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XXVI. 



VAPOUR-PRESSURE APPARATUS. 



By JAMES J. HUTCHINSON. 



[COMMUNICATED BY PROF. W. F. BARRETT, F.R.S.] 



[Bead, March 15 ; Keceived for Publication, April 22 ; 

 Published, Sept. 17, 1904.] 



The simple method usually employed in determining the vapour- 

 pressure of a liquid at various temperatures below its boiling-point, 

 consists of the following : — 



A round-bottomed flask is fitted with a cork, through which pass 

 a thermometer and long tube, the latter bent twice at right angles ; 

 the shorter arm of the tube passes just through the cork, the other 

 arm dipping into a mercury-tank. On boiling the liquid in the 

 flask, vapour is forced out through this tube, and escapes through the 

 mercury- valve into the atmosphere. The pressure of the vapour 

 is thus transmitted to the mercury through the vapour itself. On 

 removing the source of heat, the pressure within the flask falls 

 below that of the atmosphere ; and the mercury rises in the tube, 

 and serves to indicate the pressure at various temperatures as 

 recorded by the thermometer. The method has serious defects: 

 one is, that, as the vapour condenses in the tube, a head of liquid 

 is formed on the mercury-column ; and, as condensation continues, 

 this head of water is constantly changing, so that exact correction 

 for this error is impossible. The following method, in part due 

 to suggestions received from Professor Barrett, gets over this 

 difficulty. The flask is now provided with a cork, through which 

 three holes are bored, as shown in the figure, p. 326. Through 

 one hole passes a short piece of tubiug, bent at right angles, and 

 provided with a tap or press-clip. The second hole allows a 

 thermometer to pass into the vapour, while through the third 

 hole passes the long pressure-tube. This tube is similar to the 



