( 539 ) 



Lately, however, a paper by Guye and Drouginine ') appeared, in 

 which some vapour press.ure determinations of nitrogen tetroxide 

 are mentioned. These vahies, which were obtained by a statical 

 method, however, deviate perceptibly from the vahies found dyna- 

 mically by Ramsay and Young ; Guye and Drouginine's line lies at 

 lower temperature, resp. higher pressure than that of Ramsay and 

 Young, and these deviations come particularly into the foreground 

 in the temperature region where only the two statical values marked 

 in the above table with an asterisk, have been given. 



If, accordingly, these two values, which have both been found 

 indirectly, were not correct, and if those of Guye and Drouginine 

 on the contrary, were, we should have found an example in the 

 nitrogen peroxide for which the velocity in the homogeneous equili- 

 brium is not (as is usually the case) very great with respect to the 

 velocity of evaporation: 



This last consideration led us to seek a statical method which 

 would enable us to determine the vapour tensions of the nitrogen- 

 tetroxide accurately, and to remove the said contradiction. 



Such an investigation was the more attractive as the P-7'-line 

 of the nitrogentetroxide possesses a point of inflection according to 

 Guye and Drouginine's determinations. Vapour pressure curves of 

 simple substances have, as far as is known, without exception P-T- 

 lines which are all convex seen from the temperature axis. For a 

 dissociating substance, however, as nitrogen-tetroxide, whose internal 

 vapour equilibi'ium between the molecules jV^^^ and N0^_ is accu- 

 rately known by numerous investigations, the existence of such a 

 point of inflection does not' seem impossible however. If we namely 

 consider the system NO^ — ^V/)^ as a pseudo-binary system, we 

 shall expect that the vapour pressure curve of the JSfO^, which 

 would indicate the coexistence between liquid and vapour, which 

 both consist exclusively of molecules xVO, lies at higher pressure, 

 resp. lower temperatures than the tension curve of the N^O^, as is 

 indicated in the figure 1. The vapour curve, which indicates the 

 stable coexistence of the equilibrium liquid and vapour, will lie close 

 to the N^Ot curve at low temperatures; at higher temperatures, 

 however, the dissociation taking place endothermically, it will approach 

 the NO, curve, and in this way it may give rise to a point of 

 inflection G. Guye and Drouginine's line actually shows such a point 

 of inflection between 0° and the boiling-point. Also for this reason 

 it seemed important to us to repeat the determinations of the vapour 



1) Journ. de Ch. ph. 8. 473 (1910). 



