CORRESPONDING PRESSURES 47 



empirical relations, as was done for volumes by Mathias' 

 law of the rectilinear diameter. The following has been 

 suggested l ; 



log '-/' 



which we may test by the numbers for fluor-benzene : 



i Pk 

 log - "P 



0-59 0-929 3-00 



0-236 0-823 2 '9 2 



0-089 '733 2>8 9 



0-0295 0-656 2-94 



0-0118 0-606 2-97 



0-00059 0.486 3-05 



Whilst most substances are approximately united by 

 these relations, striking exceptions are to be noted, being 

 found especially in those bodies which are regarded as 

 partly or wholly double-molecular, on the ground of the 

 osmotic methods. We must especially notice the behaviour 

 of bodies such as ethyl alcohol, which behaves normally in 

 the critical state (p. 22), and therefore probably possesses 

 normal molecular weight ; retaining the same in the form 

 of saturated vapours at low temperatures, as shown by the 

 density; but appearing as double-molecular in the liquid 

 form on cooling, as judged by the osmotic methods. 



Let us therefore compare the vapour pressure of ether 

 a normal liquid with alcohol at corresponding tempera- 

 tures, fluor-benzene being taken as the standard substance : 



-^- C 6 H 5 F (C 2 H 5 ) 2 C 2 H 5 OH 



i i 0.798 1-411 



0-928 i 0-804 1-253 



0-822 i 0-785 0-962 



0-733 i 0-761 0-681 



0-639 * 0-740 0-409 



The saturation pressure of ethyl alcohol thus falls off 



1 V. d. Waals, Continuity, p. 148. 



