Book I. VEGETABLE CHEMISTRY. 2ia 



ferent species of coloring matter are classed according to their effect in the art of dyeing. The principal 

 and fundamental colors in this art are the blue, the red, the yellow, and the brown. 



1389. Thefiiieiit of all vegetable bluet is that which Is known by blue by the action of the atmosphere. The blue color of In- 

 the name of indigo. It is the produce of the indigofera tinctoria, digo, therefore, is owing to its combination with oxygen. 

 lAn., a shrub which is cultivated for the sake of the dye it 1590. The principal red colors are such as are found to exist 

 attords, in Mexico and the East Indies. The plant reaches in the root, stem, or flower, of the five following plants : rubia 

 maturity in about six months, when its leaves are gathered tinctorum, lichen, roccella and parellus, carthamus tinctorius, 

 and immersed in vessels filled with water till fermentation caesalpinia crista, and hasmatoxylon campechianum. 

 takes place. The water then becomes opaque and green, ex- 1391. Yellow, which is a color of very frequent occurrence 

 haling an odor like that of volatile alkali, and evolving bubbles among vegetables, and the most permanent among flowers, is 

 of carhonic acid gas. When the fermentation has been con- extracted for the purpose of dyeing, from a variety of plants, 

 tinned long enough, the liquid is decanted and put into other It is extracted from the reseda luteola, Lin., by the decoction of 

 vessels, where it is agitated till blue flakes begin to appear. its dried stems. The coloring matter is precipitated by 

 Water is now poured in, and flakes are precipitated m the means of alum, and is much used in dyeing wool, silk, and 

 form of a blue powdery sediment, which is obtained by de- cotton. It is also obtained from the moms tinctoria, bixa 

 cantation ; and which, after being made up Into small lumps orellana or amotta, serratula tinctoria, genista tinctoria, rhus 

 and dried in the shade, is the indigo of the snops. It is insolu- cotinus, rhamnus infectorius, and quercus tinctoria, or quer- 

 ble in water, though slightly soluble in alcohol. But its true citron, the bark 'of which last affords a rich and iiermanent 

 solvent is sulphuric acid, with which it forms a fine blue dye, yellow that is at present much in use. 

 known by the name of liquid blue. It affords by distillation 1392. The bro7vn coloring matter -'' 

 carbonic acid gas, water, ammonia, some oily and acid matter, particularly in astringent plants, 

 and much charcoal ; whence its constituent principles are of the walnut-tree, and rind of the walnut ; as also from the 

 most probably carbon, hydrogen, oxygen, and nitrogen. sumac and alder, but chiefly from nut-galls, which are ex- 

 Indigo may be procured also from several other plants besides crescences formed upon the leaves of a species of quercus, 

 indii?ofera tinctoria, and particularly from isatis tinctoria or indigenous to the south of Europe, in conseqiience of the punc. 

 woad, a plant indigenous to Britain, and thought to be the ture of insects. The best in quality are brought from the 

 plant with the juice of which the ancient Britons stained their Levant, They are sharp and bitter to the taste, and extremely 

 naked bodies, to make them look terrible to their enemies. If astringent ; and soluble in water by decoction when ground or 

 this plant is digested in alcohol, and the solution evaporated, grated to a powder. The decoction strikes, with the solution 

 white crystalline greuns, somewhat resembling starch, will be of iroft, a deep black, that forms the basis of ink, and of most 

 left behind ; which grains are indigo, becoming gradually dark colors used in dyeing cloths. 



1393. Tannin. If a quantity of pounded nut-galls, or bruised seeds of the grape, is taken and dissolved in 

 cold water, and the solution evaporated to dryness, there will be left behind a brittle and yellowish sub- 

 stance of a highly astringent taste, which substance is tannin, or the tanning principle. It is soluble both 

 in water and alcohol, but insoluble in ether. With the salts of iron it strikes a black. And when a so- 

 lution of gelatine is mixed with an aqueous solution of tannin, the tannin and gelatine fall down in com- 

 bination, and form an insoluble precipitate. When tannin is subjected to the process of distillation, it 

 yields charcoal, carbonic acid, and inflammable gases, with a minute quantity of volatile alkali, and seems 

 accordingly to consist of the same elements with extract, from which, however, it is distinguished by the 

 peculiar property of its action upon gelatine. Tannin may be obtained from a great variety of other vege- 

 tables also, as well as those already enumerated, but chiefly from their bark j and of barks, chiefly from 

 those that are astringent to the taste. The following table exhibits a general view of the relative value 

 of different species of bark, as ascertained by Sir Humphry Davy. It gives the average obtained from 

 4801b. of tlie entire bark of a middle-sized tree of the several different species, taken in the spring, when 

 the quantity of tannin is the largest. 



Black thorn - - - 16 



Coppice oak - - - - 32 



Inner rind of oak-bark - - 72 



Oak cut in autumn - - - 21 



Larch cut in autumn - - 8 



1394. Tannin is of the very first utility in Us application to medicine and the arts ; being regarded by 

 chemists as the general principle of astringency. The medical virtues of Peruvian bark, so celebrated as 

 a febrifuge and antiseptic, are supposed to depend upon the quantity and quality of its tannin. In conse- 

 quence of its peculiar property of forming an insoluble compound with gelatine, the hides of animals are 

 converted into leather, by the important art of tanning. The bark of the oak-tree, which contains tannin 

 in great abundance, is that which is most generally used by the tanner. The hides to be tanned are pre- 

 pared for the process by steeping them in lime-water, and scraping off the hair and cuticle. They are then 

 soaked, first in weaker and afterwards in stronger infusions of the bark, till at last they are completely 

 impregnated. This process requires a period of from ten to eighteen months, if the hides are thick ; and 

 four or five pounds of bark are necessary on an average to form one pound of leather. 



1395. Bitter principle. The taste of many vegetables, such as those employed in medicine, is extremely 

 bitter. The quassia of the shops, the roots of the common gentian, the bark and wood of common broom, 

 the calyx and floral leaves of the hop, and the leaves and flowers of chamomile, may be quoted as ex- 

 amples. This bitter taste has been thought to be owing to the presence of a peculiar substance, different 

 from every other vegetable substance, and has been distinguished by the name of the bitter principle. 

 When water has been digested for some time over quassia, its color becomes yellow, and its taste in- 

 tensely bitter ; and if it is evaporated to drj-ness, it leaves behind a substance of a brownish yellow, with 

 a slight degree of transparency, that continues for a time ductile, but becomes afterwards brittle. This 

 substance Dr. Thomson regards as the bitter principle in a state of purity. It is soluble in water and in 

 alcohol ; but the solution is not much affected by re-agents. Nitrate of silver and acetate of lead are the 

 only two that occasion a precipitate. The bitter principle is of great importance, not only in the practice 

 of medicine, but also in the art of brewing ; its influence being that of checking fermentation, preserving 

 the fermented liquor, and when the bitter of the hop is used, communicating a peculiar and agreeable 

 flavor. The bitter principle appears to consist principally of carbon, hydrogen, and oxygen, with a little 

 nitrogen. 



1396. Narcotic principle. There is a species of medical preparations known by the name of narcotics, 

 which have the property of inducing sleep ; and if administered in large doses, of occasioning death. 

 They are obtained from the milky and proper juices of some vegetables, and from the infusion of the 

 leaves or stem of others, all which have been supposed to contain in their composition some common in- 

 gredient, which chemists have agreed to designate by the name of the narcotic principle. It exists in 

 great abundance in opium, which is the concrete juice of papaver album, or the white poppy, from which 

 it is obtained pure, in the form of white crystals. It is soluble in boiling water and in alcohol, as well as 

 in all acid menstrua ; and it appears that tne action of opium on the animal subject depends on this prin- 

 ciple. When distilled it emits white vapors, which are condensed into a yellow oil. Some water and 

 carbonate of ammonia pass into a receiver ; and at last carbonijc acid gas, ammonia, and carburetted 

 hydrogen, are disengaged, and a bulky charcoal left behind. Many other vegetable substances besides 

 opium possess narcotic qualities though they have not yet been minutely analysed. The following are 

 the most remarkable : the inspissated juice of lettuce, which resembles opium much in its appearance, is 

 obtained by the same means, and possesses the same medical virtues ; the leaves of atropa belladonna, or 

 deadly nightshade, and indeed the whole plant ; the leaves of digitalis purpurea, or foKglove ; and lastly, 

 the following plants, hyoscyamus niger, conium maculatum, datura stramonium, and sedum palustre, with 

 many others belonging to the Linnaean natural order of Lurida-. 



1897. Acids. Acids are a class of substances that may be distinguished by their exciting on the palate 

 the sensation of sourness. They exist, not only in the animal and mineral, but also in the vegetable 

 kingdom ; jind such of them as are peculiar to vegetables have boon denominated vegetable acids. Of 

 acids peculiar to vegetables chemists enumerate the following : the oxalic, acetic, citric, malic, gallic, 

 tartaric, benzoic, and prussic, which exist ready formed in the juices or organs of the plant, and are as- 



