igo4\ Mills — Molecular Attraction. 147 



in the constant fi' as the critical temperature was approached. 



For ethyl oxide Profs. Ramsay and Young- 1 obtained at vol- 



S P 

 ume 4.00 the -5-^=- = 413.7, and it is evident from their paper 



o 1 



on ethyl oxide 2 that the values they used for calculating- the 



latent heat of vaporization point to a value at this point of 



about 490, a fact which sufficiently explains the rise noted in 



the value of /*'. (The values Profs. Ramsay and Young used 



8P 

 for -«-=- at this point are not in accord with the Biot formula 



they publish and this prevents quantitive comparison here. 

 See p. 175.) 

 We have therefore direct proof that the equation, 



T T£ 



Q/ J = constant, is applicable in the immediate 



yd — y D 



neighborhood of the critical temperature. 



THE PRODUCT OF THE PRESSURE AND THE VAPOR DENSITY. 



In examining- the data used to discover if possible the 

 immediate source of variations in // it proved impracticable 

 to plot either the pressure or the volume of the vapor directly 

 against the temperature. But their product, PV, varied more 

 slowly and the values were plotted and gave regular curves 

 except for di-isobutyl, brom-benzene, iodo-benzene, hexa- 

 methylene, and water. This result is so interesting that the 

 curves are given below, Diagrams 1 to 3. The numerical 

 results are given in Tables 1 to 21. For water the break in 

 the regularity of the curve occurs at 100° C, and since differ- 

 ent formulas were used for the vapor pressure above and 

 below that temperature it would seem certain, in view of 

 remarks already quoted (second paper, p. 396), that the 

 formulas for the pressure need adjustment. For di-isobutyl, 



iPhil. Mag., May, 1887. 

 2 Phil. Trans., 1887 A, p. 57. 



