Vapour-Densities of some CavhoN Compounds, 517 



the liquid state, exhibits, if ether is taken to possess the 

 theoretical moU'cuhir weight, a divergence of only 1 part in 

 588 ; whereas the hydrocarbons, which show no such ten- 

 dency, are characterized by a greater discrepancy, all except 

 hexane. 



Another supposition which deserves consideration is that 

 although there may be no molecular groupings, yet the mole- 

 cules attract one another, and thus lower the value of the 

 product Pi\ And it is conceivable that the nearer the boiling- 

 point of the substance to the temperature of experiment, 

 129*6°, the more this attraction should manifest itself. This 

 possibility finds support to some extent by the results. The 

 order of divergence does present a general correspondence 

 with the order of boiling-points ; but the numbers are reversed 

 between three pairs of substances, as may be seen from the 

 table which follows : — 



Ether. 



Hexaue. 

 833 



35<: 



Methyl Di-iso- 

 alc. butyl. 

 588 455 

 66^ 109° 



Benzene. 



Octane. 



Toluene. 



333 



80° 



182 

 126° 



159 

 110° 



1 part in 



Boiling-pt. .. 



The boiling-points of methyl-alcohol and hexane, of benzene 

 and di-isobutyl, and of toluene and octane follow the wrong- 

 order. Still, if substances belonging to the same class be 

 compared, hexane, di-isobutyl, and octane show correspondence; 

 and so, too, do benzene and toluene. 



Another plan of attacking the problem is to compare the 

 values of the expression d{Pv)/dF for all the substances at 

 certain definite pressures and temperatures, the same for all ; 

 in other words, to compare the slopes of the curves with the 

 divergences from the " theoretical '^ vapour-densities. This 

 is done in the following table : — 



i 



Discrepancy 

 1 part in 



Terap. 



Eelative values of diVvy'dV at the pressures. 



500 nims. 



300 mms . 



100 mms. 1 



Me. Ale... 



588 



130° 



0-0000197 



0-0000160 



0-0000114 



Ether ... 





130 



0-0000282 



00000271 



0-0000184 







100 



365 



347 





Benzene... 



333 



130 



288 



278 



208 







100 



376 



348 





Hexane ... 



833 



130 



379 



321 



277 







115 



422 



402 









100 



480 



434 





Toluene... 



159 



130 



466 



420 



282 







115 



548 



512 









100 



664 







Di-iaob. ... 



455 



130 



623 



595 



480 







115 



740 



710 





Octane ... 



182 



130 



720 



656 



593 



