Note 18 — Chapter IX · COMPOUNDS OF CARBON WITH OXYGEN AND NITROGEN

Percarbonic acid, H2CO4 (= H2CO3 + O) is supposed by A. Bach (1893) to be formed from carbonic acid in the action of light upon plants, (in the same manner as, according to the above scheme, sulphuric acid from sulphurous) with the formation of carbon, which remains in the form of hydrates of carbon: 3H2CO3 = 2H2CO4 + CH2O. This substance CH2O expresses the composition of formic aldehyde which, according to Baeyer, by polymerisation and further changes, gives other hydrates of carbon and forms the first product which is formed in plants from CO2. And Berthelot (1872) had already, at the time of the discovery of persulphuric (Chapter XX.) and pernitric (Chapter VI., Note 26) acids pointed out the formation of the unstable percarbonic anhydride, CO3. Thus, notwithstanding the hypothetical nature of the above equation, it may be admitted all the more as it explains the comparative abundance of peroxide of hydrogen (Schöne, Chapter IV.) in the air, and this also at the period of the most energetic growth of plants (in July), because percarbonic acid should like all peroxides easily give H2O2. Besides which Bach (1894) showed that, in the first place, traces of formic aldehyde and oxidising agents (CO3 or H2O2) are formed under the simultaneous action of CO2 and sunlight upon a solution containing a salt of uranium (which is oxidised), and diethylaniline (which reacts with CH2O), and secondly, that by subjecting BaO2, shaken up in water, to the action of a stream of CO2 in the cold, extracting (also in the cold) with ether, and then adding an alcoholic solution of NaHO, crystalline plates of a sodium salt may be obtained, which with water evolve oxygen and leave sodium carbonate; they are therefore probably the per-salt. All these facts are of great interest and deserve further verification and elaboration.