COMPOUNDS OF CARBON WITH OXYGEN AND NITROGEN 395 



it splits up into carbonic anhydride and marsh gas = C0 2 + CH 4 . But 

 conversely it can also be obtained from those components into which it 

 decomposes. If one equivalent of hydrogen in marsh gas be replaced 

 (by indirect means) by sodium, and the compound CH 3 Na is obtained, 

 this directly absorbs carbonic anhydride, forming a salt of acetic acid, 

 CH 3 Na + CO 2 = C 2 H 3 NaO 2 ; from this acetic acid itself may be 

 easily obtained. Thus acetic acid decomposes into marsh gas and 

 carbonic anhydride, and conversely is obtainable from them. The 

 hydrogen of marsh gas does not, like' that in acids, show the property 

 of being directly replaced by metals ; i.e. CH 4 does not show any acid 

 character whatever, but on combining with the elements of carbonic 

 anhydride' it acquires the properties of- an acid. The investigation of 

 all other organic acids shows similarly that their acid character depends 

 on their containing the elements of carbonic anhydride. For this 

 reason there is no organic acid containing less oxygen in its molecule 

 than there is in carbonic anhydride ; every organic acid contains in its 

 molecule at least two atoms of oxygen. In order to express the rela- 

 tion between carbonic acid, H 2 C0 3 , and organic acids, and in order to 

 understand the reason of the acidity of these latter, it is simplest to 

 turn to that law of substitution which shows (Chapter VI.) the rela- 

 tion between the hydrogen and oxygen compounds of nitrogen, and 

 permits us (Chapter VIII.) to regard all hydrocarbons as derived 

 from methane. If we have a given organic compound, A, which 

 has not the properties of an acid, but contains hydrogen connected 

 to carbon, as in hydrocarbons, then AC0 2 will be a monobasic 

 organic acid, A2CO 2 a bibasic, A3CO 2 a tribasic, and so on that is, 

 each molecule of CO 2 transforms one atom of hydrogen into that 

 state in which it may be replaced by metals, as in acids. This 

 furnishes a direct proof that in organic acids it is necessary to 

 recognise the group HCO 2 , or carboxyl. If the addition of CO 2 raises 

 the basicity, the removal of C0 2 lowers it. Thus from the bibasic 

 oxalic acid, C 2 H 2 O 4 , or phthalic acid, C 8 H 6 O 4 , by eliminating CO 2 

 (easily effected experimentally) we obtain the monobasic formic acid, 

 CH 2 O 2 , or benzoic acid, C 7 H 6 O 2 , respectively. The nature of carboxyl 

 is directly explained by the law of substitution. Judging from what 

 has been stated in Chapters VI. and VIII. concerning this law, it 

 is evident that CO 2 is CH 4 with the exchange of H 4 for O 2 , and that 

 the hydrate of carbonic anhydride, H 2 CO 3 , is CO(OH) 2 , that is, 

 methane, in which two parts of hydrogen are replaced by two parts of 

 the water radicle (OH, hydroxyl) and the other two by oxygen. 

 Therefore the group CO(OH), or carboxyl, HCO 2) is a part of carbonic 

 acid, and is equivalent to (OH), and therefore also to H. That is, it 



