388 PRINCIPLES OF CHEMISTRY 



into carbonic oxide (C + CO 2 =CO + CO), i s considered a reversible 

 reaction, because at a high temperature the carbonic oxide splits up 

 into carbon and carbonic anhydride, as Sainte-Claire Deville showed by 

 using the method of the ' cold and hot tube.' Inside a tube heated in 

 a furnace another thin metallic (silvered copper) tube is fitted, through 

 which a constant stream of cold water flows. The carbonic oxide 

 coming into contact with the heated walls of the exterior tube forms 

 charcoal, and its minute particles settle in the form of lampblack on 

 (the lower part) of the cold tube, and, since they are cooled, they do not 

 act further on the oxygen or carbonic anhydride formed. 24 A series 

 of electric sparks also decomposes carbonic oxide into carbonic anhy- 

 dride and carbon, and if the carbonic anhydride be removed by 

 alkali complete decomposition may be obtained (Deville). Aqueous 

 vapour, which is so similar to carbonic anhydride in many respects, 

 acts, at a high temperature, on charcoal in an exactly similar way, 

 C + H 2 O = H 2 + CO. From 2 volumes of carbonic anhydride with 

 charcoal 4 volumes of carbonic oxide (2 molecules) are obtained, and 

 precisely the same from 2 volumes of water vapour with charcoal 



21 The first product of combustion of charcoal is always carbonic anhydride, and not 

 carbonic oxide. This is seen from the fact that with a shallow layer of charcoal (less 

 than a decimetre if the charcoal be closely packed) carbonic oxide is not formed at all. 

 It is not even produced with a deep layer of charcoal if the temperature is not above 500, 

 and the current of air or oxygen is very slow. With a rapid current of air the charcoal 

 becomes red hot, and the temperature rises, and then carbonic oxide appears (Lang 1888). 

 Naumann and Pistor determined that the reaction of carbonic anhydride with carbon 

 commences at about 550, and that between water and carbon at about 500. At the latter 

 temperature carbonic anhydride is formed, and only with a rise of temperature is car- 

 bonic oxide formed (Lang) from the action of the carbonic anhydride on the carbon, and 

 from the reaction CO 2 + H 2 = CO + H 2 O. Kathke (1881) showed that at no temperature 

 whatever is the reaction as expressed by the equation CO 2 + C = 2CO complete ; a part of 

 the carbonic anhydride remains, and Lang determined that at about 1000 not less than 

 3 p.c. of the carbonic anhydride remains untransformed into carbonic oxide, even after 

 the action has been continued for several hours. The endothermal reactions, C + 2H 2 O = 

 CO 2 + 2H 2 , and CO + H 2 O = CO 2 + H 2 , are just as incomplete. This is made clear if we 

 note that on the one hand the above-mentioned reactions are all reversible, and therefore 

 bounded by a limit ; and, on the other hand, that at about 500 oxygen begins to combine 

 with hydrogen and carbon, and also that the lower limits of dissociation of water, carbonic 

 anhydride, and carbonic oxide lie near one another between 500 and 1200. For water 

 and carbonic oxide the lower limit of the commencement of dissociation is unknown, but 

 judging from the accumulated data (according to Le Chatelier, 1888) that of carbonic 

 anhydride may be taken as about 1050. Even at about 200 half the carbonic anhydride 

 dissociates if the pressure be small, about O'OOl atmosphere. At the atmospheric 

 pressure, not more than 0'05 of the carbonic anhydride decomposes. The influence of 

 pressure is hei-e evident, for the reason that the splitting up of carbonic anhydride into 

 carbonic oxide and oxygen is accompanied by an increase in volume (as in the case of 

 the dissociation of nitric peroxide. See Chapter VI. Note 46). As in stoves and 

 lamps, and also with explosive substances, the temperature is not higher than 2000 to 

 2500, it is evident that although the partial pressure of carbonic anhydride is small, still 

 its dissociation cannot be considerable, and probably does not exceed 5 p.c. 



