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LVII. On the Determination of the Critical Volume. 

 By Sydney Young, D.Se., University College, Bristol*. 



ABOUT a year ago I read a paper before the Physical 

 Society (Phil. Mag. Feb. 1892, p. 158) on the " Gene- 

 ralizations of Van der Waals regarding Corresponding Tem- 

 peratures, Pressures, and Volumes ; " and at the close of the 

 paper, after calling attention to the extreme difficulty attend- 

 ing the determination of the critical volume, I described 

 a method by which the relative critical volumes of various 

 substances could be ascertained. At that time it appeared 

 to me that the direct determinations of the critical volumes 

 of benzene, fluorbenzene, chlorobenzene, and acetic acid were 

 probably approximately correct; and I was supported in 

 this view by the fact that the ratios of the observed volumes 

 to that of fluorbenzene were practically identical with the 

 ratios obtained by the graphical method then described. 



I therefore assumed that the observed critical molecular 

 volume of fluorbenzene was correct, and calculated the 

 volumes of the other substances from their ratios to that 

 of fluorbenzene, and gave these values as the probable 

 critical molecular volumes of the other eleven substances. 

 It was observed, however, that the critical volumes of the three 

 alcohols and of ether obtained by this method were consider- 

 ably lower than those previously deduced by Dr. Ramsay and 

 myself, and were also lower than those calculated by M. Guye 

 [Compt. Bend. cxii. p. 1257). Finally, I pointed out that if 

 these values are taken as correct it would appear that the 

 ratio of the actual critical density to the theoretical (for 

 a " perfect " gas) is for many substances about 4*4. 



Since then, M. E. Mathias has shown (Compt. Bend. cxv. 

 p. 35) that the critical densities of the substances referred to 

 in my paper may be determined by the method described 

 by MM. Cailletet and Mathias {Compt. Bend. cii. p. 1202, 

 civ. p. 1563). The method has also been found by 

 M. Amagat to give excellent results with carbon dioxide, 

 and by Mr. G. L. Thomas and myself with five of the lower 

 esters ; and I have therefore much pleasure in acknow- 

 ledging the correctness both of the method (except, perhaps, 

 in the case of the alcohols) and of the critical densities cal- 

 culated by M. Mathias, and I have no hesitation in accepting 

 these values in place of those previously adopted. 



I have myself made an independent calculation of the 

 critical densities of the twelve substances by this method, 

 and the results 1 have obtained are practically identical 

 * Communicated by the Physical Society : read October 28, 1892. 



