28 PHYSICAL PROPERTIES AND COMPOSITION 



4. The Rectilinear Diameter 1 . 



Since it appears from the preceding that the volume 

 relations of various substances are inter- related we will 

 return to one of them, fluor-benzene, as type ; we then find 

 that despite the failure of the reduced characteristic equation. 

 a very simple relation of volumes holds, which, however, 

 must so far be treated as purely empirical. It is the 

 so-called rule of the rectilinear diameter which states that 

 the sum of the densities of liquid and saturated vapour is 

 a linear function of the temperature, as may be seen from 

 the following data in which the critical values are chosen 

 as units of temperature and density : 



I Vk I Vk 



- = - (liquid) -- = -: (sat. rap.) 



A I + - 



I 



0-266 



0-094 



0-035 



0-012 



0-006 



O-OO2 



1.82 

 1-85 

 r.86 

 1-92 

 1.88 

 1.88 



reproduces this relation, expressed directly in 

 density and temperature : AB 

 expresses the density of the 

 liquid D h BO that of the satu- 

 rated vapour D B the critical 

 density D k ; the diameter BD ex- 

 presses the values of \ (A + -A,-)> 

 its straight course is therefore 

 the expression of the law in 

 question, which may also be 

 represented by the equation 



When double molecules are 

 found in the liquid, as in the 

 case of alcohol according to the 



Mathias, J. de Phys. (3) 2. 1893, n ; (3) i. 1892, 53. 



