228 LECTURE XII. 



the products of each successive decomposition being more 

 and more highly oxidised. A suggestion as to the possible 

 nature of this oxidative decomposition in the case of carbo- 

 hydrates is afforded by Hoppe-Seyler's researches upon the 

 decomposition of carbohydrates by the action of alkalies 

 and by heating in closed tubes with water. He obtained 

 formic and ethylidene-lactic acids, an aromatic substance 

 termed pyrocatechin, and carbon dioxide was evolved. It 

 appears to be probable that oxalic acid is formed in the 

 process, and that it is decomposed into formic acid and carbon 

 dioxide, for the commercial preparation of oxalic acid is 

 effected by heating cellulose (sawdust) with a mixture of 

 potassium and sodium hydrates. We may represent the 

 formation of oxalic acid in this way by the following equa- 

 tion, 



pyrocatechin oxalic acid 



2C 6 H 12 O 6 + 4O 2 = C 6 H 6 O 2 + 3C 2 H 2 O 4 + 6H 2 O. 



It is not probable, however, that so highly oxidised an acid 

 as the oxalic is at once produced ; it is more likely that a 

 less highly oxidised acid, such as the succinic, is first formed, 

 and that from this, by successive oxidative decomposition, 

 the more highly oxidised acids are derived. The formation 

 of succinic acid may be represented thus, 



succinic acid 



aCeH^Oe + 2 = C 6 H 6 2 + 3 C 4 H 6 4 + 3 H 2 O. 



Moreover all the more complex acids yield the simpler 

 ones on oxidation ; thus, when succinic acid is fused with 

 potassium hydrate, a mixture of potassium propionate and 

 carbonate is formed ; when malic, tartaric, or citric acid is 

 similarly treated, it yields a mixture of potassium acetate 

 and oxalate. 



It must not be assumed that the decomposition of carbohydrate 

 always takes place in the manner described above ; this instance is only 

 cited to illustrate the point that the production of acids from carbo- 

 hydrates is not effected by simple oxidation. 



There is, however, reason to believe that the particular decompo- 

 sition in question does take place in plants. Karl Kraus has found that 

 the external dry scales of the Onion contain pyrocatechin and crystals 

 of calcium oxalate, but no grape-sugar ; whereas the internal succulent 



