36 PHYSICAL PROPERTIES AND COMPOSITION 



critical pressures roughly corresponding temperatures *, 

 as may be seen from the following : 



From this point of view it was a fortunate idea of Kopp 

 (1855) to compare molecular volumes at the boiling point. 

 It appeared on the whole that these might be regarded 

 as the sum of values peculiar to each element. With the 

 assumption of the following atomic volumes 



C O H Cl Br I S 



II 12-2 5-5 22-8 27-8 37-5 22-6 



the molecular volume (M r ), and therefore the density at 

 the boiling point, is roughly calculable from the molecular 

 weight (M & \ as e.g. for acetone (C 3 H fi O) 



M v = 3 x 1 1 + 6 x 5-5 + 1 2-2 = 78-2, 

 Jf0 = 3x13 + 6 +16 =58; 

 the density at the boiling point (56-5) should therefore be 



- = 0-742, whilst 0-75 is the observed value. 

 70-2 



A striking confirmation of the values thus calculated 

 from organic compounds is the following : 



1 Guldberg, Zeilschr. f. Phys. Chem. 5. 376. 



