Properties of Liquid Mixtures. 



49 



pure substances, both mixtures containing alcohol show a 

 maximum — a very flat one. In the case of alcohol and benzene 

 it occurs at about 40 per cent, (molecular) of alcohol, and is 

 406 mm., being 406/(271 + 220) = 83 per cent, of the sum of 

 the pressures of the pure substances; while for alcohol and 

 toluene it is at about 74 percent, of alcohol, and has the value 

 249 mm., which is 249/(93 + 220) = 80 per cent, of the sum 

 of the pressures. 



With regard to the variation of pressure with temperature, 

 we may, as already remarked, regard logp as linear in t over 

 a few degrees. But the value of ~di}ogp)l~ftt for the pure 

 substances varies somewhat ; this is partly due to the fact that 

 50° is a different fraction of the critical temperature in each 

 case ; it diminishes as the u reduced " temperature rises, and 

 consequently has a higher value for toluene at 50° than for 

 benzene or carbon tetrachloride at the same temperature. 

 Alcohol, though its critical point is the lowest of the four, 

 has the highest value of the coefficient — a marked instance of 

 its exceptional behaviour. The numbers are : — 



d(log^)/d*. 



Toluene 0'0189 



Benzene 0'0164 



Carbon tetrachloride . . . 0*0156 

 Alcohol .... 0-0209 



The coefficients found for the mixtures are necessarily (on 

 account of the small range of temperature) less reliable than 

 the vapour-pressures themselves : they are in all cases nearly 

 those that would be calculated by interpolation from the coeffi- 

 cients of the pure substances, even when the vapour-pressure 

 curve shows a maximum, i. e. they are additive. The differ- 

 ences from the additive values are not greater than the. 

 probable errors of experiment. I have included the values of 

 the coefficients in the tables, but lay no stress on them. 



Alcohol and Benzene : Vapour-pressures at 50°. 



Per cent. 



Mol. per cent. 



Saturation-press. 



d(logp) 



alcohol. 



alcohol. 



Po0> 



dt 











270 9 



00164 



432 



7-11 



339 8 





1111 



17-49 



389-9 



00172 



18-87 



28-28 



404-3 



179 



23-20 



3387 



406-4 



179 



38-81 



51-80 



400-3 



178 



46-75 



59-80 



3940 



188 



60-33 



7205 



375-3 



182 



72-37 



81-61 



345-8 



184 



81-32 



88-07 



318-8 



175 



90-61 



94-21 



274-3 



193 



100 



100 



219-5 



209 



Phil. Mag. S. 5. Vol. 46. No. 278. July 1898. 



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