400 PRINCIPLES OF CHEMISTKY 



of electric sparks also decomposes carbonic oxide into carbonic anhydride 

 and carbon, and if the carbonic anhydride be removed by alkali com- 

 plete decomposition may be obtained (Deville). 24 bis 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 

 4 volumes of a gas consisting of hydrogen and carbonic oxide (H 2 -I- CO) 

 are formed. This mixture of combustible gases is called water gas** 



and also that the lower limits of dissociation of water, carbonic anhydride, and carbonic 

 oxide lie near one another between 500 and 1,200. For water and carbonic oxide the 

 lower limit of the commencement of dissociation is unknown, but judging from the pub- 

 lished data (according to Le Chatelier, 1888) that of carbonic anhydride may be taken 

 as about 1,050. 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 

 U'05 p.c. of the carbonic anhydride decomposes. The reason of the influence of pressure 

 is here evidently that the splitting up of carbonic anhydride into carbonic oxideand 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 2,000 to 2,500, it is evident that although 

 the partial pressure of carbonic anhydride is small, still its dissociation cannot here be 

 considerable, and probably does not exceed 5 p.c. 



t>is Besides which L. Mond (1890) showed that the powder of freshly reduced 

 metallic nickel (obtained by heating the oxide to redness in a stream of hydrogen) is able, 

 when heated even to 350, to completely decompose carbonic oxide into CO 2 and carbon, 

 which remains with the nickel and is easily removed from it by heating in a stream of 

 air. Here 2CO = COo + C. It should be remarked that heat is evolved in this reaction 

 (Note 25), and therefore that the influence of ' contact ' may here play a part. Indeed, 

 this reaction must be classed among the most remarkable instances. of the influence of 

 contact, especially as metals analogous to Ni (Fe and Co) do not effect this reaction 

 (see Chapter II., Note 17). 



'- ft A molecular weight of this gas, or 2 volumes CO (28 grams), on combustion 

 (forming CO..,) gives out 68,000 heat units (Thomsen 67,960 calories). A molecular weight 

 of hydrogen, H.> (or 2 volumes), develops on burning into liquid water 69,000 heat units 

 (according to Thomsen 68,300), but if it forms aqueous vapour 58,000 heat units. Char- 

 coal, resolving itself by combustion into the molecular quantity of CO 2 (2 volumes), 

 develops 97,000 heat units. From the data furnished by these exothermal reactions it 

 follows : (1) that the oxidation of charcoal into carbonic oxide develops 29,000 heat units j 

 (2) that the reaction C + COo = 2CO absorbs 39,000 heat units; (3) C + H 2 O = H 2 -i-CO 

 absorbs (if the water be in a state of vapour) 29,000 calories, but if the water be liquid 

 40,000 calories (almost as much as C + CO.,,); (4) C + H.,O = CO.> + 2H 2 absorbs (if the 

 water be in a state of vapour) 19,000 heat units; (5) the reaction CO + H 2 = CO 2 + Hg 

 develops 10,000 heat units if the water be in the state of vapour ; and (6) the decomposi- 

 tion expressed by the equation 2CO = C + CO 2 (Note 24 bis) is accompanied by the evolii* 

 tion of 39,000 units of heat. 



Hence it follows that 2 volumes of CO or Ho burning Into C0 2 or H 2 develop 

 almost the same amount of heat, just as also the heat effects corresponding with the 

 equations 



c+c 2 =co+co 



are nearly equal. 



