1894.] Molecular Surface-energy of Mixtures of Liquids. 183 



that each should evaporate to nearly the same extent. For this 

 reason the following liquids were chosen: 



f Toluene U0'6 at 761'2 mm. pressure. 



I Piperidine 105 106 0< 2 at 769 mm. pressure. 



* f Benzene Constant, about 80. 



1 Carbon tetrachloride Constant 77. 



f Chlorobenzene Constant 132. 



I Ethylene dibromide . Within O5, about 131. 



f Carbon disulphide . . . Constant, about 46'2. 

 I Chloroform Constant, about 62. 



The amount of toluene distilled was 750 c.c. The thermometer did 

 not vary during the distillation by the fiftieth part of a degree. The 

 amount of piperidine was much less, about 75 c.c. The alteration of 

 boiling point appears to be due, in part at least, to its eager absorp- 

 tion of carbon dioxide. The benzene was part of a large stock which 

 had been repeatedly frozen and thawed. It was free from thiophene, 

 and had an absolutely constant boiling point. The carbon tetra- 

 chloride boiled constantly while 400 c.c. passed over. The chloroben- 

 zeue was part of a stock used for securing constant temperatures, 

 and had been repeatedly fractionated ; it boiled with absolute con- 

 stancy while 750 c.c. passed over. The ethylene dibromide was not 

 quite so pure ; the rise of 0'5, however, was spread over 200 c.c. ; 

 while the purity of the carbon disulphide and the chloroform was 

 guaranteed by the constancy of boiling point while large quantities 

 distilled. 



The molecular surface-energies of the pure substances were first 

 determined. They are given in the tables which follow : 



t = temperature. 



h = rises in centimetres in capillary tube. 

 p = density of liquid. 



7 = surface-tension calculated by the equation 7 = \rgln>. 

 r = radius of tube. 



7(Mw)* = molecular surface-energy, where 

 M = molecular weight, and 

 v = volume of one gram. 



* The constancy of the boiling-point of a liquid is the guarantee of its purity, 

 provided a considerable quantity boils at a constant temperature. The determina- 

 tion of the actual temperature involves the accuracy of the thermometer. 



