VEGETABLE PRODUCTS. 



151 



leaving it to concrete in tlie sun, or by the fire. 

 A second layer is added to the first, and others in 

 succession, till the vessel acquires the thickness 

 that is wanted. The mould is tlien broken, and 

 the vessel fit for use; and in this state it is gen- 

 erally brought to Europe. It is brought over in 

 its milky state also, by being excluded from the 

 action of the air. If the pulpy juice be exposed 

 to the air, an elastic pellicle is formed on the 

 surface. If it is confined in a vessel containing 

 oxygen gas, the pellicle is fonned sooner. If oxy- 

 iniuiatic acid is poured into the milky juice, the 

 caoutchouc precipitates immediately. This ren- 

 ders it probalile that the fonnation of the caout- 

 chouc is owing to the absorption ot oxygen. 



When pure this substance is of a white colour, 

 without taste or smell. The black colour which 

 it commonly presents is owing to the method of 

 drying the different layers upon the moulds on 

 which they are spread. They are dried by being 

 exposed to smoke. The black colour of the 

 caoutchouc, therefore, is owing to the smoke or 

 soot alternating with its different layers. It is 

 soft and pliable like leather, and extremely elas- 

 tic, so that it may be stretched to a very great 

 length, and still recover its foi-mer size. Mr 

 Gough of Manchester has made some curious ex- 

 periments on the changes of temperature which 

 this sulistance undergoes on its being stretched 

 out suddenly, from which it would appear that 

 ductility, or elasticity, as well as fluidity, de- 

 pends upon latent heat. It is not altered by ex- 

 posure to the air ; is insoluble in water ; but if 

 boiled in this fluid for some time, its edges be- 

 come so soft as to adhere if pressed together 

 closely for some time. It is insoluble in alco- 

 liol ; but soluble in pure ether and some of the 

 volatile oils, as also the alkalies. Naptha, or the 

 essential oil derived from tar or coal gas, is a 

 ready solvent of this substance. It is also acted 

 on by acids ; and from their decomposition, it 

 seems to consist of the elementary constituents 

 of carbon, hydrogen, oxygen, and nitrogen. 



Caoutchouc exists in many vegetables, com- 

 bined with other ingredients. From resins it 

 may be separated by their solution in alcohol ; 

 and from the berries of the misscltoe it may be 

 separated by the addition of water. Opium and 

 mastic contain a portion of it, but not a suffi- 

 cient proportion to compensate the labour of its 

 extraction. It is applied to many useffil pur- 

 poses. Rolled out into very thin plates it fonns 

 a tenacious covering to jars and phials, and is 

 quite impervious to water and most liquids. It is 

 also used to construct tubes and cylinders for 

 chemical and surgical purposes; and dissolved 

 in naptha, and spread over various kinds of cloth, 

 it forms the Mackintosh fabrics, now so univer- 

 sally used. An immense quantity of this sub- 

 stance is now imported into this country for this 

 manufacture, and consequently its price has been 



greatly raised. In the countries where it is pro- 

 duced the natives manufacture it into bottles, 

 make of it boots and shoes, and often bum it in- 

 stead of candles. 



Cork. This well known and useful article is 

 the outer bark of the quereus suber, or cork tree, 

 a kind of oak that grows abundantly in France, 

 Italy, and Spain. To prevent its natural exfo- 

 liation, which is always irregular, and to disen- 

 gage it in convenient portions, a longitudinal in- 

 cision is made in the bark from the root to the 

 top of the stem, and a transverse and circular in- 

 cision at each extremity ; the outer layer, which 

 is cork, is then stripped off, and to flatten and 

 reduce it to sheets, it is put into water, and 

 loaded with weights. The tree continues to 

 thrive though it is thus stripped of its cork once 

 in two or three years. 



Cork is a light, soft, and elastic substance. Its 

 colour is a sort of light tan. It is very inflam- 

 mable, and burns with a bright white flame, 

 leaving a black and bulky charcoal behind. 

 When distilled it yields a small quantity of am- 

 monia. Nitric acid corrodes and dissolves it, 

 changing its colour to yellow, and finally decom- 

 poses it, converting it partly into an acid, and 

 partly into a soft substance resembling wax or 

 resin. The acid which is thus formed is deno- 

 minated the suberic acid, and has been ascer- 

 tained to be one of a peculiar nature. It seems 

 probable that cork exists in the bark of some 

 other trees as well as the quercus suber. There 

 is a variety of the ulmus campestris, common in 

 hedge rows, whose bark assumes something of 

 the external appearance of cork, which it re- 

 sembles in its thickness, softness, and elasticity, 

 and in its loose and porous texture, as well as in 

 its chemical properties. Fourcroy seems indeed 

 to regard the epidermis of all trees to be a sort 

 of cork, but does not say on what grounds his 

 opinion is founded. 



Wood)/ fibre. The principal part of the stem, 

 root, and branches of trees, is known under the 

 denomination of wood; but this term is too 

 general for the purpose of analytical distinction, 

 as the part so designated often includes a great 

 proportion of the substances that have already 

 been described. It remains, therefore, to be con- 

 sidered whether there exists in the plant any in- 

 dividual substance different from those already 

 described, and constituting more immediately 

 the fabric of the wood. If a piece of wood is 

 well dried and digested, first in water and then 

 in alcohol, or such other solvent as shall produce 

 no violent effects upon the insoluble parts, and 

 if the digestion is continued till the liquid is no 

 longer coloured, and dissolves no more of the 

 substance of the plant, there remains behind a 

 sort of vegetable skeleton, which constitutes the 

 basis of the wood, and which has been called 

 woody fibre. It is composed of bundles of Ion- 



