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XLVII. 



N 1 5 IP?' 



ON BROWN'S FORMULA FOR DISTILLATION. 



By SYDNEY YOUNG, D.Sc., F.R.S., 



University Professor of Chemistry, Trinity College, Dublin. 



Read December 16, 1919. Published January 2, 1920. 



The subject of fractional distillation was investigated by F. D. Brown during 

 the years 1879 to 1881 (Trans. Chem. Soc, xxxv, 547 ; xxxvii, 49 ; xxxix, 304 

 and 517), and he arrived at the conclusion that the relation between the 

 composition of a liquid mixture and that of the vapour evolved from it could 

 in general be expressed by the formula 



m' A = m A P A 



m' B «IePb 

 where m' A and m' B are the relative masses of the two substances in the 

 vapour, m A and m B their relative masses in the liquid mixture, and 

 P A and P B the vapour-pressures of the pure substances at the boiling 

 point of the mixture. He considered, however, that a better agreement 

 between the calculated and observed results was obtained by substituting 

 a -constant c for the ratio P A /P B - 



It is frequently convenient to write the equation 



—± = c — 



M' B ~ M B ' 



where m' a , m' b , and M A , M B are the relative number of molecules of A and B 

 in the vapour and the liquid respectively. 



As a matter of fact, Brown's experimental results did not agree by any 

 means well with the formula 



M^ = c Ma 

 M' B M b ' 



but it has been pointed out by the author (" Fractional Distillation," p. 93) 

 that the experimental evidence which so far has been obtained points to the 

 conclusion that the formula is applicable to those liquid mixtures for which 

 the relation P = mP a + (1 - m)P e holds good. [P is the vapour -pressure of the 

 mixture, and P A and P E the vapour-pressures of the two pure substances at 

 the same temperature, and m is the molar fraction of the substance A.] It 

 also seemed probable that in such cases c = P A /P B - 



The number of such mixtures investigated is, however, very small, and 

 further evidence is required before definite statements can be made. 



SCIENT. PEOC. R.D.S.. VOL. XV., NO. XLVII. 5 L 



