COMPOUNDS OF CARBON WITH OXYGEN AND NITROGEN 39$ 



mixture the oxygen (and not the carbonic anhydride) be removed, 

 and a series of sparks be again passed, the decomposition is renewed, 

 and terminates with the complete dissociation of the carbonic 

 anhydride. Phosphorus is used in order to effect the complete absorp- 

 tion of the oxygen. In these examples we see that a definite mixture 

 of changeable substances is capable of arriving at a state of stable 

 equilibrium, destroyed, however, by the removal of one of the sub- 

 stances composing the mixture. This is one of the instances of the 

 influence of mass. 



Although carbonic anhydride is decomposed On heating, yielding 

 oxygen, it is nevertheless, like water, an unchangeable substance aV 

 ordinary temperatures. Its decomposition, as effected by plants, is 

 on this account all the more remarkable ; in this case the whole 

 of the oxygen of the carbonic anhydride is separated in the free 

 state. The mechanism of this change is that the heat and light 

 absorbed by the plants are expended in the decomposition of the 

 carbonic anhydride. This accounts for the enormous influence of 

 temperature and light on the growth of plants. But it is at present 

 not clearly understood how this takes place, or by what separate in- 

 termediate reactions the whole process of decomposition of carbonic 

 anhydride in plants into oxygen and the carbohydrates (Note 1) 

 remaining in them, takes place. It is known that sulphurous anhy- 

 dride (in many ways resembling carbonic anhydride) under the action 

 of light (and also of heat) forms sulphur and sulphuric anhydride, S0 3 , 

 and in the presence of water, sulphuric acid. But no similar decompo : 

 sition has been obtained directly with carbonic anhydride, although it 

 forms an exceedingly easily decomposable higher oxide percarbonic 



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 CO.j is decomposed when the tempera- 

 ture is about 3,000, and at 1,500 less than 1 per cent. (Krafts) ; whilst under a pressure 

 of 10 atmospheres about 84 per cent, is decomposed at 8,800 (Mallard and Le Chatelier). 

 It follows therefore that, under very small pressures, the dissociation of CO 2 will be 

 considerable even at comparatively moderate temperatures, but at the temperature ofc 

 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 

 CO.> (i.e. the amount of heat required to raise 44 units of weight of CO- 2 1), according 

 to the determinations and calculations of Mallard and Le Chatelier, for a constant 

 volume C v = 6'26 + 0'0087< ; for a constant pressure C p = C v +2 (see Chapter XIV., Note 7),' 

 i.e. the specific heat of CO 2 increases rapidly with a rise of temperature : for example, at 

 ' (per 1 part by weight), it is, at a constant pressure = 0'188, at 1,000 = 0'272, at 2,000, 

 about 0-856. 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 2,600), 

 beyond which th specific heat begins to fall. 



