Young — Vapour- Pressures, ^"c, of Thirty Pure Substances. 389 



previously been purified by repeated crystallizations. It was distilled, and 

 after the first two-thirds of the total quantity of 600 c.c. had come over the 

 boiling-point remained perfectly constant. 



Specific gravity at 0^/4°, . . ] -0697 



Boiling-point under normal pressure, . 1I8'5° 



Proofs of Purify. 



A liquid was not regarded as pure unless it fulfilled the following con- 

 ditions : — 



1. There was no perceptible rise of temperature during distillation. 



2. The vapour-pressure at each temperature was independent of the 

 relative volumes of liquid and vapour. 



There are certain mixtures which boil at a perfectly constant temperature, 

 and in this respect behave like pure liquids ; but the composition of the 

 mixture of constant boiling-point varies with the pressure under which the 

 liquid is distilled. Consequently, if the vapour-pressure of the liquid were 

 determined at its boiling-point under the pressure at which it was distilled, 

 no change would be produced by altering the relative volumes of liquid 

 and vapour ; but if the determination were made at a much higher 

 temperature, the mixture would not have the composition corresponding to 

 that temperature ; and the vapour-pressure would fall when the volume of 

 vapour was increased, and rise when it was diminished. 



3. The vapour-pressures of each liquid at temperatures below the 

 boiling-point under normal pressure were determined by a dynamical 

 method ; at higher temperatures by a statical method. The continuity of 

 the two vapour-pressure curves affords strong evidence of the purity of the 

 substance investigated. 



4. In the determination of the specific volumes of liquid and saturated 

 vapour at the highest temperatures, the values calculated for different 

 readings of the actual volumes of liquid and saturated vapour showed good 

 agreement. Such a result would not be obtained if impurities were present, 

 even in very small amount. 



5. The phenomena observable at and in the immediate neighbourhood of 

 the critical point were quite normal. It is usually quite easy to detect the 

 presence of an impurity by observing the disappearance and reappearance 

 of the meniscus, and the formation of mist and so on at and very near the 

 critical point. 



The only cases in which these tests are inconclusive are those in which the 

 impurity boils at very nearly the same temperature as the pure liquid, and is 



SOLENT. PROC. E.D.S., VOL. XII., NO. XXXI. 3 H 



