Book I. VEGETABLE CHEMISTRY. 219 



and hence wool and silk assume a deeper dye, and retain it longer, than cotton or linen. Colouring 

 matter exhibits a great variety of tints, as it occurs in different species of plants ; and as it combines 

 with oxygen, which it absorbs from the atmosphere, it assumes a deeper shade ; but it loses at the same 

 time a portion of its hydrogen, and becomes insoluble in water; and thus it indicates its relation to ex- 

 tract. Fourcroy reduced colours to the four following sorts : extractive colours, oxygenated colours, carbo- 

 nated colours, and hydrogenated colours ; the first being soluble in water, and requiring the aid of saline or 

 metallic mordants to fix them upon cloth ; the second being insoluble in water, as altered by the absorp- 

 tion of oxygen, and requiring no mordant to fix them upon cloth ; the third containing in their compo- 

 sition a great proportion of carbon, but soluble in alkalies ; and the fourth containing a great proportion 

 of resin, but soluble in oils and alcohol. But the simplest mode of arrangement is that by which the dif- 

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

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



1415. The finest of all vej^etaUe blues is that which is known by by the action of the atmosi)Iiere. The blue colour of indigo, 

 the name of indigo. It is the produce of the Indigdfera tinct6ria therefore, is owing to its combination with oxygen. 

 Lin., a shrub which is cultivated in Mexico and the East 1416. The principal red cvltmrs are such eis are found to exist 



Indies for the sake of the dye it affords. The plant reaches in the root, stem, or flower, of the five following plants ; iiubia 

 maturity in about six months, when its leaves are gathered tinctbrum.RocceViu tinct6ria,Lecan6rapa^^;lla,C'arttan*tinc- 

 and immersed in vessels filled with water till fermentation t6rius, Caesalpfnia crista, and HajmatiSxylon campechi^num. 

 takes place. The water then becomes opaque and green, ex- 1417. Yellow, which is a colour of very frequent occurrence 



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

 of cafbonic acid gas. AVhen 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 tne /iesdda Lut^ola Lin., by the decoction 

 vessels, where it is agitated till blue flakes beg^n to appear. of its dried stems. The colouring matter is precipitated by 

 Water is now poured in, and flakes are precipitated in 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 A/6rus tinctoria, Btxa 

 cantation ; and which, after being made up into small lumps Orellana or amotta, ierratula tinctoria. Genista tinctoria, 



shade. Is the indigo of the sho - - .... ... ., . . , .. ... 



and dried in the shade. Is the indigo of the shops. It is insolu- iihus Ci5tinus, rthamnus infectiirius, and Qut-rcus tinct6ria, 



ble In water, though slightly soluble in alcohol ; but its true or quercitron, the bark of which last affords a rich and per- 



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



known by the name of liquid blue. It affords by distillation liXH.Tlie brown colouring matter of vegetables iiyerjabxaidLaxit, 

 carbonic acid gas, water, ammonia, some oily and acid mat- particularly in astringent plants. It is obtained from the root 

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

 are most probably carbon, hydrogen, oxygen, and nitrogen. sumach 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 Qu^rcus, 

 Indigdfera tinctoria, and particularly fi-om /satis tinctoria or indigenous to the south of Europe, in consequence of thepunc- 

 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 s'oluble 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 grains, somewhat resembling starch, will be of iron, a deep black, that forms the basis of ink, and of most 

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



1419. 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 yellov.'ish 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. W ith 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 ; 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 Huiiii)hry Davy. It gives the average obtained from 

 480 lbs. of the entire bark of a middle-sized tree of the different species, taken in the spring, when the 

 quantity of tannin is the largest : 



ih. 



lb. 

 Blackthorn - - - 16 



Coppice oak - - - 32 



Inner rind of oak bark - - 72 



Oak cut in autumn - - 21 



Larch cut in autumn - - 8 



1420. 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 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 im- 

 pregnated. 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. 



1421. 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 colour becomes yellow, and its taste in- 

 tensely 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. Thompson 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 

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

 nitrogen. 



142'2. 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 Papkver somniferum var. 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 vapours, 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 



