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SCIENCE PROGRESS 



densities at equal fractions of the critical pressures are equal 

 multiples of the critical densities. 



Owing to certain irregularities which occur at or near the 

 critical points, the author has indicated {Phil. Mag. S. 6, vol. xiv. 

 No. 81, Sept. 1907, pp. 340-4; vol. xvi. No. 91, July 1908, 

 pp. 87-9) the advantage of making the densities or volumes 

 at some reduced pressure like P/P* = '01 1795 the basis of 

 comparison. 



Comparison of the Volumes of a number of Hydrocarbons at 

 Corresponding Pressures. 



The Law of Coincident States is thus approximately realised, 

 as Young has shown. The divergences therefrom have been 

 connected by him with differences in constitution. 



(b) Prideaux, as already stated, has given some data under 

 conditions of equal pressure (normal atmospheric), showing that 

 the volumes or densities are comparable with similar data for 

 a pressure of 200 mm. in the case of non-associated compounds. 



Thus 



V m (76o) 



V m (200) 



CeHg 



PCI3 



HC1 



2 



C2HUBr 



const. 



1-050 

 1-050 

 1-049 

 1-048 

 1*048. 



It appears that constant relations are also obtained by sum- 

 ming the atomic volumes of the free elements under the two or 

 more equal pressures and comparing them : 



V m ( 7 6o) 2V a ( 7 6o) 



V ra (200) 



PCh 

 ICl' 



1-050 



] "041 



2V a (2oo) 

 P + Cl 3 1-050 

 I + CI 1 -043 



