ro.Mi'orxns OK CAKIION AVJTII O\Y<;I;N AND NITROGEN 383 



many other plants. Sometimes these acids exist in a free state in the 

 plants, and sometimes in the form of salts ; for instance, tartaric acid 

 is met with in grapes as the salt known as cream of tartar, but in the 

 impure state called argol, or tartar. C 4 H 5 KO 6 . In sorrel we find the 

 so-called salts of sorrel, or acid potassium oxalate, C 2 HKO 4 . There is 

 a very clear connection between carbonic anhydride and the above- 

 mentioned organic acids namely, they all, under one condition or 

 another, yield carbonic anhydride, and can all be formed by means of it 

 from substances destitute of acid properties. The following examples 

 afford the best demonstration of this fact : if acetic acid, C 2 H 4 O 2 , the 

 acid of vinegar, be passed in the form of vapour through a heated tube, 

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

 it can also be obtained conversely 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 ;{ Na + CO 2 = C 2 H 3 Na0 2 ; from this acetic acid itself may be 

 easily obtained in a similar way to that by which nitric acid may 

 be obtained from nitre. Therefore 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 ; the gas itself does not 

 show any acid character whatever, but on combining with the elements 

 of carbonic anhydride it acquires the properties of an acid. The investi- 

 gation 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 mole- 

 cule 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 CO 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. The facts of the matter are as follows : if we have a 

 given organic compound, A, which has not the properties of an acid, but 

 as the derivative of a hydrocarbon contains hydrogen combined 

 with carbon, then ACO 2 will be a monobasic organic acid, A2CO 2 

 a bibasic, A3CO 2 a tribasic, and so on that is, each molecule, CO 2 , 

 transforms one atom of hydrogen into that state in which it may be 

 replaced by metals, as in acids. Tljis already furnishes proof that in 

 organic acids it is necessary to recognise the group HCO 2 , or carboxyl. 



