the Nature of Liquids. 



137 



and isothermals at 0°, 21°, 49 a 7, 73°*7, 99°-8, 129°«9, and 

 151°'4. The limit of pressure was 800 millim. Now the 

 boiling-point of nitric peroxide is, from our measurements, 

 21 0, 8; hence the densities of the saturated vapour are de- 

 ducible from only the first three of the Messrs. Natansons' 

 isothermals. We have plotted their results on curve-paper; 

 this has shown us the regularity and trustworthiness of their 

 observations ; and by continuing the curves in the direction 

 in which they run until they intersect the straight lines 

 denoting vapour-pressures at the temperatures at which their 

 measurements were made (using for this purpose the vapour- 

 press ares determined by us), the density of the saturated 

 vapour is determined with but small error. 

 The Natansons' numbers are as follows : — 



Temp. 



Pressure. 



Density. 



Temp. 



Pressure. 



Density. 



-12-6 



millim. 

 115-4 



millim. 

 52-54 



o 



21 



millim. 

 491-60 



millim. 

 38-74 



00 



37-96 



35-84 





516-96 



3901 





86-57 



38-59 





55350 



3915 





172-48 



40-71 





639-17 



39-64 





250-66 



41-90 









At — 12°*6 the vapour-pressure of nitric peroxide is 125 mm. 

 The density of the saturated vapour must therefore be a little 

 above 52 54. Now the theoretical density of N 2 4 is 46. It 

 may be that the higher density is due to experimental error; 

 but from graphic representation of the Natansons' results 

 this appears improbable. If the measurement is correct, it 

 would imply that the chemical combination of molecules of 

 N0 2 is not complete when the molecular complexity is repre- 

 sented by the formula (N0 2 ) 2 , but may extend to (N0 2 ) 3 , or 

 even further. At 0° the vapour-pressure is 255 millim.; again 

 the density found by the Natansons must be nearly that of 

 saturation. At 21° the vapour-pressure is about 700 millim. ; 

 and a prolongation of the curve constructed from the above 

 numbers would cut the line representing the large alteration 

 of volume with no rise of vapour-pressure at a vapour-density 

 of about 40. It is evident, then, that with nitric peroxide, as 

 with acetic acid, the density of the saturated vapour rises with 

 fall of pressure and temperature. Now it is known that nitric 

 peroxide dissociates, for the physical properties (colour, &c.) 

 change, on change of (N0 2 )« into w(N0 2 ) ; and the similarity 

 of behaviour between nitric peroxide and acetic acid renders 

 Phil. Mag. S. 5. Vol. 23. No. 141. Feb. 1887. L 



