PROCEEDINGS OE THE CANADIAN INSTITUTE. S? 



The Vapor Tensions of Liquid Mixtures. By W. Lash Miller, Ph.D., 

 and t. r. rosebrugh, m.a. 



(Read April 24, 1897). 



Much of the recent remarkable progress in physico-chemical work is due to 

 experimental and theoretical investigations on the vapor tensions of solutions, and 

 Professor Van 't Hofif's paper showing the relations between the tensions, freezing- 

 points, boiling-points, osmotic pressures and compositions of solutions marks a new 

 epoch in the science. In his celebrated monograph " On equilibrium in hetero- 

 geneous systems" Prof. Willard Gibbs has deduced an equation (i) trom which may 

 be obtained a relation between the alterations produced in the vapor tensions of 

 the components of a liquid mixture by altering the composition of the mixture. A 

 close examination of this result of Gibbs' and of the method by which it was ob- 

 tained, shows that his equation contains as special cases many of the results of 

 Van 't Hoff referred to above; it is consequently very desirable to subject the 

 equation in its most general form to a direct comparison with experimental results. 

 Such a control would be afforded by a set of measurements of the tensions and 

 compositions of the vapors given off at any constant temperature by mixtures of 

 two liquids in different proportions, but curiously enough no complete set of meas- 

 urements of the nature referred to seems as yet to have been .published. We have 

 undertaken to supply the requisite data by an investigation of the case of mixtures 

 of alcohol and water ; the present paper contains a short description of the ap- 

 paratus employed, the results of the measurements, and their comparison with the 

 theory will form the subject of a subsequent communication. 



The apparatus, as finally constructed, consists of a cylindrical vessel to hold five 

 litres, made of tinned copper, and provided with five openings. Of these, the first 

 is fitted with a thermometer, the second with a means of filling and. emptying the 

 vessel, the third with means of electrical communication to a heating coil sus- 

 pended in the, liquid ; while through the fourth passes a glass tube to convey the 

 vapors to a condenser, from which the condensed liquid drops back through the 

 fifth opening into the apparatus. When desired, small quantities of the condensed 

 vapor may be removed and their composition ascertained; these analyses, together 

 with a knowledge of the composition of the contents of the copper vessel and meas- 

 urements of the temperature and pressure, give all the data necessary for testing 

 the accuracy of the equation of Gibbs referred to above. 



In order to protect the vapor from partial condensation (and consequent 

 fractionation) on the way to the condenser, the tube through which it passes is 

 wound with insulated wire and may thus be kept hot, electrically; errors due to 

 splashing of the boiling liquid have been provided against by a special construction 

 at the bottom of the tube ; condensation on the walls of the copper vessel itself 

 is prevented by surrounding the latter with a tin cylinder wound with wire and kept 

 at the temperature desired by means of an adjustable, electrical current, while the 

 absence of super-heating, and a thorough equilibrium between vapor and liquid, 

 are secured by the use of perforated copper plates (under the surface of the liquid) 

 through which the vapor must find its way. 



(i) Trans. Connec. Acad. III. 143. Egn. No. 97. 



