394 PRINCIPLES OF CHEMISTRY 



acid ; 18 and perhaps that is the reason the oxygen separates. On 

 the other hand, it is known that plants always form and contain 

 organic acids, and these must be regarded as derivatives of carbonic 

 acid, as is seen by all their reactions, of which we will shortly treat. 

 For this reason it might be thought that the carbonic acid absorbed by 

 the plants first forms (according to Baeyer) formic aldehyde, CH 2 O, 

 and from it organic acids, and that these latter in their final trans- 

 formation form all the other complex organic substances of the plants. 

 Many organic acids are found in plants in considerable quantity ; for 

 instance, tartaric acid, C 4 H 6 O 6 , found in grape-juice and on the acid 

 juice of many plants ; malic acid, C 4 H 6 5 , found not only in unripe 

 apples but in still larger quantities in mountain ash berries ; citric 

 acid, C 6 H 8 7 , found in the acid juice of lemons, in gooseberries, 

 cranberries, <fcc. ; oxalic acid, C^B^O^ found in wood-sorrel and 

 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 HK0 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, 



* 8 Percarbonic acid, H 2 CO 4 ( = H 2 CO 3 + O) is supposed by A. Bach (1898) 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: 3H 2 CO 5 = 2H 2 CO 4 + CH 2 O. This 

 substance CH 2 O 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 CO 2 . 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, COj. 

 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 (Schone, 

 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 H 2 O 2 . 

 Besides which Bach (1894) showed that, in the first place, traces of formic aldehyde 

 and oxidising agents (CO 3 or H 2 O 2 ) are formed under the simultaneous action of CO 2 

 and sunlight upon a solution containing a salt of uranium (which is oxidised), and diethyl- 

 aniline (which reacts with CH 2 O), and secondly, that by subjecting BaO 2 , shaken up in 

 water, to the action of a stream of CO 2 in the cold, extracting (also in the cold) with 

 ether, and then adding an alcoholic sShxtion of NaHO, crystalline plates of a sodium 

 salt may be obtained, which with wate* 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. 



