26 PHYSICAL PROPERTIES AND COMPOSITION 



volumes equal multiples of the critical volume, whilst the 

 ratio of liquid and vapour volumes, or densities, is a con- 

 stant quantity for each value of y. 



To confirm this rule we will apply it e. g. to hexane, and 

 calculate the density as liquid and as saturated vapour at 

 a given temperature, say 82. Since the critical tempera- 

 ture is 235 the fraction in question, 



82 



. 

 373 + 2 35 



From the preceding table for fluor-benzene we find that 

 at this fraction of the critical temperature the liquid volume 

 (^j) is 0396 of the critical volume, while the volume of the 

 saturated vapour is 44 times the latter. Since the critical 

 density of hexane has been found to be 0-2343, those of the 

 liquid and vapour at 82 are 



02343 , 0-2343 



y -^- = 0-592 and 2ZP 0-0053, 

 0-396 44 



whilst 0-599 an d 0-005 were actually found. 



Let us see next how far the behaviour in question gives 

 a point of departure for judging of molecular weights. 

 Striking deviations are observed in the alcohols. If 

 we take ethyl alcohol, for example, and compare it with 

 benzene, choosing the ratio of volumes at the critical points 

 as unit : 



rn nt /j / 



7 = -^7 (liquid] -^- (saturated vapour} 



Tk P! P 2 



ii i 



0-928 0-99 1-23 



0-822 0'99 1.74 



0-733 099 2 !39 



0-656 3-51 

 0-639 i .01 



The second column gives the volume of the liquid alcohol 

 referred to that at the critical point, divided by the corre- 

 sponding quotient for benzene : the third column gives the 

 same for saturated vapour. The rules of corresponding states 

 require that all these quotients should be unity. This is 



