Generalizations of Van der Waals. 



181 



may again point to the existence of complex molecules of 

 acetic acid in both the liquid and gaseous states, but of methyl 

 alcohol in the liquid state only, except possibly very near the 

 critical point. 



With ethyl and propyl alcohols the ratios of the molecular 

 volumes of liquid to those of fluorbenzene are also low, though 

 not nearly to the same extent as with methyl alcohol. 



Note on the Determination of Critical Constants. 



Of the three critical constants, temperature, pressure, and 

 volume, the first is the most easily determined, for by em- 

 ploying as heating-agents the vapours of pure liquids boiling 

 under known pressures, the temperature is perfectly under 

 control and is easily measured. Moreover, the presence of a 

 very small amount of impurity does not influence the critical 

 temperature of a substance to nearly the same extent as the 

 critical pressure. 



The critical pressure may also as a rule be determined 

 without very much difficulty, provided that absence of impurity 

 is ensured, but this point is of the utmost importance. In 

 the case of substances that attack mercury at high tem- 

 peratures, such as carbon tetrachloride and stannic chloride, 

 the ordinary method of operation requires modification, and 

 the calculations become more laborious, but otherwise the 

 difficulty is not greatly increased. 



The determination of the critical volume of a substance, 

 even when perfectly pure and without action on mercury, 







Fig. ] 



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Fig. 



2. 











r^ 



\ 













Me 

 40 



30 



J 



20 



















/ 

















bees 













































1 































1 









































k 



CCs. 



> 



i. 





2( 



' 

















CCs. 



is, however, a much more difficult matter. That this is so is 

 evident from the form of the curves representing the relation 



