380 VAPOR-DENSITIES. 



objection would apply to the supposition of any error in the deter- 

 mination of the weight of the flask when filled with air alone.) 

 But if we should attribute the divergences to an impurity which 

 diminishes the density (as air), we should be driven to the conclusion 

 that the first series of Deville and Troost gives the most correct 

 results, and that all the best attested numbers at temperatures 

 below 90 are considerably in the wrong. It does not seem possible 

 to account for these discrepancies by any causes which would apply 

 to cases of normal or constant density. They are illustrations of 

 the general fact that when the density varies rapidly with the 

 temperature, determinations of density for the same temperature 

 and pressure by different observers, or different determinations by 

 the same observer, exhibit discordances which are entirely of a 

 different order of magnitude from those which occur with substances 

 of normal or constant densities, or which occur with the same 

 substance at temperatures at which the density approaches a 

 constant value. In some cases such results may be accounted for 

 by carelessness on the part of the observers, not controlled by a 

 comparison of the result with a value already known. But such an 

 explanation is inadequate to explain the general fact, and evidently 

 inadmissible in the present case. 



It is probable that these discrepancies are in part attributable 

 to a circumstance which has been noticed by M. Wurtz, in his 

 account of his experiments upon the vapor-density of bromhydrate 

 of amylene, in the following words : " Le temps pendant lequel la 

 vapeur est maintenue a la temperature ou Ton determine la densite 

 n'est pas sans influence sur les nombres obtenus. C'est ce qui result 

 des deux experiences faites a 225 degrees avec des produits identiques. 

 Dans la premiere, la vapeur a ete portee rapidement a 225 degre's. 

 Dans la seconde elle a ete maintenue pendant dix minutes a cette 

 temperature. On voit que les nombres trouves pour les densites ont 

 e'te fort difieYents. (The numbers were 4*69 and 3*68 respectively.) 

 Ce resultat ne doit point surprendre si Ton considere que le pheno- 

 mene de decomposition de la vapeur doit absorber de la chaleur, et 

 que les quantity's de chaleur necessaires pour produire et la dilatation 

 et la decomposition ne sauraient etre fournies instantanement."* 



It is not difficult to form an estimate of the quantities of heat 

 which come into play in such cases. With respect to peroxide of 

 nitrogen, it was estimated in the paper already cited that the heat 

 absorbed in the conversion of a unit of N 2 4 into NO 2 under 

 constant pressure is represented by 7181 a 2 . (The heat is supposed 

 to be measured in units of mechanical work.) Now the external 



* Comptes Rendua, t. Ix, p. 730. 



