142 



HISTORY OF THE VEGETABLE KINGDOM. 



wliicli is tannin. Pure tannin is colourless, its 

 taste is excessively astringent, without bitterness. 

 It has no smell, water dissolves it freely, and 

 the solution reddens paper stained with litmus. 

 It decomposes the alkalies with effervescence, 

 forms with most of the metallic solutions a pre- 

 cipitate, and with the salts of iron a deep black 

 ink. 



Wlien tannin is distilled it yields charcoal, 

 carbonic acid, and inflammable gases, with a 

 minute quantity of volatile alkali, and seems 

 accordingly to consist of the same elements as 

 extract; from which, however, it is distinguished 

 by the peculiar property of its action upon gela- 

 tine. 



Gall nuts were kno'mi to the ancients, and 

 were employed by them in medicine; but they 

 seem to have had no accurate idea of their origin, 

 as they considered them to be the fruit of the 

 oak. They are now ascertained to be the nidus 

 or nest of the young of the cynips gallm tinc- 

 torice, and of the cynips quercus folios, insects 

 which live on the oak. 



Proust was the first chemist who procured 

 tannin from gaUs in a separate state, and who 

 accurately described its properties. Chemists 

 have since enumerated several varieties of it. 

 The purest kind is that obtained from the seeds 

 of the grape. It forms a white precipitate with 

 the solution of isinglass, and in most of its quali- 

 ties resembles the tannin of nut galls. Catechu 

 contains another kind; its precipitate by gelatine 

 has a brown colour. The substance known 

 under the name of dragon's blood yields a tannin 

 also, with some distinctive peculiarities. That 

 got from sumac by drying and grinding the 

 shoots of the plant to a powder, yields a white 

 sediment when precipitated by gelatine. A 

 sixth kind is got from the wood of the morus 

 tinctoria by maceration in water or alcohol. 

 It is precipitated by a solution of common salt. 

 A seventh is got from the hino of the shops, 

 which is an extract from the cocoldba urifera. 

 Its solution throws down gelatine of a rose 

 colour, and forms with salts of iron a deep green 

 precipitate. Tannin is now reckoned an acid, 

 and other acids are procured from the various 

 substances just mentioned. 



Tannin exists in a gi-eat many vegetables, and 

 chiefly in the barks of various trees. The fol- 

 lowing table by Sir H. Davy, exhibits the rela- 

 tive value of different kinds of barks. It gives 

 the average obtained from 480 lb. of the entire 

 bark of a middle sized tree, of the several species 

 taken in spring when the quantity of tannin is 

 largest : 



Oak .... 

 Spanish chestnut 

 Leicester willow, (large) 

 Elm 



lb. 

 23 

 21 

 33 

 13 



lb. 



Common willow, (large) . . . 11 



Ash IR 



Beech 10 



Horse-chestnut 9 



Sycamora 11 



' Lombardy poplar 15 



Birch ...... 8 



Hazel 14 



Black thorn 16 



Coppuci oak 32 



Inner rind of oak hark ... 72 



Oak cut in autumn • ... 21 



The use of tannin in the arts is its property of 

 combining with the gelatine of the skins of 

 animals, and thereby rendering the leather 

 prepared in this way thick and impervious to 

 water. 



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 hau- 

 and cuticle. They are then soaked first in weaker 

 infusions, and afterwards in stronger infusions 

 of the bark, tiU 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 neces- 

 sary on an average to form one pound of leather. 

 Some recent improvements have shortened the 

 process of steeping the hides. Bark is used in 

 medicine, in the various forms, in which it is 

 found, as a tonic. 



Bitter and alkaline principles of vegetables. 

 Many vegetables have an extremely bitter taste, 

 such as quassia, peruvian bark, gentian, &c. 

 This bitter principle has been ascertained, by the 

 researches of modem chemists, to be of an al- 

 kaline nature. There are twenty-one of these 

 alkaline substances now ascertained. They are 

 all compounds of the following elementary sub- 

 stances : carbon, hydrogen, azote, and oxygen. 



Quinine. This is one of the most important 

 of the vegetable alkaline bitters. It was first 

 discovered by Vauquelin in 1811, and its pre- 

 paration on a large scale pointed out by PeUetier 

 and Caventon, in 1820. It is obtained by boiling 

 the yellow bark, {cinchona cordifolia) in water 

 and sulphuric acid, and then treating it with lime 

 and alcohol, when the quinina is precipitated in 

 the form of a white powder. It is a pure bitter, 

 possessing all the medicinal virtues of the Per- 

 uvian bark. The annual produce of this sub- 

 stance in Paris exceeds 120,000 ounces. 



Strychnina exists in the seeds or fruits of 

 several species of stryclinos, as the nux vomica. 

 Its taste is intensely bitter, it leaves an impres- 

 sion in the mouth similar to that produced by 

 certain metallic salts, and it acts with great energy 

 on the animal economy as a virulent poison. 



Narcotic principle. This well known medi- 



