﻿572 Vapour-Pressure of a Liquid at Constant Temperature. 



by compression the air remains to a large extent in the vapour 

 above the liquid and dissolves but slowly in the liquid formed; 

 consequently, as the volume of vapour diminishes the undis- 

 solved air becomes more and more compressed, and its pres- 

 sure increases and becomes considerable when the vapour is 

 nearly all condensed. Similarly, with an impurity more 

 volatile than the pure substance, a partial " fractionation " 

 of the liquid takes place, the less volatile pure substance 

 condensing first, whilst the vapour becomes more and 

 more contaminated with the more volatile impurity. 



On the other hand, when the readings are taken with 

 increasing volume, the air or volatile impurity is at first 

 uniformly distributed through the liquid, and it escapes but 

 slowly through the long column of liquid into the vapour 

 above. Thus when a reading is taken with a relatively small 

 volume of vapour, the air present has a much greater influence 

 in the first case than the second, but when the volume of 

 vapour is relatively large its influence is small in both 

 cases. I have noticed repeatedly that the freer a liquid is 

 from air or permanent gas, the more closely do the readings 

 during compression and expansion agree. 



The presence of a small quantity of impurity less volatile 

 than the pure substance may escape detection so long as the 

 volume of liquid, relatively to that of the vapour, is fairly 

 large ; but its influence becomes very marked when the 

 quantity of liquid is relatively very small. Thus, on one 

 occasion, the isopentane had come in contact for a moment 

 with an indiarubber tube and had, no doubt, dissolved a 

 minute quantity of it. It was noticed in this case that the 

 mercury was slightly tarnished (owing in all probability to 

 the sulphur in the indiarubber), but the vapour-pressures, 

 when there was plenty of liquid present, agreed among them- 

 selves and with previous determinations. When, however, 

 the volume was increased until nearly the whole of the liquid 

 was evaporated, the observed vapour-pressures were far too 

 low, and diminished rapidly as the volume was increased. 



In this case and in those in which the presence of any 

 permanent gas was detected, the experimental results were 

 rejected and the tube was refilled. 



