Note 47 — Chapter VI · TTHE COMPOUNDS OF NITROGEN WITH HYDROGEN AND OXYGEN

Liquid nitrogen peroxide is said by Geuther to boil at 22°–26°, and to have a sp. gr. at 0° = 1·494 and at 15° = 1·474. It is evident that, in the liquid as in the gaseous state, the variation of density with the temperature depends, not only on physical, but also on chemical changes, as the amount of N2O4 decreases and the amount of NO2 increases with the temperature, and they (as polymeric substances) should have different densities, as we find, for instance, in the hydrocarbons C5H10 and C10H20.

It may not be superfluous to mention here that the measurement of the specific heat of a mixture of the vapours of N₂O₄ and NO₂ enabled Berthelot to determine that the transformation of 2NO₂ into N₂O₄ is accompanied by the evolution of about 13,000 units of heat, and as the reaction proceeds with equal facility in either direction, it will be exothermal in the one direction and endothermal in the other; and this clearly demonstrates the possibility of reactions taking place in either direction, although, as a rule, reactions evolving heat proceed with greater ease.