CHANGE OF VAPOUR PRESSURE 



49 



Accuracy attainable in measuring vapour pressures. 

 Finally, something may be said as to carrying out the 

 measurement of vapour pressure, and the accuracy attain- 

 able in doing so. As compared with the method of 

 measuring the osmotic pressure directly, the vapour pressure 

 method stands at a considerable disadvantage. The one per 

 cent, sugar solution, that at ordinary temperatures exercises 

 f atmosphere osmotic pressure, causes a fall of only 0-4 mm, 

 in the vapour pressure at 100. But on the other hand, 

 the measurement of osmotic pressure has met with such diffi- 

 culties in practice, that several attempts 

 have been made recently to obtain more 

 accurate results by the vapour pressure 

 method 1 . An important improvement 

 lies in increasing the difference of level 

 which corresponds to the pressure differ- 

 ence to be measured. The substitution 

 of oil for mercury (Frowein-Bremer 

 tensimeter 2 ) is a great advantage. But 

 lately Smits has applied a suggestion of 

 Kretz (Jamin, Cours de Physique, III, 

 vol. iv, p. 218) in using two liquids 

 differing but little in density. One the 

 heavier was aniline, which, when moist, has at 20 the 

 density 



s a = 1-022; 

 the other was water, whose density at 20 is 



8 b = 0-998. 



The heavier liquid occupies the lower part of the narrow 

 tube AB (Fig. 9) ; the water, for the most part, the reservoirs 

 c and D (and is covered with a thin layer of oil, not shown 

 in the diagram, to prevent evaporation). Then suppose an 



1 Dieterici, Wied. Ann. 62. 617 ; Smits, Arch. Neerl 1897 ; Boldingh, 

 MaandLl. voor Natuurw. 21. 181. 



2 Zeitschr.f. Phys. Chem. i. 424. 



Fig. 9. 



