52 



Prof. R. A. Lehfeldt on the 



heating was then sufficient to keep the top of the apparatus 

 at least as hot as the small flame kept the bottom, and at the 

 same time provided an excellent light to read the thermo- 

 meters by. About 30 c.c. of mixture was placed in the 

 tube ; the temperature of the bath adjusted to between 51° 

 and 52°, and then the pressure lowered till the liquid boiled 

 tranquilly at as near as possible 50° ; as the distillation pro- 

 ceeded, the thermometer of course tended to rise ; this was 

 corrected by an occasional stroke of the pump as required. 

 Three lots of about 1 c.c. each were usually distilled, and 

 examined separately by the Pulfrich re fracto meter ; from 5 

 to 10 minutes being required to distil 1 c.c. 



In order to interpret the results it was first needful to 

 construct tables of the refractive index of the mixtures used ; 

 the refractive index is far from additive in any of the four 

 cases, and cannot be satisfactorily expressed by the parabolic 

 formula used in Part I. for mixtures of benzene with metbyl 

 and ethyl acetates ; it is therefore worth while to record the 

 results by a short extract from the tables used, which were 

 themselves derived from a smoothed curve based on experi- 

 ment. 



Refractive Indices (at 18° for sodium light). 



Per cent, 

 of alcohol 

 (or CC1 4 ). 



Alcohol- 

 benzene. 



Alcohol- 

 toluene. 



Carbon 



tetrachloride- 



benzene. 



Carbon 



tetrachloride- 



toluene. 



. 



15024 



1-4970 



1-5024 



1-4970 



10 



1 -4869 



1-4823 



1-5008 



1-4956 



20 



1-4716 



1-4680 



1-4985 



1-4937 



3Q 



1-4568 



1-4539 



1-4958 



1-4914 



40 



1-4425 



1-4401 



1-4929 



1-4889 



50 



1-4283 



1-4265 



1-4894 



1-4860 



60 



1-4146 



1-4131 



1-4853 



1-4827 



70 



1-4011 



1-4000 



1-4807 



1-4787 



80 



1-3878 



1-3873 



1-4755 



1-4742 



90 



1-3749 



1-3747 



1-4692 



1-4685 



100 



1-3622 



1-3622 



1-4618 



1-4618 



The numbers recorded in an experiment on the composition 

 of the vapour were (1) the refractometer reading, with the 

 corresponding temperature for the liquid before distillation ; 

 (2) that for each of the distillates ; (3) that of the residue. 

 From them were calculated the refractive indices, reduced to 

 18°, and then, from the tables quoted above, the percentage 

 composition of the liquid (mean between the reading before 

 and after), and of the vapour (mean of the three distillates). 

 The results are shown in the following tables : — 



