802 PRINCIPLES OF CHEMISTRY 



But only a portion of the carbonic acid gas undergoes this change, 

 therefore the result will be a mixture of carbonic anhydride, carbonis 

 oxide, hydrogen, and water, which does not suffer further change under 

 the action of heat. 17 Although, like water, carbonic anhydride is ex* 

 ceedingly stable, still on being heated it partially decomposes into car* 

 bonic oxide and oxygen. Deville showed that such is the case if car- 

 bonic anhydride be passed through a long tube containing pieces of 

 porcelain and heated to 1,300. If the products of decomposition 

 namely, the carbonic oxide and oxygen be suddenly cooled, they can b& 

 collected separately, although they partly reunite together. A similar 

 decomposition of carbonic anhydride into carbonic oxide and oxygeiv 

 takes place on passing a series of electric sparks through it (|$n 

 instance, in the eudiometer). Under these conditions an increase' of 

 volume occurs, because two volumes of C0 2 give two volumes of CO and 

 one volume of 0. The decomposition reaches a certain limit (less than 

 one-third) and does not proceed further, so that the result is a 

 mixture of carbonic anhydride, carbonic oxide, and oxygen, which 

 is not altered in composition by the continued action of the sparks. 

 This is readily understood, as it is a reversible reaction. If thg* 

 carbonic anhydride be removed, then the mixture explodes when a? 

 spark is passed and forms carbonic anhydride,- 17 Ws If from an identical 



I? Hydrogen and carbon are near akin to oxygen as regards affinity, but it ought to 

 be considered that the affinity of hydrogen is slightly greater than that of carbon, be- 

 cause during the combustion of hydrocarbons the hydrogen burns first. Some'' idea of 

 this similarity of affinity may be formed by the quantity of heat evolved. Gaseous 

 hydrogen, EU, on combining with an atom of oxygen, = 16, develops 69,000 heat-units 

 if the water formed be condensed to a liquid state. If the water remains in the fonn of 

 H gas (steam) the latent heat of evaporation must be subtracted, and then 58,000 calories 

 will be developed. Carbon, C, as a solid, ou combining with 02 = 82 develops about 

 97,000 calories, forming gaseous CO 2 . If it were gaseous like hydrogen, and only 

 contained C 3 in its molecule, much more heat would be developed, and judging 

 by other substances, whose molecules on passing from the solid to the gaseous 

 state absorb about 10,000 to 15,000 calories, it must be held that gaseous carbon on 

 forming gaseous carbonic anhydride would develop not less than 110,000 calories thai 

 is, approximately twice as much as is developed in the formation of water. And since 

 there is twice as much oxygen in a molecule of carbonic anhydride as in a molecule of 

 water, the oxygen develops approximately the same quantity of heat on combining with 

 hydrogen as with carbon. That is to say, that here we find the same close affinity (see 

 Chapter II., Note 7) determined by the quantity of heat as between hydrogen, zinc, and 

 iron. For this reason here also, as in the case of hydrogen and iron, we ought to expecl 

 an equal distribution of oxygen between hydrogen and carbon, if they are both In 

 excess compared with the amount of oxygen ; but if there be an excess of carbon, it wiD 

 decompose water, whilst an excess of hydrogen will decompose carbonic anhydride. Ever* 

 if these phenomena aiid similar ones have been explained in isolated cases, a complete 

 theory of the whole subject is still wanting in the present condition of chemical know- 



17 bis The degree ov relative magnitude of the dissociation of C0 2 varies with the 

 temperature and pressure that is, it increases with the temperature and as the pressure 



