174 Prof. W. Ramsay. The Complexity and the [June 14, 



of non-dissociating liquids and frequently cut those of dissociating 

 liquids. The fact, then, that the vapour-pressure curve of a liquid 

 cuts those of undeniably simple substances, such as benzene, carbon 

 tetrachloride, &c., may be taken as a proof that that liquid contains 

 complex molecules. 



/. This relation may also be expressed by the factor in van der 

 Waals' equation for calculating vapour-pressures, viz. : 



Tc T 



where pc is the critical pressure ; Tc the critical temperature ; and p 

 some other pressure at temperature T. The constant / has a value 

 close to 3 for all non-associating compounds. Thus from Young's 

 results the following values of / are calculated : 



/ / 



Benzene ............ 2'89 Propyl formate .... 3 04 



Chlorobenzene ...... 2'95 Methyl acetate .... 3'07 



Fluorobenzene ...... 2 '99 Ethyl acetate ...... 3' 26 



Carbon tetrachloride 2'81 Propyl acetate .... 3'22 



Tin tetrachloride .... 3'0l Methyl propionate. . 3'13 



Ethyl oxide ......... 3'00 Ethyl propionate . . 3'22 



Methyl formate ..... 3'00 Methyl butyrate ____ 3'25 



Ethyl formate , ..... 2'97 Methyl isobutyrate. . 3'15 



The mean value is 3'06. 



But for liquids with complex molecular groupings the values are 

 considerably higher, and, moreover, are not constant. 



/ 

 Methyl alcohol ....... 3'56 to 377 



Ethyl alcohol ........ 3'58 4'02 



Propyl alcohol ....... 3'49 3-77 



Acetic acid .......... 3'36 3'49 



Water .............. 3'20 3'24 



Other relations besides those mentioned by Guye, of whose 

 memoir the preceding pages give an abstract, also point towards 

 the molecular complexity of the alcohols and acids. Among them 

 may be mentioned the ratios of the volumes of saturated vapour at 

 some chosen pressure to that at the critical pressure, as shown in 

 p. 1257 of Young's memoir (loc. cit.) ; the greater values of the ex- 

 pression (dp/dfyT for the alcohols and for water compared with those 

 of other substances (see Ramsay and Young, ' Proc. Phys. Soc.,' VII, 

 p. 303) ; this really means the greater heat of vaporisation for unit 

 increase of volume, for (dpjdt)T is equivalent to L/(Sj S 2 ). This 



