Maximum or Minimum Vapour-Pressure. 121 



which has been fully investigated by Barus *. A mixture 

 kept at a temperature above 200° C. is constantly losing water 

 which is absorbed by the glass ; the tube gradually becomes 

 quite opaque. The composition of the mixture is thus always 

 changing, and it is obviously impossible, under these circum- 

 stances, to make accurate measurements about the relation 

 between vapour-pressure and composition. We had, therefore, 

 to content ourselves with the results obtained. 



2. Acetone and Chloroform. 



According to v. Zawidski f mixtures of these two substances 

 have a minimum vapour-pressure at ordinary and low pres- 

 sures. As our success with mixtures of hydrochloric acid 

 and methyl ether J, which also have a minimum vapour- 

 pressure, had been only partial, owing to chemical action at 

 high temperatures, we resolved to undertake the investigation 

 of these mixtures in the neighbourhood of the critical point. 



We began by testing v. Zawidski's observations, and found 

 them confirmed. In our first experiments at high tempera- 

 tures, we inclosed the substances in the usual way in a high- 

 pressure tube of a Ducretet apparatus above mercury §, but 

 unfortunately it appeared that at high temperatures a che- 

 mical action takes place between the chloroform and the 

 mercury. We then carefully dried the chloroform (Schering's 

 chloral chloroform) with calcium chloride, but were unable 

 to reduce the action to any extent. The nature of the 

 action is such as to generate a gas which reduces the 

 critical temperature and raises the pressure. It is so rapid > 

 that in a short time the tube becomes opaque and useless. 

 The acetone (obtained from Kahlbaum, made from the 

 bisulphite compound) gave no difficulty. 



The results obtained bv this method are contained in the 

 following table (VI.) (p. 122). 



The table only contains the earlier observations of each 

 series, which are naturally more trustworthy than the later 

 ones, although even these show that the chloroform is 

 gradually changing into a mixture. From the vapour- 

 pressures at 251 0, 3 and the critical data, we find by inter- 

 polation the following vapour-pressures : — 



Temperature. Pressure. 



Chloroform. Mixture. 



257-6 50*1 51*4 



259-6 51-5 53-3 



Comparing these, it appears that the pressure of the mixture 



* Phil. Mag. xlvii. pp. 104, 461. t ZeiUchr.physik. Chemie, xxxv. p. 129. 

 X Phil. Mag. [6] i. p. 593. § Phil. Mag. [5] xlviii. p. 187. 



