444 Mr. J. H. Shaxby on Vapour Pressures 



the first seven columns are as follows : — (1) temperature,. 

 f C. ; (2) density of liquid, d A ; (3) density of vapour, d 2 ; 



1 9TJT J 



(4) b, i. e. -j y ; (5) A, i. e. w l°g«:r 5 (6) calculated 



saturated vapour pressure ; (7) observed saturated vapour 

 pressure. It will be seen that the equation represents the 

 facts with fair accuracy. The calculated value is usually 

 too high at low pressures, then becomes somewhat low until 

 the temperature is within a few degrees of the critical ; at 

 the latter temperature it is again too high. 



The corresponding Equation of State thus becomes 



2i < 



• • • (2> 

 The Isothermals of Isopentane, Hexane, Ethyl Ether, and 

 Water have been calculated from this equation for the 

 specific volumes quoted in Winkelmann, vol. iii. pt. 2, 

 pp. 1114-1135. 



Table II. gives the result for Water and Isopentane ; those 

 for the other substances are quite similar. Column 1 gives the 

 Specific Volume, column 2 the calculated pressures, and 

 the last column (5) the actual pressures, as given by experi- 

 ment. It will be noted that the agreement is fair for low 

 temperatures. 



At the critical temperature the equation assumes the form 



{ V ~2d) = 



RT c e 



For temperatures above the critical this same form has been 

 used, except that d c has been replaced by l 2 and the 



value of the- latter obtained by extending the linear relation 

 of Mathias to the given temperature. For the present we 

 cannot assign any physical meaning to (d 1 + d 2 ) for tempera- 

 tures above the critical ; we return to this point later. 

 The table shows that the equation still represents the facts 

 moderately well for the smaller pressures, but completely 

 breaks down at the higher, L e. when the specific volumes 

 are small. 



Since this paper was written (and in part read at the 

 Cardiff meeting of the British Association, 1920), Professor 

 Porter has called my attention to a paper bv F. H. Mac- 

 Dougall (J. Amer. Chem. Soc. xxxviii. no. 3,"l916, p. 528} 



