232 PHYTOCHEMY. 



rived from hydrogen ; (Braconnot, in Berlin Jahrbuch der 

 Pharmacie, b. 20. s. 205.) 



Tanning matter is produced by such a change in extracli% c 

 matter as renders it fit to coagulate animal jelly, and to 

 cause a black deposition of salts of iron. There is also a 

 dark green deposition of salts of iron, by means of the common 

 extractive matter ; and tanning matter may be almost com- 

 pletely changed into extract, by repeatedly evaporating a so- 

 lution of it, by means of heat. Caustic alkali also takes 

 from tannin its astringent taste, and it is hence also probable, 

 that a difference in the proportion of oxygen distinguishes 

 those two substances from each other. This is also in part 

 evident from the disengagement of the gallic acid during the 

 mouldincss which takes place on a solution of the gall-nut ; 

 because, during the production of this matter, the characters 

 of tannin are lost, and that acid probably does not exist in 

 the matter previously, but is produced during the change. 



We have already stated, when speaking of the green co- 

 lour of leaves, (317.), and of the tints of blossoms, (327.), 

 that the colouring matters of plants are of very different 

 kinds. They can sometimes be removed by water, and 

 sometimes by spirit of wine ; very frequently they are united 

 with a residuary powder, or a substance containing azote, which 

 gives the putrid smell to the colours of many plants, and with 

 ft weak nitric acid even produces azotic gas. 



355. 



An important ingredient of plants is the mild oil, which 

 we procure in its purest state from the fruit of the Olive and 

 Beech, but which we also very often obtain from the coty- 

 ledons, and alliuminous bodies in the seed. As, according 

 to corresponding analyses, mild oil consists of seventy-live 

 parts of carbon, and twenty-five of hydrogen ; as nature also 

 prepares the same matter from the saccharine juices of fruits, 

 and art can again change this oil into saccharine matter, by 

 treating it with the oxide of lead, it is extremely probable 

 that it owes its origin to a complete deoxidation of the saccha- 

 rine matter, and to an increased proportion of hydrogen. 



