44 Prof. R. A. Lehfeidt on the 



a fact which I also had overlooked. Margules's chief new 

 contribution is, therefore, the empirical formula he proposes 

 for the relation between composition of liquid and vapour, as 

 will be mentioned below. 



Measurements of Vapour-Pressure. 



The measurements made form two distinct groups, those of 

 vapour-pressure and those of composition of vapour, which 

 were carried out separately, but on material from the same 

 source, and prepared in identical manner. The materials 

 were as follows : — 



Benzene. — Kahlbaum's thiophene-free : its boiling-point 

 was nearly constant, but it was fractionated twice from calcium 

 chloride, and then stood over sodium. B.p. (corrected and 

 reduced to 760 mm.) = 80°'0. Vapour-pressure at 50° = 



270-9 mm. Density ^ = 0'8803. n D at 18°=1'5024 by 



spectrometer. 



Toluene. — Baird and Tatlock's purest. Fractionated from 



calcium chloride and stood over sodium. B.p. (corr. and red.) 



18 

 = 110 o, 0. Vapour-pressure at 50°= 93*0 mm. Density -j- 



=0-8667. fi v at 18°= 1*4970. - 4 



Carbon Tetrachloride. — Baird and Tatlock. Slightly 



yellow when obtained ; on fractional distillation it became 



colourless, and on redistilling the best fraction nearly the 



whole came over within 0°'l. B.p. 76°*6 (corr. and red.). 



1 o 



Vapour-pressure at 50° = 310*2 mm. Densitv -r =1-5979. 

 /ul d at 18°= 1-4618. 4 



Ethyl Alcohol.— Baird and Tatlock's " absolute." Digested 

 on a water-bath with lime and baryta till a yellow colour 

 appeared ; then distilled with special care to avoid moisture. 

 B.p. (corr. and red.) = 78°'3. Vapour-pressure at 50°= 219*5 



mm. Density ^ = 0-7929. fi v at 18°= 1-3622. 



The density is not so low as it should be according to 

 Mendeleef ; but it is probable that if a purer alcohol had been 

 obtained it would have absorbed some moisture in the course 

 of the inevitable manipulation of the mixtures. Distillation 

 from sodium was also tried, but did not give any better 

 results. 



To measure the vapour-pressure of the mixtures, the 

 dynamic method was adopted. The most interesting part of 



