance ; and 

 he showed that the results of that formula agreed better with 

 experiment than those of any other formula containing three 

 constants only. In a series of papers on the Mechanical Action 

 of Heat, read to the Royal Society of Edinburgh in 1850 and 

 subsequent years, and in other publications also, the same for- 

 mula is explained, and its use exemplified in various ways. 



The first division of the present paper gives the results of the 

 computation of the values of the constants A, B, and C for 

 several fluids for which they had not previously been computed, 

 the data being taken from the second volume of M. Regnault's 

 Relation des Experiences, &c, published in 1862 ; and it is also 

 shown that by the formula a conclusion was anticipated which 

 M. Regnault has deduced from his experiments, viz. that " the 

 elastic force of a vapour does not increase indefinitely with the tem- 

 perature, but converges towards a limit which it cannot exceed" 

 [Relation des Experiences, vol. ii. p. 647). 



The second division of the paper is occupied chiefly with a 

 comparison between the actual values of the pressures of satura- 

 tion of the vapours of various fluids, and the values which those 

 pressures would have if the vapours were perfectly gaseous. 



In the first of the papers already referred to, read to the 

 Royal Society of Edinburgh, and published in their ' Transac- 

 tions ' in 1850, the author proved, from the principles of ther- 

 modynamics, that the " total heat " of evaporation of a perfectly 

 gaseous vapour must be represented in dynamical units by the 

 expression 



Jb + Jc'tf, 



* Abstract of a paper read to the Royal Society of Edinburgh in April 

 1865. Communicated by the Author, 

 t ib in this expression corresponds to a. in the preceding paper. 



P2 



