150 



SCIENCE OF GARDENING. 



Part IL 



655. The finest of all vegetable bhuce Is that which is known by 

 thename of indigo. Itisthe produce of the indigofera tinctoria, 

 Lin., a shrub which is cultivated for the sake of the dye it 

 affords, in Mexico and the East Indies. The plant reaches 

 maturity in about six months, when its leaves are gathered 

 and immersed in vessels filled with water till fermentation 

 takes place. The water then becomes opaque and green, ex- 

 haling an odor like that of volatile alkali, and evolving bubbles 

 of carbonic acid gas. When the fermentation has been con- 

 tinued long enough, the liquid is decanted and put into other 

 vessels, where it is agitated till blue flakes begin to appear. 

 Water is now poured in, and the flakes are precipitated in the 

 form of a blue powdery sediment, which is obtained by de- 

 cantation ; and which, after being made up into small lumps 

 and dried in the shade, is the indigo of the shojis. It is insolu- 

 ble in water, though slightly soluble in alcohol. But its true 

 solvent is sulphuric acid, with which it forms a fine blue dye, 

 known by the name of Uquid blue. It affords by distillation 

 carbonic acid gas, water, ammonia, some oily and acid matter, 

 and much charcoal ; whence its constituent principles are 

 most probably carbon, hydrogen, oxygen, and nitrogen. 

 Indigo may be procured also from several other plants besides 

 indigofera tinctoria, and particularly from isatis tinctoria or 

 woad, a plant indigenous to Britain, and thought to be the 

 plant with the juice of which the ancient Britons stained their 

 naked bodies, to make them look terrible to their enemies. If 

 this plant is digested in alcohol, and the solution evaporated, 

 white crystalline grains, somewhat resembling starch, will be 

 left behind; wliich grains are indigo, becoming gradually 

 blue by the action of the atmosphere. The blue color of in- 

 digo therefore is owing to its combination with oxygen. 



656. The principal red colors are such as are found to exist in the 

 root, stem, or fiower, of the five following plants : rubra tinc- 

 torum, lichen roccella and parellus, carthamus tinctorius, 

 cse&alpinia crista, and hjematoxylon campechianum. 



657. Yellow, which is a color of very frequent occurrence 

 among vegetables, and the most permanent among flowers, is ex- 

 tracted for the purpose of dyeing, from a variety of plants. It is 

 extracted from the reseda luteola, Lin., by the decoction of its 

 dried stems. The coloring matter is precipitated by means 

 of alum, and is much used in dyeing wool, silk, and cotton. 

 It is also obtained from the morus tinctoria, bixa orellana, or 



659. 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 veget- 

 ables also, as well as those already enumerated, but chiefly from their bark ; 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 barks, as ascertained by Sir Humphry Davy. It gives the average obtained from 

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

 the quantity of tannin is the largest. 



arnotta [fig. 54.), serratula tinctoria, genista tinctoria, rhus 

 cotinus, rhamnus infectorius, and quercus tinctoria, or quer- 

 citron, the bark of which last affords a rich and permanent 

 yellow that is at present much in use. 



658. The brorvn coloring matter aj' vegetables is very abundant, 

 particularly in astringent plants. It is obtained from the root of 

 the walnut-tree, and rind of the walnut ; as also from the 

 sumac and alder, but chiefly from nut galls, which are ex- 

 crescences formed upon the leaves of a species of quercus, 

 indigenous to the south of Europe, in consequence of the punc- 

 ture of insects. The best in quality are brought from the 

 Levant. They are sharp and bitter to" the taste, and extremely 

 astringent ; and soluble in water by decoction when ground or 

 grated to a powder. The decoction strikes, with the solution 

 of iron, a deep black, that forms the basis of ink, and of most 

 dark colors used in dyeing cloths. 



Oak - . - - 



Spanish chestnut 

 Leicester willow (large) 

 Elm - 



Common willow (large) 

 Ash - 



Beech 



Horse-chestnut 

 Sycamore - 

 Lombardy poplar 

 Birch 

 Hazel 



6(30. Tannin is of the very first utility in its 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 consequence 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 prepared for 

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

 first in weaker infusions, 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. 



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

 bitter. The quassia of the shops, the roots of 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 examples. 

 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 intensely bitter ; and 

 if it is evaporated to dryness, 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. 



662. 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 ingredient, 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 the action of opium on the animal subject depends on this principle. 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 carbonic 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 inspis- 

 sated juice of lettuce, which resembles opium much in its appearance, is obtained by the same means, and 

 possesses the same medical virtues j the leaves of atropa belladonna, or deadly nightshade, and indeed the 



