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A Simple Laboratory jMetiiod of I\Ieasitking Vapor Tension. 



By a. E. Caswell. 



About a year ago I designed a slight modification of the ordinary 

 barometer tube apparatus for measuring vapor pressure of less than an 

 atmosphere. This has been used in connection with a heat course for 

 engineers with very satisfactory results, the accuracy attainable being 

 about the same as by the usual metliods. 



The general arrangement of the apparatus is shown in the accompany- 

 ing figure. A, is a piece of glass tubing about 2 cm. in diameter and 10.1 

 cm. long, graduated at suitable intervals. B, is a metal tube of slightly 

 larger cross section than A, and ending in the reservoir C. This is pro- 

 \ided with a tripod support. The length from the bottom of the tube to 

 the top of the reservoir may be 15 cm. less than the length of A. The 

 top of the tube A is surrounded by the vessel D, which may consist simply 

 of a metal or glass tube fitted with a rubber stopper E. The vessel D, 

 together with the tube A, to which it is rigidly attached, is raised or 

 lowered by means of a clamp attached either to a rigid support attached 

 to the tube B, or to a common laboratory support. When the tubes A 

 and B are being filled with mercury about 5 cm. of the length of the tube. 

 A is filled with the liquid whose vapor tension is to be measured. Tlie 

 vapor space can be varied by raising or lowering D, and by noting the 

 corresponding change in height of the mercury column the necessary cor- 

 rection for any contained air may be determined. Ten centimeters is a 

 convenient length for the vapor space. I) is equipped with suital>le ther- 

 mometer and stirrer. 



This method involves determining the temperature corresponding to a 

 given vapor pressure. The vessel D, is filled with water, or other liquid, 

 Iteated to a temperature above that at which the determination is to be 

 made, and raised or lowered until the mercury surface in the tube is be- 

 low graduation G, which is about 1~> cm. from the upper end of the tube, 

 and another graduation coincides with the level of the mercury surface 

 in the reservoir C. The liquid D, is kept well stirred and allowed to cool 



