HEAT OF FORMATION OF DINITROBENZENE. 273 



order to estimate correctly the influence of these two effects 

 which would enter into subsequent operations, it was necessary 

 to make several series of experiments. In the first place, it 

 was convenient to measure directly, two experiments being 

 made in each case, the heat disengaged by the mixing of the 

 nitric acid 



(HN0 3 - -335 H 2 0) 



with the boiled sulphuric acid, in four different proportions, 

 chosen so as to comprise within their limits all the cases 

 possible in the experiments which were performed. In this 

 way the curve was obtained for the quantities of heat produced 

 for the whole series of intermediate mixtures. 



Then considerable quantities of each of these liquids were 

 prepared and proportions of water added to them, increasing 

 according to distinct ratios, which also comprised within their 

 limits all the cases possible in the experiments. Each time the 

 heat disengaged was measured, and curves constructed for the 

 heats of hydration of these various systems of mixtures. 



Thus were obtained the elements necessary for calculating by 

 interpolation the quantity q, in all cases included within the 

 limits of the experiments. 



This method is somewhat tedious, but it seemed to be the 

 most suitable for the object in view, viz. the study of a series 

 of analogous formations. If, however, there were only one 

 experiment of this kind to make, it would be preferable to 

 measure the heat given off in three cases only, viz. the mixing 

 of the two acids in their initial proportions; the mixing of 

 the two acids in their final proportions, in which they 

 exist after the performing of the experiments; and lastly, 

 by the addition of water (in the proportion furnished by this 

 experiment) to the mixture of the two acids corresponding to 

 the final proportions. 



Lastly, the quantity, q 1 (p. 271), was measured directly, by 

 dissolving a known weight of crystallised dinitrobenzene in a 

 mixture of the two acids and water of proportions similar to 

 those of the final condition of the liquid, that gives rise to the 

 dinitrobenzene. This quantity is negative, as generally happens 

 when solid bodies are dissolved. It was found equal to - 2 -6 9 

 for C 6 H 4 (N0 2 ) 2 . 



We thus arrive definitely at the quantity 



Q - q. - ft- 



But this quantity relates to the formation of dinitrobenzene by 

 means of the nitrosulphuric acid of the experiments. In order 

 to apply the reaction to pure nitric acid, we must, in addition, 

 take into account the heat given off by the previous combina- 

 tion of the two acids, and also that by the union of HN0 3 with 

 335 H 2 0. 



T 



