((^ïny 



tension with groat care, because witli accurate knowledge of these 

 values it is also possible to calculate the change of the degree of 



FIG.l 



dissociation of the vai)oiir along the line of coexistence by theaid<of' 

 the dala concerning the internal xapour 0(|nilil)i-inni, as will appear 

 further on. 



2. The reason that onr knowledge of the va|)our tensions of 

 nitrogen tetroxide is still small is to be found in the first place 

 in the violent action which takes place between NjO^ and mercury, 

 even when both have been dried by phosphorpentoxide. Hence the 

 use -of a mercury manometer in which the mercury of the mano- 

 meter comes into contact with the N^O^, is excluded. Guye and 

 Drouginine have evaded this difficulty by applying an air cushion 

 between the mercury manometer and the vessel with nitrogen tetroxide, 

 so that the pressure of the nitrogen tetroxide was transferred by the 

 air to the manometer. This method is, however, certainly attended 

 with drawbacks. Even apart fi-om the solution of air in the nitrogen 

 tetroxide, an eventual mixture of air with the gaseous tetroxide 

 produces already an increase of pressure. X^e determinations can be 

 correct only when the vapour in the iminediate neighbourhood of 

 the liquid (resp. solid substance) is pure tetroxide, i. e. when the 

 air has not penetrated so far. • 



For the above reason we have rejected the use of a mei-cury 

 manometer. As we mentioned above, Ramsay and Young carried out 

 their statical determinations by making use of the compressibility of 

 a' thin-walled glass reservoir. When in May of this year we -were 

 endeavouring to find a similar method of measuring tTre^pi-essTirér 

 which rests on the deformation of a thin glass wall, a paper appeared 



