Young — Vapour- Pressures, ^c, of Thirty Pure Substances. 403 



appreciable amount of air or other permanent gas is present in the tube. 

 The amount of permanent gas must be exceedingly small — otherwise accurate 

 results cannot be obtained. 



After tlie reading has become constant the pressure is again slowly raised 

 — care being taken not to drive down the vapour below the top of the sealed 

 tube — until the next temperature is reached. If the top of the tube is 

 cooled, condensation takes place in it, and there is danger of liquid 

 remaining in contact with the walls of the tube, and of the apparent volume 

 of liquid being therefore too small. For the same reason it is advisable to 

 begin heating the tube in the manner described ; for if the tube were placed 

 in its final position at the beginning of the experiment, the liquid would boil 

 and would collect at the top of tlie tube. 



Theory of the Method. — When a sealed tube containing a liquid and its 

 saturated vapour is heated, the volume of liquid tends, on the one hand, to 

 increase owing to expansion, and, on the other hand, to diminish owing to 

 evaporation. The observed volume of liquid may therefore increase, remain 

 constant, or diminish as the temperature rises; and the actual change depends 

 on two unknown factors, neither of which can be calculated from the data 

 afforded by the determinations. 



The difficulty may be overcome by either of two methods, both of which 

 have actually been employed. 



1. The simpler plan is to make another series of determinations at the 

 same temperatures with a second tube (or with the first refilled) containing a 

 different quantity of liquid, so that the relative volumes of liquid and 

 vapour in the two tubes shall be widely different. 



Let m and m' be the weights of substance in the two tubes. At the 

 temperature t, let vi and v{ be the observed volumes of liquid, and v„ and z-/ 

 the observed volumes of saturated vapour ; and let R be the ratio of the 

 specific volume of saturated vapour to that of liquid. 



The total volumes of liquid, if all the vapour were condensed, would be 

 respectively 



vi + - and vi \ — 

 li R 



and the volumes of a gram of liquid would be 



Hence R = — = ^ , (5) 



Vim - vi Vpn - vi 



and Yi = ^^^^LZf^ (6) 



