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A Simple Laboratory ^Method of ^Ieasurtng ^'"apor Tension. 



By a. E. Caswell. 



About ;i year a.i.'<i I desifined a slight UKuliticat ion of tlu» ordinary 

 barometer tuhf aiijiaralus for nieasurini: vapor i)ressiire of less tban an 

 atmof-phere. This has Ijeeu used in connection with a lieat course for 

 engineers with very satisfactory results, the accuracy attainable bein.j; 

 about the same as by the usual nietluxls. 



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 IOj 

 cm. long, graduated at suital)le intervals. B. is a metal tube of slightly 

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

 %ided with a tripod supiiort. 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 iiiljber 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 o cm. of the length of the tube. 

 A is filled with the li(inid wliose vapor tension is to be measured. The 

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

 corresponding change in height of the mercury colunui the necessary coi-- 

 reetion for any contained air may l)e determined. Ten centimeters is a 

 convenient length for the vapor space. D is equi])ped with suitable ther- 

 mometer and stirrer. 



This method involves determining the temperature correspojiding to u 

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

 heated 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 bo- 

 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 li<iui(l I>. is kei)t well stirred and allowed to cool 



