xns OF CARBON WITH OXYGEN AND NITROGEN 381 



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

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

 oxide, hydrogen, and water, which does not change under the action of 

 heat. 17 Although, like water, carbonic anhydride is exceedingly stable, 

 still on being heated it partially decomposes into carbonic oxide and 

 oxygen. Deville showed that such is the case if carbonic anhydride 

 be passed through a long heated tube containing pieces of porcelain 

 and heated to 1300. If the products of decomposition namely, 

 the carbonic oxide and oxygen be suddenly cooled, they can be 

 collected separately, although they partly reunite together. A similar 

 decomposition of carbonic anhydride into carbonic oxide and oxygen 

 takes place on passing a series of electric sparks through it (for 

 instance, in the eudiometer). With this an increase of volume 

 occurs, because two volumes of CO 2 give two volumes of CO and one 

 volume of O. 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 the 

 carbonic anhydride be removed, then the mixture explodes when a 

 spark is passed and forms carbonic anhydride. If from an identical 

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

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



17 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, Ho, on combining with an atom of oxygen, O = 16, develops 69000 heat-units 

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

 a gas (steam) the latent heat of evaporation must be subtracted, and then 58000 calories 

 will be developed. Charcoal taken in the solid form on combining with Oo = 32 develops 

 about 97000 calories, forming gaseous CO 2 . If the charcoal were gaseous like the 

 hydrogen, and only contained C. z 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 10000 to 15000 calories, it must be held that gaseous carbon on 

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

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

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

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

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

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

 For this reason here also, as in relation to hydrogen and iron, we ought to expect an evi- 

 dent 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 will decom- 

 pose water, whilst an excess of hydrogen will decompose carbonic anhydride. If similar 

 relations of mutual action are made clear in isolated cases, still the full theory of the 

 subject is wanting in the present condition of chemical knowledge. 



