ASSIMILATION. 105 



therefore, that, by just so much as plants grow, they take carbonic 

 acid from the air, they retain its carbon, and return its oxygen. 



351. In the production of that modification of cellulose called 

 Llgnine (42), which abounds in wood (if this be really a simple 

 product, and not a mixture), not only must a larger amount of car- 

 bonic acid be decomposed, but a small portion of water also, with 

 the liberation of its oxygen. For the composition attributed to it 

 shows that it contains less oxygen than would suffice to convert its 

 hydrogen into water.* 



352. The whole class of fatty substances, including the Oils, Wax, 

 Chlorophyll (84, 88, 92), &c, contain, some of them no oxygen at 

 all (such as caoutchouc and Pine-oil), and all of them less oxygen 

 than is requisite to convert their hydrogen into water. In their 

 direct formation, if this be supposed, not only all the oxygen of the 

 carbonic acid has been given out, but also a portion belonging to the 

 •water. If formed by a further deoxidation of neutral ternary pro- 

 ducts, the same result is attained as respects the liberation of oxy- 

 gen gas, but by two or more steps instead of one. The Resins, 

 doubtless, are not direct vegetable products, but originate from the 

 alternation and partial oxidation of the essential oils. Balsams, 

 which exude from the bark .of certain plants, are natural solutions of 

 resins in their essential oils, as rosin, or Pine-resin, in the oil of tur- 

 pentine. 



3o3. An opposite class, the Vegetable Acids (8G), contain more 

 oxygen than is necessary for the conversion of their hydrogen into 

 water, but less than the amount which exists in carbonic acid and 

 water. Indeed, the most general vegetable acid, the oxalic (which 

 may be formed artificially by the action of nitric acid on starch), 

 has no hydrogen, except in the atom of water that is connected with 

 it. Acids are sometimes formed in the leaves, as in the Sorrel, the 



although the chemist is unable to transform starch, sugar, &e. into cellulose, yet 

 he readily effects the opposite change, by reconverting woody fibre, &c. (under 

 the influence of sulphuric acid) into dextrine and sugar. The plant docs the 

 same thing in the ripening of fruits, during which a portion of tissue is often 

 transformed into sugar. Starch-giains and cellulose can never be formed arti- 

 ficially, because they are not merely organizable matter, but have an organic 

 structure. 



* According to Pa yen, ligninc, separated as much as possible from cellulose, 

 consists of Carbon 53.8, Hydrogen 6.0, and Oxygen 40 2 per cent, = C&, Hsi, 

 0*. 



