122 COMPRESSIBILITY OF GASES. 



MENDELEEF 1 — 1872-76. 



Briefly described, the apparatus devised by Mendeleef 

 consisted of a reservoir of mercury, in the upper part of 

 which was the air to be submitted to low pressure, com- 

 municating by means of a capillary tube with a branch 

 of a siphon barometer. The height of the mercury in 

 this barometer indicated the pressure at work. By 

 withdrawing mercury from the reservoir the pressure 

 was reduced, and the weight of the mercury withdrawn 

 gave the data from which to calculate exactly the volume 

 of the expanded gas. 



This method is very simple, but its application de- 

 manded the utmost care and skill. Various and nice 

 precautions were needed in order to eliminate sources of 

 error and to reveal minute details in the results. 



Among these sources of error are some which even 

 Regnault had neglected. The surface of mercury in 

 any tube is convex. Mendeleef took account of the air 

 which lay below the plane across its summit. 



Every change of pressure not only changes the volume 

 of the air, but it changes also the capacity of the vessel 

 which contains it. This compressibility of the reservoir 

 had to be determined and introduced as a correction of 

 the results. 



The last traces of air had to be expelled from the mer- 

 cury and from the barometer. Mendeleef devised a 

 new mode of filling the barometer for this purpose. 



The tubes had to be calibrated with the greatest exact- 

 ness ; the joints of the apparatus had to be tight to the 

 last degree of perfection ; the gas had to be desiccated 

 with care ; and, finally, the temperature of the whole 



1 " On the Elasticity of Gases, (Russian,) Mendeleef. 

 Russian Journal of Artillery, 1872. 

 Bulletin St. Petersburg Acad, of Sciences, 1874. 

 Annalcs de Chemie de Physique, 1876. 

 Nature, xv. p. -155. 



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