Note 17 bis — Chapter IX · COMPOUNDS OF CARBON WITH OXYGEN AND NITROGEN

The degree or relative magnitude of the dissociation of CO2 varies with the temperature and pressure—that is, it increases with the temperature and as the pressure decreases. Deville found that at a pressure of 1 atmosphere in the flame of carbonic oxide burning in oxygen, about 40 per cent. of the CO2, is decomposed when the temperature is about 3,000°, and at 1,500° less than 1 per cent. (Krafts); whilst under a pressure of 10 atmospheres about 34 per cent. is decomposed at 3,300° (Mallard and Le Chatelier). It follows therefore that, under very small pressures, the dissociation of CO2 will be considerable even at comparatively moderate temperatures, but at the temperature of ordinary furnaces (about 1,000°) even under the small partial pressure of the carbonic acid, there are only small traces of decomposition which may be neglected in a practical estimation of the combustion of fuels. We may here cite the molecular specific heat of CO2 (i.e. the amount of heat required to raise 44 units of weight of CO2 1°), according to the determinations and calculations of Mallard and Le Chatelier, for a constant volume Cv = 6·26 + 0·0037t; for a constant pressure Cp = Cv + 2 (see Chapter XIV., Note 7), i.e. the specific heat of CO2 increases rapidly with a rise of temperature: for example, at 0° (per 1 part by weight), it is, at a constant pressure = 0·188, at 1,000° = 0·272, at 2,000°, about 0·356. A perfectly distinct rise of the specific heat (for example, at 2,000°, 0·409), is given by a comparison of observations made by the above-mentioned investigators and by Berthelot and Vieille (Kournakoff). The cause of this must be looked for in dissociation. T. M. Cheltzoff, however, considers upon the basis of his researches upon explosives that it must be admitted that a maximum is reached at a certain temperature (about 2,500°), beyond which the specific heat begins to fall.