FORMATION OF CARBONIC ACID. 327 



If we examine the action of oxygen upon a sub- 

 stance containing a large quantity of hydrogen,, such 

 as alcohol, we find most distinctly, that the direct 

 formation of carbonic acid is the last stage of its 

 oxidation, and that it is preceded by a series of 

 changes, the last of which is a complete combustion 

 of the hydrogen. Aldehyde, acetic, formic, oxalic, 

 and carbonic acids, form a connected chain of pro- 

 ducts arising from the oxidation of alcohol ; and the 

 successive changes which this fluid experiences from 

 the action of oxygen may be readily traced in them. 

 Aldehyde is alcohol minus hydrogen ; acetic acid is 

 formed by the direct union of aldehyde with oxygen. 

 Formic acid and water are formed by the union of 

 acetic acid with oxygen. When all the hydrogen is 

 removed from this formic acid, oxalic acid is pro- 

 duced ; and the latter acid is converted into car- 

 bonic acid by uniting with an additional portion of 

 oxygen. All these products appear to be formed 

 simultaneously, by the action of oxidizing agents on 

 alcohol ; but it can scarcely be doubted, that the 

 formation of the last product, the carbonic acid, does 

 not take place until all the hydrogen has been ab- 

 stracted. 



The absorption of oxygen by drying oils certainly 

 does not depend upon the oxidation of their carbon ; 

 for in raw nut-oil, for example, which was not free 

 from mucilage and other substances, only twenty-one 

 volumes of carbonic acid were formed for every 146 

 volumes of oxygen gas absorbed. 



It must be remembered, that combustion or oxida- 

 tion at low temperatures produces results quite simi- 

 lar to combustion at high temperatures with limited 

 access of air. The most combustible element of a 

 compound, which is exposed to the action of oxygen, 

 must become oxidized first, for its superior combus- 

 tibility is caused by its being enabled to unite with 

 oxygen at a temperature at which the other elements 

 cannot enter into that combination ; this property 

 having the same effect as a greater affinity. 



