222 SCIENCE OF AGRICULTURE. Paet If. 



liar diade. It is known to gardeners by the name of bloom. When rulAed off, it Is again reproduced, though sknvly. It 



It is easily rubbed off by the fingers ; and when viewed un- resists the action of dews and rUns, and is consequently inso- 



der the microscope seems to be composed of small opaque luble in water. But it is soluble in spirits of wine ; from 



and unpolished granules, somewhat similar to the powder of which circumstance it has been suspected, with some pro- 



irtarcb; out with a high magnifying power it appears transparent. bability, to be a resin. 



1445. The use qf resins in the arts is very considerable ; but their medical virtues are not quite so great 

 as has been generally supposed. They are employed in the arts of painting, varnishing, embalming, and 

 perfumery; and they furnish us with two of the most important of all materials to a naval power, pitch 

 and tar. 



1446. Gum-resins. This term is employed to denote a class of vegetable substances, which have been 

 regarded by chemists as consisting of gum and resin. They are generally contained in the proper vessels 

 of the plant, whether in the root, stem, branches, leaves, flowers, or fruit. But there is this remarkable 

 difference between resins and gum-resins, that the latter have never been known, like the former, to ex- 

 ude spontaneously from the plant. They are obtained by means of bruising the parts containing them, 

 and expressing the juice, which is always in the state of an emulsion, generally white, but sometimes of a 

 different color ; or they are obtained by means of incisions from which the juice flows. This juice, which 

 is the proper juice of the plant, is then exposed to the action of the sun, by which, in warm climates, it is 

 condensed and inspissated, and converted into the gum-resin of commerce. Gum-resins, in their solid 

 state, are brittle, and less transparent than resins. They have generally a strong smell, which is some- 

 times alliaceous, and a bitter and nauseous taste. They are partially soluble both in water and in alcohol. 

 When heated, they do not melt like the resins, nor are they so combustible. But they swell and soften by 

 heat, and at last burn away with a flame. By distillation they yield volatile oil, ammonia combined with 

 an acid, and have a bulky charcoal. The principal species of gum-resins which have been hitherto applied 

 to any useful piuT)ose are : 



1447. Galbanum, obtained from the stem of the bubon gal- 1455. Myrrh, the plant yielding which grows in Abyssinia 

 banum. and Arabia. Bruce savs, it belongs to the genus mimosa ; 



1448. .Ammoniac, brought from Africa in the form of small but however this may be, myrrh is the juice of the plant 

 tears ; the plant which yields it is thought to be a species of concreted in the form of tears. Its color is yellow, its odor 

 ferula. strong but agreeable, and its taste bitter ; it is employed in 



1449. Scammony, the produce of the convolvulus scammonia. medicine, and is esteemed an excellent stomachic. 



1450. Opoponaj;, obtamed from the pastinaca opoponax. 145C. Assafcetida, a substance which is well known for its 



1451. Buphorbium, the prodttce of the euphorbia officinalis ; strong and fetid smell, is obtained from the ferula assafoutida. 

 its taste is caustic ; it is considered as a poison, but is occa- At four years old the plant is dug up by the root. The root 

 sionally employed in medicine. is then cleaned, and the extremity cut off; a milky juice 



1452. O/Jianum is obtained from the juniperus lycia, which exudes, which is collected; and when it ceases to flow an- 

 grows in Arabia, particularly by the borders of the Red Sea. other portion is cut off, and more juice extricated. The pro- 

 It is the frankincense of the ancients. It exudes from in- cess is continued till the root is exhausted. The juice which 

 cisions made in the tree, and concretes into masses about the has been collected soon concretes, and constitutes assafoetida. 

 size of a chestnut. It is brought to Europe in small agglutinated grains of dif- 



1453. Sa^apenuw is supposed to be obtained from the ferula ferent colors, white, red, yellow. It is hard, but brittle. Its 

 persica. taste is bitter, and its smell insufferably fetid; the Indians 



1454. Gamboge, or gumgutt, the produce of the mangostana use it as a seasoning for their food, and call it the food of 

 cambogia. the gods. In Europe, it is used in medicine as an antispas- 

 modic. 



1457. Balsams. The substances known by the name of balsams are resins united to the benzoic acid. 

 They are obtained by means of incisions made in the bark, from which a viscous juice exudes, which is 

 afterwards inspissated by the action of the fire or air, or they are obtained by means of boiling the part 

 that contains them. They are thick and viscid juices, but become readily concrete. Their color is brown 

 or red; their smell aromatic when rubbed; their taste acrid; their specific gravity 1 '090. They are un- 

 alterable in the air after becoming concrete. They are insoluble in water, but boiling water abstracts part 

 of their acid; they are soluble in the alkalies and nitric acid. When heated they melt and swell, evolv- 

 ing a white and odorous smoke. The principal of the balsams are the following : benzoin, storax, styrax, 

 balsam of tolu, balsam of Peru. 



1458. Benzoin is the produce of the styrax benzoin. 1461.'BaZ*ani o/io/u is obtained from the toluiferabalsamum. 



1459. Storax is obtained from the styrax officinale. 1462. Balaam of Peru is obtained from the myroxylon perui- 



1460. Styrax is a semi-fluid juice, the produce of a tree said ferum. 

 to be cultivated in Arabia. 



1463. Camphor. The substance known by the name of camphor is obtained from the root and stem of 

 the laurus camphora, by distillation. When pure it is a white brittle substance, forming octagonal crystals 

 or square plates. Its taste is hot and acrid ; its odor strong but aromatic ; its specific gravity 0'9887. 

 When broke into small fragments and put into water, on the surface of which it swims, a singular 

 phenomenon ensues. The water surrounding the fragments is immediately put into commotion, advanc- 

 ing a,nd retiring in little waves, and attacking the fragments with violence. The minuter fragments are 

 driven backwards and forwards upon the surface as if impelled by contrary winds. If a drop of oil is let 

 fall on the surface of the water it produces an immediate calm. This phenomena has been attributed to 

 electricity. P'ourcroy thinks it is merely the effect of the affinities of the camphor, water, and air, enter- 

 ing into combination. Though camphor is obtained chiefly from the laurus camphora, yet it is known to 

 exist in a great many other plants, particularly labiate plants, and has been extracted from the roots of 

 Eodoary, sassafras, thyme, rosemary, and lavender. 



1464. Caoutchouc. The substance denominated caoutchouc was first introduced into Europe about the 

 beginning of the eighteenth century. But from a use to which it is very generally applied of rubbing out 

 the marks made upon paper by a black-lead pencil, it is better known to most people in this country by the 

 name of Indian rubber. It is obtained chiefly from haevea caoutchouc and jatropha elastica, trees indi- 

 genous to South America ; but it has been obtained also from several trees which grow in the East Indies, 

 such as ficus indicus, artocarpus integrifolia, and urceola elastica. If an incision is made into the bark of 

 any of these plants a milky juice exudes, which, when exposed to the air, concretes and forms caoutchouc. 

 As the object of the natives in collecting it had been originally to form it into vessels for their own use, it 

 IS generally made to concrete in the form of bags or bottles. This is done by applying the juice, when 

 fluid, in thin layers to a mould of dry clay, and then leaving it to concrete in the 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 then broken and the vessel fit for use, and in this state it is generally brought in- 

 to Europe. It has been brought, however, even in its milky state, by being confined from the action of 

 the air. If the milky juice is exposed to the air, an elastic pellicle is formed on the surface. If it is con- 

 fined in a vessel containing oxygene gas, the pellicle is formed sooner. If oxymuriatic acid 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 oxygene. Caoutchouc, when pure, is of a white color, with- 

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

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

 t smoke. The black color 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 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 Man- 

 chester, has made some curious and important experiments on the connection between the temperature 

 of caoutchouc 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 close- 

 ly together. It is insoluble in alcohol, but soluble in ether. It is soluble also in volatile oiU and in alka- 



