Hutchinson — Vapour- Pressure Apparatus. 



327 



and vapour are allowed to escape through the short tube a ; and 

 when all air is expelled from the flask, the tap is closed. The 

 pressure developed inside the flask will now force the liquid up 

 the pressure-tube b and into the mercury-tank, driving out all the 

 air in the tube. Quickly removing the flame, and allowing the 

 pressure to fall, hastening it if necessary by chilling the vapour 

 in the flask, the mercury is forced back into the tube. A column 

 of water now completely fills the part of the tube above the 

 mercury, so that the vapour-pressure of the liquid is found from 

 the mercury-column, less this column of water from the level in 

 the flask to the surface of the mercury in the tube. The level of 

 the liquid in the flask A may be regarded as constant, so that this 

 column of water is accurately measured by the reading of the 

 mercury surface. 



The method has been adapted to determining pressures greater 

 than atmospheric, by fitting another tube into the mercury-tank. 

 Here, in the case of high pressures, the liquid forced over from 

 the flask cannot escape, and forces the mercury up the second tube. 

 The apparatus is thus capable of giving, from one experiment, 

 pressures above and below that of the atmosphere, and thus takes 

 the place of the two methods, distinct and apart, which are usually 

 employed. 



The method is in daily use in the Physical Laboratory of the 

 Eoyal College of Science, and yields results, even in the hands of 

 inexperienced workers, which are accurate and concordant. The 

 following figures have been taken from the work of students using 

 the apparatus for the first time : — 



Temp. 

 C. 



104° 



101 

 90 

 80 

 20 



