154 



SCIENCE OF 'Gardening. 



Paiit II. 



in a vessel containing oxygene gas, the pellicle Is fonncd sooner. If ox>Tnuriatic acki is poured into the 

 milky juice, the caoutchouc precipitates immediately. This renders it probable that the formation of the 

 caoutchouc is owing to the absorption of oxygen. Caoutchouc, when pure, is of a white color, without 

 taste and without smell. The black color of the caoutchouc of commerce is owing to the method of dry- 

 ing the different layers upon the moulds on which they are spread. They are dried by being exposed to 

 smoke. The black color of the caoutchouc, therefore, is owing to the smoke or soot aiternating with its 

 different layers. It is soft and pliable like leather, and extremely elastic, so that it may be stretched to a 

 very great length, and still recover its former size. Its specific gravity is 0'9335. Gough, of Manchester, 

 has made some curious and important experiments on the connection between the temperature of caout- 

 chouc and its elasticity, from which it results that ductility as well as fluidity is owing to latent heat. 

 Caoutchouc is not altered by exposure to the air. It is perfectly insoluble in water ; but if boiled in water 

 for some time its edges become so soft that they will cement, if pressed and kept for a while closely toge- 

 ther. It is insoluble in alcohol, but soluble in ether. It is soluble also in volatile oils and in alkalies. 

 And from the action operated upon by acids it is thought to be composed of carbon, hydrogen, oxy- 

 gen, and azote. It seems to exist in a great variety of plants combined with otlier ingredients. It 

 may be separated from resins by alcohol. It may be separated from the berries of the misseltoe by means 

 of water, and from other vegetable substances by other processes. It is said to be contained both in opium 

 and in mastic. But from these substances it cannot "be extracted in sufficient quantities to make it wortli 

 tlie labor. It is applied to a great many useful purposes both in medicine and the arts, to which, from 

 its great pliability and elasticity, it is uncommonly well adapted. In the countries where it is produced 

 the natives make boots and shoes of it, and often use it by way of candle. 



693. Cork. The substance known by the name of cork is tiie outer and exfoliated bark of the quercus 

 suber or cork-tree, a species of oak that grows in great abundance in France, Spain, and Italy. But to 

 prevent its natural exfoliation, which is alwaj s irregular, and to disengage it in convenient portions, a longi- 

 tudinal incision is made in the bark from the root to the top of the stem ; and a transverse and circular in- 

 cision at each extremity. Tlie 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, distingu'shed 

 by the following properties : — Its color is a sort of light tan. It is very inflammable, and burns w.th a 

 bright white flame, leaving a black and bulky charcoal behind. When distilled it yields a small qu:intity 

 of ammonia. Nitric acid corrodes and dissolves it, changing its color to yellow, and finally decomposes it, 

 converting it partly into an acid, and partly into a soft substance resembling wax or resin. The acid 

 which is thus formed is denominated the suberic acid, and has been proved by the experiments of La- 

 grange to be an acid of a peculiar nature. It seems probable that cork exists in the bark of some other 

 trees also, as well as the quercus suber. The bark of the ulnius subcrosa assumes something of the exter- 

 nal appearance of cork, which it resembles in its thickness, softness, and elasticity, and in its loose and 

 porous texture, as well as also in its chemical properties. Fourcroy seems, indeed, to regard the epider- 

 mis of all trees whatever to be a sort of cork, but does not say on what grounds his opinion is founded. 



697. Woodi/ fibre. The principal body of the root, stem, and branches of trees, is designated by the 

 appellation of wood. But the term is too general for the purpose of analytical distinction, as tlie part 

 designated by it often includes the greater part of the substances that have been already enumerated. It 

 remains, therefore, to be ascertained whether there exists in the plant any individual 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 

 colored, 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 denominated woody fibre. It is 

 composed of bundles of longitudinal threads, which are divisible into others still smaller. It is somewhat 

 transparent. It is without taste and smell, and is not altered by exposure to the atmosphere. It is inso- 

 luble in water and alcohol ; but the fixed alkalies decompose it with the assistance of heat. When heated 

 in the open air it blackens without melting or frothing, and exhales a thick smoke and pungent odor, 

 leaving a charcoal that retains the form of the original mass. When distilled in a retort it yields an em- 

 pyreumatic oil, carburetted hydrogene gas, carbonic acid, and a portion of ammonia, according to Four- 

 croy, indicating the presence of nitrogen as constituting one of its elementary principles ; and yet this 

 ingredient does not appear in the result of the later analysis of Gay Lussac, and Thenard, which is, car- 

 bon, 52-53 ; oxygen, 41 78 ; hydrogen, 5.69; total 100. 



698. Cfiarconl. When wood is burnt with a smothered flame, the volatile parts are driven off" by 

 the heat, and there remains behind a substance exhibiting the exact form, and even the several layers of 

 the original mass. This process is denominated charring, and the substance obtained, charcoal. As it is 

 the woody fibre alone which resists the action of heat, while the other parts of the plant are dissipated, it 

 is plain that charcoal must be the residuum of woody fibre, and tliat the quantity of the one must depend 

 upon the quantity of the other, if they are not rather to be considered as the same. Charcoal may be ob- 

 tained from almost all parts of the plant, whether solid or fluid. It often escapes, however, during com- 

 bustion, under the form of carbonic acid, of wliich it constitutes one of the elements. From a variety of 

 cx})eriments made on different plants and on tlieir ditierent parts, it appears that the green parts contain 

 a greater proportion of charcoal than the rest. But this proportion is found to diminish in autumn, when 

 the green parts begin to be deprived of their glutinous and extractive juice. The wood contains more 

 charcoal than the alburnum, the bark more than both. But this last result is not constant in all plants, 

 because the bark is not a homogeneous substance, the outer parts being affected by the air and the inner 

 parts not. The wood of the quercus robur, separated from the alburnum, yielded from 100 parts of its dried 

 substance 19'75 of charcoal ; the alburnum, 17'5 ; the bark, 26; leaves gathered in May, 80; in Septem- 

 ber, 26. But the quantity of charcoal differs also in different plants, as well as in different parts of the 

 same. According to the experiments of Mushet, 100 parts of the following trees afforded as follows : — 



Lignum vitas 

 ]\Iahof;any 

 Laburnum 

 Chestnut 



Oak - . - - 



American black birch 



"Walnut - 



HoUy 



Beech 



American maple 

 Elm 



- 20-6 



- 19-9 



- 19-9 



- 19-9 



Norway fir 

 Sallow 

 Ash 

 Birch 



Scotch pine 



19-2 

 18-4 

 17-9 

 17-4 

 16-4 



699. The properties of charcoal are insolubility in water, of which however it absorbs a portion when 

 newly made, as also of atmospheric air. It is incapable of putrefaction. It is not altered by the most 

 violent heat that can be applied, if all air and moisture are excluded ; but when heated to a"bout8tX) it 

 burns in atmospheric air or oxygene gas, and if pure, without leaving any residuum. It is regarded by 

 chemists as being a triple compound, of which the ingredients are carbon, hydrogen, and oxygen. Char- 

 coal is of great utility both to the chemist and artist as a fuel for heating furnaces, as well as" for a variety 

 of other purposes. It is an excellent filter for purifying water. It is a very good tooth-powder ; and is 

 also an indispensable ingredient in the important manufacture of gunpowder. 



700. The sap. If the branch of a vine is cut asunder early in the spring, before the leaves have begun 

 to expand, a clear and colorless fluid will issue from the wound, which gardeners denominate the tears 

 of the vine. It is merely, however, the ascending sap, and may be procured from almost any other plant 

 by the same or similar means, and at the same season ; but particularly from the maple, birch, and walnut- 

 tree, by means of boring a hole in the trunk. It issues chiefly from the porous and mixed tubes of the 



