INDIA-RUBBER, or CAOUTCHOUC, consists of the dried coagulated milky juice of various trees and shrubs, belong-ing chiefly to the natural orders Emphorbiaceae, Moraceae, Artocai-pacem, and Apocynacem. Although a milky juice is found in plants of many other families, it does not in all cases yield caoutchouc, nor do different species of the same genus yield an equal quantity or quality of that substance. On the other hand, there are many plants which afford a good rubber, but have not yet been sought out for com-mercial purposes. The milky juice of plants furnishing caoutchouc is contained chiefly in the middle layer of the, bark, in a network of minute tubes known to botanists as laticiferous vessels. In the Apocynaceae these vessels are found also in the inner bark, or bast layer. The milky juice above mentioned possesses the properties of a vegetable emulsion, the caoutchouc being suspended in it in the form of minute transparent globules, averaging, according to Adriani, 1/12250 inch in diameter. Like other emulsions, it is easily coagulated by the addition of an acid or saline solution,—alum or salt water being commonly used for this purpose; but it is said by Mr Bruce Warren not to be coagulated by alcohol. The caoutchouc appears to be kept in suspension in the juice by means of ammonia ; at least in some cases the fresh milk exhales an ammoniacal odour. Probably it is on this account that the addition of liquid ammonia prevents the juice from coagulating for a con-siderable length of time; and the ammonia is in certain districts added when the milk has to be carried some distance from the place of collection. The addition of salt water to the juice is to be deprecated, as it renders the caoutchonc very hygroscopic. The best rubber known is obtained by careful evaporation of the recently strained juice at a moderate heat. Trees are known to contain caoutchouc by the bark on incision yielding a milk that when rubbed between the fingers coagulates into an elastic fibre. The dried lark of such plants when broken shows between the two fractured surfaces of bark a number of silky fibres which can be stretched for some distance without breaking.
Caoutchouc differs from other vegetable products of like origin by possessing considerable elasticity, by being insoluble in water or alcohol, alkalies, and acids (with the exception of concentrated nitric and sulphuric acids). Although apparently simple in constitution, it contains, not only the elastic substance to which its commercial value is due, but a small quantity of an oxidized viscid resinous body soluble in alcohol. This latter substance varies in quantity in different kinds of rubber, those containing the smallest amount, such as the Parà and Ceara, being considered the most valuable, while those in which it is present in greatest proportion, such as the Guatemala and African rubbers, are the least esteemed. Rapid evaporation of the juice, or any means which prevents oxidation, tends to prevent the formation of this viscid resin.
The first notice of india-rubber on record was given nearly five hundred years ago by Herrera, who, in the second voyage of Columbus, observed that the inhabitants of Hayti played a game with balls made "of the gum of a tree," and that the balls, although large, were lighter and bounced better than the wind-balls of Castile (Herrera, Historia, dec. i. lib. iii. cap. iv.). Torquemada, however, seems to have been the first to mention by name the tree yielding it. In his De la Monarquia Indiana, published at Madrid in 1615, tom. ii., cap. x1iii. p. 663, he says, "There is a tree which the [Mexican] Indians call Ulequahuitl ; it is held in great estimation and grows in the hot country. It is not a very high tree; the leaves are round and of an ashy colour. This tree yields a white milky substance, thick and gummy, and in great abundance." He further states that the juice was collected and allowed to settle in calabashes, and was afterwards softened in hot water, or the juice smeared over the body and rubbed off when sufficiently dry, The tree mentioned by Torquernada has usually been identified as Castilloa elastica, Cerv., but the above account cannot apply to it, as that tree is described by Cervantes as one of the loftiest forest trees of the north-east coast of Mexico, and its leaves are not round but oblong-lanceolate. Torquemada mentions also that an oil was extracted from the "ulli," or rubber, by heat, possessing soft and lubricous properties, and of especial effect in removing tightness of the chest. It was also drunk with cocoa to stop haemorrhage. Even at that early date the Spaniards used the juice of the ulé tree to waterproof their cloaks. This fact, however, apparently did not attract attention in the Old World, and no rubber seems to have reached Europe until long afterwards. The first accurate information concerning any of the caoutchouc trees was furnished by La Condamine, who was sent in 1735 by the French Government to measure an arc of the meridian near Quito. I
In 1751 the researches of M. Fresnau, an engineer re-siding in Guiana, were published by the French Academy, and in 1755 M. Aublet described the species yielding caoutchouc in French Guiana. Nevertheless india-rubber remained for some time unknown in England except as a curiosity, for Dr Priestley, in the preface to his work on perspective, called public attention to it as a novelty for erasing pencil marks, and states that it was sold in cubical pieces of 1/2 inch for 3s. each. India-rubber was not known as a product of Asia until 1798, when a plant, afterwards named Urceola elastica, Roxb., was discovered to yield it by Mr J. Howison, a surgeon of Prince of Wales Island, and soon afterwards Assam rubber was traced by Dr Roxburgh to Ficus elastica, Roxb. It was not, however, until the beginning of the 18th century that the india--rubber industry really commenced. The rapid progress which this has made during the last twenty years may be perceived by a glance at the following table:—
Imported into England in the year 1830, 464 cwts.
Imported into England in the year 1840, 6,640 cwts.
Imported into England in the year 1850, 7,616 cwts.
Imported into England in the year 1870, 152,118 cwts.
Imported into England in the year 1879, 150,601 cwts.
It has been computed that in 1870 there were in Europe and America more than 150 manufactories, each employ-ing from 400 to 500 operatives, and consuming more than 10,000,000 1b of caoutchouc. The imports into the United States have largely increased during the last few years.
Botanical Sources, Modes of Preparation, &c.
Notwithstanding the fact that caoutchouc-yielcling trees are found in a large belt of countries around the globe, in-cluding at least 500 miles on each side of the equator, yet the demand for the best qualities of india-rubber is in excess of the supply. The varieties which are almost exclusively used when great elasticity and durability are required are the Pará, Ceara, and Madagascar rubbers.
The principal forms of caoutchouc which are imported into Great Britain may be grouped under four heads, the order in which they are here placed indicating their respective values;—South American, Pará, Ceara, Penambuco, Maranhão, Cartagena, Guayaquil; Central American—West Indian, Guatemala; African—Mada-gascar, Mozambique, West African; Asiatic—Assam, Borneo, Rangoon, Singapore, Penang, and Java. Of all these, the most important is the Pará, the imports of which, according to Messrs Hecht, Levis, & Kahn, have increased from 1670 tons in 1857 to 8000 tons in 1879. For this rubber and the Mozambique variety the demand in -increases every year,—an unerring indication of their value.
I. SOUTH AMERICAN.—Pará rubber is obtained chiefly from Heven brasiliensis, Müll. Arg., a large euphorbiaceous tree upwards of 60 feet in height, branching from the base, and having trifoliate leaves, the leaflets being lanceolate and tapering at both ends (figs. 1, 2). Other species of Hevea, as well as Micrandra siphonoides and M. minor, Benth., all of which grow abundantly in the moist steamy valleys of the Amazon and its tributaries, are also used indiscriminately by the natives to furnish Pará rubber. These trees are found in different districts, but all flourish best on rich alluvial clay slopes by the side of rivers, where there is a certain amount of drainage, and the temperature reaches from 89º to 94º at noon and is never cooler than 73º at night, while rain is rarely absent for ten days together. The genus Hevea was for-merly called Siphonia, and the tree named Pao de Xerringa by the Portuguese, from the use by the Omaqua Indians of squirts or syringes made from a piece of pipe inserted in a hollow flask-shaped ball of rubber.
The caoutchouc is collected in the so-called dry season between August and February. The trees are tapped in the evening, and the juice is collected on the following morning. To obtain the juice a deep horizontal incision is made near the base of the tree, and then from it a vertical one, extending up the trunk, with others at short distances in an oblique direction. Small shallow cups made from the clayey soil and dried in the sun are placed below the in-cisions to receive the milk, each cup being attached by sticking a piece of soft clay to the tree and pressing the cup against it. The juice, of which each tree yields only about 6 ounces in three days, has a strong ammoniacal. odour, which rapidly goes off, and inconsequence of the loss of ammonia it will not keep longer than a day unchanged, hence when it has to be carried to a distance from the place of collection 3 percent of liquid ammonia is added. The juice is said by Bruce Warren to yield half its weight of caout-chouc, but 32 percent. appears to be the usual quantity. To ob-tain the rubber the juice is heated in the following manner. A piece of wood about 3 feet long, with a flattened clay mould at one end of it, is dipped in the milk, or this is poured over it as evenly as possible. The milk is then carefully dried by turning the mould round and round in white vapour obtained by heating certain oily palm nuts, those of Attalea excelsa being much pre-ferred, and the vapour being confined within certain limits by the narrowness of the neck of the pot in which the nuts are heated. Each layer of rubber is allowed to become firm before adding an-other; a practised hand can make 5 or 6 lb in an hour. From what-ever cause, the rubber thus prepared is the finest that can be obtained. The cakes when completed are, in order to remove them from the mould, slit open with a sharp knife, which is kept wet, and are hung up to dry. The flat rounded cakes of rubber made in this manner are known in the London market as "biscuits." They rarely con-tain more than 15 per cent. of moisture. The scrapings from the tree, which contain fragments of wood, are mixed with the residues of the collecting pots and the refuse of the vessels employed, and are made up into large rounded balls, which form the inferior com-mercial quality called "negrohead," and often contain 25 to 35 per cent. of impurity. An intermediate quality is known as "entre-fine." Pará rubber is said to be sometimes adulterated with the juice of the Magandaruba tree (Mimusops elata), which might account for the great differences that have been occasionally observed in the behaviour of Pará rubber in certain stages of manufacture, the coagulated juice of the Mimusops genius resembling gutta percha rather than caoutchouc.
Previous to 1860 Pari rubber was exported only in small quan-tities, and then chiefly in the form of shoes; this variety ceased to be sent over in 1852. Occasionally "negrohead" has been im-ported in grotesque forms of animals, &c., and the better qualities in the shape of small bottles moulded in soft clay which has been afterwards washed out by water.
In British Guiana rubber is obtained from Hevea paucifolia, Müll. Arg.; in French Guiana from H. Guayanensis, Aubl., where it is known as "heve," "siringa," or "caoutchou,"—the last being the probable origin of the name caoutchouc; and in Venezuela from H. brasiliensis, there called dápi or dápiche. None is exported to England from any of these localities. Small quantities of rubber intermediate in character between that of Pará and Pernambuco are occasionally imported from Maranhão. On account of its great value as a source of caoutchouc, the cultivation of the Pará rubber tree has been attempted in India; but it has been found to be too tropical a plant for cultivation in northern and central India, although suit-able for Ceylon, Malabar, and South Burnish, according to recent reports. The seeds, which are about the size of a damson (fig. 2, d), soon lose their vitality, and cuttings do not thrive unless taken from the young wood.
Ceara rubber is considered almost next to the Pará in value, as it is a "dry" rubber, very elastic and free from stickiness; but it often contains a quantity of wood and foreign matter aris-ing from the mode of collecting it, the loss in washing previous to manufacture amounting some-times to 25 per cent. It is the produce of Manihot Glaziovii, Müll. Arg., a euph-orbiaceous tree com-mon in the province of Rio Janeiro, about 30 feet high, with a rounded head of foli-age, and greyish--green 3- to 7-lobed palmate leaves, some-what resembling the leaves of the castor--oil plant in shape and size (figs. 3, 4, 5). The trees are tapped, according to Mr R. Cross, when the trunk attains a diameter of 4 to 5 inches, i.e., when they are about two years old. The mode of collecting the rubber is as follows. After brushing away the loose stones and dirt from the root of the tree by means of a handful of twigs, the collector lays down large leaves for the milk to drop upon. He then slices off the outer layer of the bark to the height of 4 or 5 feet. The milk, which exudes in many tortuous courses, some of it ultimately falling on the ground, is allowed to remain on the tree for several days, until it becomes dry and solid, when it is pulled off in strings, which are either rolled up into balls or put into bags in loose masses, in which form it enters commerce under the name of Ceara "scrap." The amount of Ceara rubber imported in 1879 amounted to 500 cwt. The attempts which have been recently made to cultivate this rubber plant in India have been attended with signal success. In Rio Janeiro it grows in a rocky or stony and region, where a short under scrub is the only vegetation, and the atmosphere is hot and dry, the temperature ranging from 82º to 90º Fahr. It is, therefore, suited for cultivation where the Hevea will not grow. In Ceylon it has been found to thrive at an altitude of from 200 feet to 3000 feet above the sea level. At Zanzibar and Calcutta also it succeeds well. The seeds (fig. 5, c), which have a hard thick coat, take a year in germinating, unless the edges near the end bearing the car-uncular projection are rasped off. Cuttings, provided they have a single bud, strike readily.
Pernambuco or Mangabeira, rubber is obtained from Hancornia speciosa, Gom., an apocynaceous tree common on the South American plateau in Brazil from Pernambuco to Rio Janeiro, at a height of 3000 to 5000 feet above the sea, It is about the size of an ordinary apple tree, with small leaves like the willow, and a drooping habit like a weeping birch, and has an edible fruit called "mangaba," for which, rather than for the rubber, the tree is culti-vated in some districts. Only a small quantity of this rubber comes to England, and it is not much valued, being a "wet" rubber. It occurs in "biscuits" or "sheets." The caoutchouc is collected in the following manner. About eight oblique cuts are made all round the trunk, but only through the bark, and a tin cup is fastened at the bottom of each incision by means of a piece of soft clay. The cups when full are poured into a larger vessel, and solu-tion of alum is added to coagulate the juice. In two or three minutes coagulation takes place, and the rubber is then exposed to the air on sticks, and allowed to drain for eight days. About thirty days afterwards it is sent to market. Pernambuco rubber, as is the case with most rubbers coagulated by saline solutions, contains a large quantity of water.
Cartagena rubber comes from New Granada in the form of black sheets 3/4 inch thick, having a somewhat rough or "chewed" appear-ance, and is more or less "tarry" or sticky. It also occurs in the form of strips or small pieces pressed together in bags. Its botanical source is not known, but is thought to be a pinnate-leaved tree, a portion at least being derived, it is supposed, from Castilloa elastica. It loses 35 percent. of moisture when dried. The importa-tion of Cartagena rubber into Great Britain has declined from 3518 cwts. in 1875 to 1679 cwts. in 1879.
Guayaquil rubber is imported from Ecuador in large flakes or lumps, of a whitish colour in the best kinds, the inferior sorts being porous and filled with a fcetid black liquid, having an odour of cow-dung, and staining the knife and hands. It is believed to be obtained from Castilloa elastica. The amount imported into Britain has diminished from 3815 cwts. in 1875 to 482 cwts. in 1879. In washing for manufacture it sometimes loses up to 40 per cent. of its weight. The bulk of the two last-mentioned rubbers is ex-ported to the United States.
II. CENTRAL AMERICAN.—The source of all the principal rubbers exported from Central America is Castilloa elastica, Cerv., a lofty artocarpaceous tree, with a trunk 3 feet or more in diameter, and large hairy oblong lanceolate leaves often 18 inches long and 7 inches wide, those subtending the young branches being much smaller and more ovate (fig. 6). The tree grows most abundantly in a sporadic manner in the dense moist forests of the basin of the Rio San Juan, where the rain falls for line months in the year. It prefers rich fertile soil on the banks of watercourses, but does not flourish in swamps. It is found also in Costa Rica, Guatemala, Honduras, Mexico, Cuba, and Hayti, and in Panama in company with another species, C. Markhamiana, Collins, and on the west coast of South America down to the slopes of Chimborazo, the Cordilleras of the Andes separating the Castilloae from the Heveoe of Brazil, according to Mr R. Spruce.
Nicaragua rubber.—In Nicaragua the juice is collected in April, when the old leaves begin to fall and the new ones are appearing, during which time the milk is richest. The tree is tapped either in the manner as the Hevea, or by encircling the tree with a simple spiral cut at an inclination of 45º, or by two spirals in opposite directions if the tree be large. At the bottom of the spiral an iron spout about 4 inches long is driven into the tree, and the milk is received in iron pails. A tree 20 to 30 feet high to its first branches, and about 4 feet in diameter, is expected to yield 20 gallons of milk, each gallon giving about 2 lb of rubber. In the evening the milk is strained through a wire sieve and transferred to barrels. The milk is coagulated by the addition of the juice of the "acheté" plant (Ipomoea bona-nox) or of another plant called "coasso." The strained juice of either of these plants, obtained by bruising the moistened herb and subsequent expression, is added to the milk in the proportion of about 1 pint to the gallon. If these plants are not procurable, two parts ofwater are added to one of the milk, and the mixture allowed to stand for twelve hours. The coagulum is next flattened out by a wooden or iron roller to get rid of the cavities con-taining watery liquid, and the sheets are then hung up for fourteen days to dry, when they weigh about 2 lb, the sheets being usually _ to 1/8th inch thick and 20 inches in diameter. When coagulated by water, the mass is placed in vats in the ground and allowed to dry, this taking place in about a fortnight. It is then rolled into balls. That which dries on the incisions in the tree is called bola or burucba, and is said to be highly prized in New York. The loss of Nicaragua rubber in drying is estimated at 15 per cent. It is exported chiefly from San Juan del Norte, or Grey Town, and the larger proportion goes to the United States, The Castilloa appears to be suitable for cultivation only in districts where the Pará rubber would grow equally well. The deciduous lateral shoots if planted will never grow erect.
West Indian rubber is the variety usually imported into England, but in comparatively small quantity only. It occurs in the form of blocks, the finest quality consisting of thin separable sheets, and the second of "scraps," usually conglomerated and containing fragments of bark. It is the best description of Central American rubber known. It is not, as its name seems to imply, produced in the West Indies, but derives its appellation from being brought over in West Indian steamers.
Honduras rubber rarely comes over to England; it is of good quality, and free from "tarry" matter.
Mexican rubber is imported into Liverpool in small quantity only. The imports of Mexican caoutchouc decreased from 1292 cwts. in 1875 to 158 cwts. in 1879.
Guatemala rubber is a very inferior kind and very unequal in quality ; the best varieties are whitish, and the "lower" are black with a "tarry" appearance. It occurs in the form of sheets compacted together, from between which when pressed a thick resinous fluid exudes. This when evaporated leaves a hard resinous sub-stance unaffected by hot water or steam. The rubber is collected from the trees as in Nicaragua, but it is poured on mats to dry, and the thin sheets are subsequently peeled off, folded into squares, and subjected to pressure to remove as much as possible of the contained moisture. The imports of india-rubber into England from the whole of Central America amounted only to 2080 cwts. in 1879, having decreased from 5809 cwts. in 1875. The greater proportion of Central American rubber is exported to New York, especially that from Nicaragua and Panama.
Siphocampylus Caoutchouc, Don., and S. Jamesonianus, D. C., Central American plants belonging to the natural order Lobeliaceae, are also stated to yield rubber of good quality ; and at the Phil-adelphia exhibition a rubber called Durango caoutchouc, obtained from a composite plant, was exhibited.
III. AFRICAN.—India-rubber is produced throughout equatorial Africa, the chief districts of export being the Gaboon, Congo, and Benguela on the west coast, and Madagascar, Mozambique, and Mauritius on the east. The Madagascar, Mauritius, and Gaboon rubbers are, it is believed, chiefly exported to France. Those which enter into British commerce are known as Mozambique, Madagascar, and African, although the imports are described as coming from the following distiiets in the blue books—Senegambia and Sierra Leone 3808 cwts., West Coast 11, 307 cwts., East Africa 7621 cwts., Cape of Good Hope 4241 cwts., Mauritius 570 cwts., Gold Coast 12 cwts. The above imports, which are for 1879, show an increase during the past five years, except in the case of Mauritius, Mada-gascar, and the Gold Coast. Africa, in respect of the large amount exported, may now be considered as taking the second place as an india-rubber producing continent.
Mozambique rubber, which is one of the most important varieties, occurs in the form of balls about the size of an orange, and "sausages," or spindle-shaped pieces, made up of slender strings of rubber wound around a piece of wood, which is eventually removed; or sometimes it occurs in smooth pieces of irregular size known as "cake" or "liver." Madagascar rubber consists of two qualities, the best of a pink and the inferior or "lower" of a black colour, and occurs in shapeless pieces.
The other kinds included under the general name of African are amorphous lumps called "knuckles" from Congo ; small "negro--heads" or "balls" of scrap, and smooth cakes from Sierra Leone; small square pieces like dice called "thimbles," and others more irregular in shape called "nuts," and "small negro-heads|" from the Portuguese colonies; "tongues," consisting of flat pieces, usually wet and sticky, from the Gaboon; and "balls" from Liberia African rubber as a rule possesses more adhesiveriess and less elas-ticity than Pará rubber, and is inferior in value. Comparatively little is known of the plants yielding caoutchouc, in Africa or of the mode of collection. In Angola, according to Dr Welwitsch, the natives either cut off a piece of bark, and allow the milky juice to run into a hole in the ground, or placing the hand against the trunk of the tree permit the milk to trickle down their arms, going from tree to tree until the arm is covered, when the rubber is rolled back towards the hand in the form of a ring. The wood of some of the trees. according to Mr Collins, contains a gum which, if the incision penetrates below the bark, mixes with the rubber and deteriorates it. In Madagascar, accordingto M. Coignet, rubber is obtained from the "Voá-héré" or "Voá-canja," Vahea madagascariensis, Boj., the "Voá-hiné," V. comorensis, Boi., and from V. gummifera, Lam. In Senegambia it is obtained from the "Anjouan," Vahea sene-galensis, A. D. C. In Mauritius Willughbeia edulis, Roxb. (which is found also in Madagascar, and in Chittagong and Silhet in India), appears to be the chief source of rubber. All the above are climb-ing shrubs with opposite entire leaves and fleshy fruits.
In Central Africa, from Liberia on the one side to Zanzibar or the other, caoutchouc is collected from plants of genera nearly allied to Vahea, a few only of the species being known to botanists. In Angola, under the name of "Licomgue," in Golungo Alto and Cazengo, it is collected from Landolphia owariensis, Pal. de Beauv. ; from L. florida, Benth., in Angola and Liberia, and from L. Hendelotii, D. C., in Senegal. At Kew there, also exists a speci-men of india-rubber from the west coast of Africa obtained from an undescribed species of Carpodinus with hairy leaves and stems. In the basin of the Gaboon and Congo it is obtained, according to Du Chaillu, from a climbing plant called N’dambo, which gives its name to dambonite, a peculiar substance contained in this kind of rubber (see p. 840). That some African caoutchoric is yielded by species of Ficus there can be no doubt. In Sierra Leone it is collected from Ficus Brasii, R. Br. In Liberia, according to Mr Thomas Christy, the finest rubber is obtained from Urostigma Vogelii, Miq., a tall tree with large handsome leaves, and lower qualities of rubber from other species, and from Landolphia florida, Benth. In Angola on the west, and at Inhambane on the east coast, rubber is also obtained from species of Ficus. In the island of Réunion caoutchouc is said to be obtained from Periploca graeca, L.
IV. ASIATIC.—The rubbers which enter English commerce from Asia include the Assam, Borneo, Rangoon, Singapore, Penang, and Java kinds.
Assam rubber is imported chiefly from Calcutta in baskets made of split rattans, weighing about 3 cwt. each, and covered with a gunny bag. The rubber is glossy, of a bright pink colour and mottled appearance, and occurs in the form either of small balls pressed together or of irregular masses called "slabs" or "loaf" rubber. The former, being more liable to adulteration, are less in demand by manufacturers. The imports into Liverpool in 1879 were 7000 cwts. Assam rubber is obtained from Ficus elastica, Roxb., a plant too well. known as a window ornament to need de-scription. A portion also is collected from Urostigma laccifera, Miq. Ficus elastica grows in the tropical rocky valleys of the Himalayas, between 70º and 80º E. long., where there is always a hot moist atmosphere, the temperature rising to 98º F. in the shade. The trees are tapped in the most careless manner. In the lower portion of the tree and in the large aerial roots, diagonal cuts penetrating to the wood are made, from 6 to 18 inches long, and in an elliptical form so as to be about 3 inches across the centre. The milk is received either in holes made in the ground or in leaves folded in the form of a funnel, that from the smaller cuts on the branches (for the collectors scarify every portion within reach) being allowed to dry on the tree. About 50 oz. of the milk collected in August gives 15 oz. of caoutchouc, but the percentage sometimes falls as low as 10 per cent. From February to April the milk is more scanty, but richer in caoutchouc, and is consequently best collected at that time. The milk is coagulated by pouring it into boil-ing water and stirriug it until it is sufficiently firm to be carried about without being clammy; sometimes it is pressed, again boiled, and dried in the sun. In this way the "loaf" rubber in irregular masses is formed. The small "balls" are formed of the strings of rubber which have been allowed to dry on the tree.
Assam rubber, although fairly elastic, is much depreciated in value by the careless mode of collection, and of ten loses, by washing at the manufactory, as much as 35 per cent. of dirt, consisting of clay, sand, or bark. The exportation of caoutchouc from British India, exclu-sive of the Straits Settlements and Ceylon, in 1879 amounted to 9973 cwts., of which 7000 are estimated to have been produced in Assam. About three-fourths of the rubber exported froin India goes to Great Britain, and the remainder to the United States.
In consequence of the reckless destruction of the trees, the culti-vatiou of Ficus elastica has been commenced in Assam. It is calcu-lated that the trees can be tapped at the age of twenty-five years, and that after fifty years they will yield 40 1b of caoutchouc each (worth £3, 4s.) every three years, it being injurious to their health to tap them more frequently.
Palay rubber is the product of Cryptostegia grandiflora, R. Br., an asclepiadaceous plant common on the coast of India; and from Willughbeia edulis, Roxb., and W. martabanica, D. C., a rubber is obtained in Chittagong; neither of these, however, is known in Britain as a commercial variety.
Borneo rubber comes to the Liverpool market in the form of balls or shapeless masses, internally of a white or pinkish colour, and very porous and spongy, the pores being usually filled with salt water, in consequence of which it often loses 20 to 50 per cent. of its weight in drying. The imports into Great Britain amounted in 1879 to 5000 cwts. Although Borneo rubber was first made known in 1798, it was not imported into England as an article of trade till 1864, when it appeared under the name of gutta susu, i.e., in Malayan, milk-gum. The plant which yields Borneo rubber was identified by Roxburgh as Urceola elastica, Roxb., an apocynaceous climbing plant with a trunk as thick as a man’s body, and having a soft thick bark. Mr F. W. Burbidge, who recently visited the island, states that there are three varieties of the rubber plant, known to the natives as "petabo," which yields the firiest caoutchouc; "menoongan," which yields the largest quantity; and "serapit," from which the commonest rubber is obtained. the petabo variety, according to specimens at Kew, is referred to a species of Leuconotis. The rubber is obtained by cutting the plant into pieces varying from a few inches to 2 or 3 feet long, and allowing the juice to drain into buckets or jars, heat being sometimes applied to one end of the pieces when the juice flows slowly. The milk is coagulated by salt water. The Borneo rubber plant is probably one of the plants that would repay cultivation, as it grows rapidly, yields a supply of sap in three years, and after planting requires no atten-tion.
In Sumatra, caoutchouc is obtained from Willughbeia firma, and is exported to Holland, but this variety is not known in England. Malacca rubber, which is not met with in English commerce, is said to be obtained from Urceola elastica, Roxb.
Rangoon rubber, and those of Penang and Java, are imported into England in small quantities only, and are irregular in appearance. From its physical characters, a portion at least of Rangoon rubber is believed to be the produce of a species of Ficus, probably F. hispida, L. Another caoutchouc-yielding plant, Urceola (Chavon-nesia) esculenta, Benth., belonging to the Apocynaceae, has, how-ever, been recently discovered in Burnish, some specimens of which at the age of five years have stems 6 inches in diameter, while the crown covers an area of 200 square feet. It has been recommended for plantations as an available source of rubber, the cost of cultivation being very slight after the first year, and the profit cornmenc-ing in seven years, at which age the yield is calculated to be 3 1/2 lb.
Penang rubber in character resembles the Assam, and may be also supposed to be obtained from a species of Ficus. Dr Wallich, how-ever, has stated that its source is an asclepiadaceous plant, Cynan-chum ovalifolium, Wright.
Java rubber is stated by Dr De Vrij to be obtained from Ficus elastica. Like the Assam rubber it is dark and glossy, but it is of a deeper tint, and has occasional reddish streaks. It is said to be prepared by allowing the juice to dry on the incisions made in the tree. Singapore, Jav'a, and Penang rubbers are much alike in char-acter, and may be classed with the Assam rubber, having a firm texture, mottled appearance, and bright polished surface, but vary-ing in colour in a single sample from light yellowish-white to dark brown. Java rubber is also exported to France.
Caoutchouc is obtained in the Malay archipelago from Alstonia costulata, Miq.; and Alstonia scholaris, R. Br., is likewise reported to yield it. In Fiji it has been obtained from Alastonia plumosa, Labill. In North Australia caouatchouc has been prepared from Ficus macrophylla, Desf., and F. rubiginosa, Desf. ; the last-named plant has been recommended by Baron Müller as suitable for cul-tivation, being a hardy species, None of the above rubbers are as yet known in British commerce as regular articles of trade.
Bibliography.—Collins, in Journal of Botany, 1868 ; Journ. Soc. Arts, vol. xviii. p. 86; Bevan, British Manufacturing Industries, 1877, p. 97-105, and Report on Caoutchouc, 1872; Markham, in Journ. Soc. Arts, p. 476, 1876; L’Ingenieur Universel, vol. ii. p, 187 ; Bernardin, Classification de 100 Caoutchoucs et Guttaperchas, Ghent, 1872 ; Christy, New Commercial Plants; Kurz, Forest Flora of British Burmah, vol. ii. p. 184. (E. M. H.)
Chemistry, Manufacture, and Industrial Uses.
The remarkable body known as india-rubber is composed of carbon and hydrogen alone, but its exact chemical nature is not by any means known with certainty. The analyses of Faraday indicate that its ultimate composition is 87·5 per cent. of carbon and 12·5 per cent. of hydrogen ; but there appears to be good ground for regarding the substance as a polymer of the group C10H8, or as (C10H8)x There are, however, no data for estimating the value of x in this case. It will be noticed, too, that the formula given requires considerably less hydrogen than the propor-tion indicated by Faraday’s analysis; but the difficulties of obtaining such a body as caoutchouc in a fit condition for analysis are so great as to render this discrepancy a matter of comparatively small import. The action of cold and heat on india-rubber presents many points of interest. When exposed to a temperature approaching 0º C., it gradually loses its softness and ready extensibility, and finally becomes rigid and inelastic; but its normal condition may be restored by submitting it either to a tempera-ture of 35º or 40º C., or to a tension sufficient to stretch it to about twice its natural length. In the latter case it is probable that the change is really due to heat arising from the physical disturbance consequent upon the act of stretching. The effects of heat are more complex and varied than those of cold; and with caoutchouc at an ordinary temperature, say 15º C., the primary effect of heat is to increase its flexibility and elasticity. This is well illustrated by the fact that a strip of rubber stretched by a weight contracts when it is heated to a temperature of about 40º C. This diminution as regards length is, however, accompanied by a more than corresponding increase in thickness, on account of the expansion in volume due to an elevated temperature. When caoutchouc is exposed to a temperature ranging between 100º and 120º C., it becomes considerably softened, and almost entirely loses its elasticity; but, if of good quality, it slowly recovers its former condition under the influence of a moderate degree of cold. When, however, the heat is pushed to 150º, it becomes viscous, and at 200º it fairly melts, forming a thick liquid which possesses the same composition as ordinary caoutchouc, but has no tendency to resume its original condition even when exposed to cold for a prolonged period. At a still higher temperature, caoutchouc yields a variety of volatile hydrocarbons; and, on subjection to dry distillation in a retort, its conversion into these bodies is tolerably complete, only a trifling carbonaceous residue remaining behind. Among the most notable volatile products resulting from the dry distillation of caoutchouc may be mentioned caoutchin, an oil-like body having a composition and vapour volume corresponding to the formula C10H8, and boiling at 171º C,; and isoprene, another hydrocarbon oil identical in composition with caoutchin and with caoutchouc itself, and boiling at 38º C. Other hydrocarbon oils are also formed, as, for example, heveene and caoutcheuc,—these being members of the CnR2n, series. The former boils at 228º, and the latter at 14º·5. The mixed products of the dry distillation of caoutchouc, often described under the name caoutchoucin, form an excellent but rather expensive solvent of this body. When exposed to the air, caoutchouc gradually oxidizes and undergoes deterioration; the oxidation is often much favoured by exposure to sunlight or to alternate conditions of dampness and dryness. The deteriorated caoutchouc is either somewhat soft and deficient in tensile strength, or brittle and resinous in its nature. Spiller found 27·3 per cent. of oxygen in a resinous product resulting from the decay of caoutchouc. Ozone rapidly attacks and destroys the substance.
Dilute acids or alkalies have little or no action on caoutchouc, but strong and hot sulphuric acid chars, and concentrated nitric acid rapidly oxidizes and destroys it. The moderate action of either chlorine, bromine, or iodine hardens or vulcanizes it; but, if allowed to act freely, they completely destroy it. The action of sulphur will be con-sidered below.
Caoutebouc, when pure, is odourless and nearly white, and possesses a specific gravity of ·915. It is porous and cellular in texture, and absorbs from 10 to 25 per cent. by weight of water when long soaked in it. Alcohol is similarly taken up. Up to this point caoutebouc has been referred to as if it consisted of one substance only; but as a matter of fact all ordinary samples contain two distinct modifications, viz.,. the hard or fibrous and the soft or viscous. These two caoutchoucs are identical in com-position, and similar as regards general properties and reactions. On subjecting a piece of raw caoutchouc, however, to the action of such a solvent as cold benzol, the essential difference between the two forms manifests itself. The fibrous or hard constituent merely swells up to many times its original bulk, but the viscous yields a true solu-tion. In a high class rubber, such as that imported from the province of Pará, the former modification is the prin-cipal factor; in a caoutchouc of low quality, such as "African tongue," the lattex. Freshly cut surfaces of caoutchouc unite together firmly, and this circumstance is due to the presence of the viscous variety ; vulcanization, by hardening this, destroys the adhesive property.
Certain liquids, such as benzol and its homologues, car-bon disulphide, petroleum, ether, volatile oils, chloroform, and melted naphthalene, dissolve caoutchouc more or less perfectly ; but unless the substance has been subjected to the process of mastication, its fibrous constituent appears, not to dissolve in the strict sense of the term, but rather to swell up, forming a paste analogous to starch which has been acted on by hot water. Carbon disulphide and chloroform, however, exercise a more powerful solvent action on the fibrous parts of india-rubber than benzol or essential oils; and Payen has found that carbon disulphide to which 5 per cent. of absolute alcohol has been added forms one of the best solvents. One part of masticated caoutchouc dissolved in thirty parts of this solvent forms a liquid which can be filtered through paper, and which leaves a film of exquisite tenuity and purity when allowed to dry on a level glass plate.
Most fatty matters exercise a remarkable destructive action on caoutchouc, causing it to become first soft, and afterwards hard and brittle. It has often happened that traces of fatty oils in the liquids employed for dissolving india-rubber, or fatty matters in the textile basis, have led to the destruction of waterproof goods. A like cause has in many cases led to the rapid deterioration of the caoutchouc threads in elastic webbing.
In the industrial working of india-rubber, the first matter to be attended to is the removal of the various impurities present in the crude material. These are in some cases natural products which have originated with the caoutchouc, while in other cases they owe their presence to careless collection or to adulteration. Among the impurities of the former class may be mentioned various gum-like or mucilaginous matters, and acid products arising from their decay or oxidation. A remarkable volatile body, which is probably of the nature of a polyatomic alcohol, has been discovered by Gerard1 in the crude caoutchouc from the Gaboon. This substance, called by the discoverer dambonite, has a composition corresponding to the formula C4H8O3, is sweetish to the taste and soluble in water, and crystallizes in needles which melt at 190º C. and volatilize between 200º and 210º. The admixtures may range from fragments of bark or wood to stones or large lumps of clay, such as are sometimes introduced into negrohead rubber,—hay or a similar substance being also placed inside to make the mass about equal in specific gravity to the genuine article. Alum and sulphuric acid are often employed to effect the coagulation of the juice; and traces of the latter remain-ing in the rubber appear, in some instances, to work mischief.
All the above-mentioned impurities are in actual practice very efficiently removed by the following process. The lumps of crude caoutchouc are first softened by the prolonged action of hot water, and then cut into slices by means of a sharp knife,—generally by hand, as thus any large stones or other foreign substances can be removed. The softened slices are now repeatedly passed between grooved rollers, known as the washing rollers (fig. 7), a supply of hot or cold water being made to flow over them. Solid impurities speedily become crushed, and are carried away by the water, while the rubber takes the form of an irregular sheet perforated by numerous holes. The washed product contains in its pores a notable proportion of water, which is removed by hanging the rubber for some days in a warm room. . It is now ready either for incorporation with sulphur and other solid bodies, or for agglomeration into solid masses by means of the masticating machine,—an apparatus which consists of a strong cylindrical cast--iron casing, inside which there revolves a metal cylinder with a fluted or corrugated surface. Some of the rub-ber having been placed in the annular space between the inner cylinder and the outer casing, the former is made to revolve; and the continued kneading action to which the rubber is subjected works it into a solid mass, something like a gigantic sausage. Before commencing the mastication it is generally necessary to warm the apparatus by means of steam. but as the operation proceeds the heat produced requires to be moderated by streams of cold water flowing through channels provided for the purpose. The inner cylinder is generally placed somewhat excentric-ally in the outer casing, in order to render the kneading more perfect than would otherwise be the case.
To convert the masticated rubber into rectangular blocks, it is first softened by heat, and then forced into iron boxes or moulds. The blocks are cut into thin sheets by means of a sharp knife, which is caused to move to and fro about two thousand times per minute, the knife being kept moistened with water, and the block fed up to it by mechanical means. Cut sheets are largely used for the fabrication of certain classes of rubber goods,—these being made by cementing the sheets together with a solution of rubber in coal-naphtha or benzol. Most articles made of cut sheet rubber would, however, be of very limited utility were they not hardened or vulcanized by the action of sulphur or some compound of that element. After vulcanization, rubber is no longer softened by a moderate heat, a temperature of 160º C. scarcely affecting it, nor is it rendered rigid by cold, and the ordinary solvents fail to dissolve it, It must, however, be distinctly understood that it is not the mere admixture but the actual combination of sulphur with india-rubber that causes vulcanization. If an article made of cut sheet be immersed for a few minutes in a bath of melted sulphur, maintained at a temperature of 120º C., the rubber absorbs about one--tenth of its weight of that element, and, although somewhat yellowish in colour from the presence of free sulphur, it is still unvulcanized, and unaltered as regards general proper-ties. If, however, it be now subjected for an hour or so to a temperature of 140º C., true combination sets in, and vulcanized caoutchouc is the result. When a manufactured article has been saturated with sulphur in the melted- sulphur bath, the heat necessary for vulcanization may be obtained either by high-pressure steam, by heated glycerin, or by immersion in a sulphur-bath heated to about 140º C. In this last case absorption of the sulphur and its intimate combination with the rubber occur simultaneously. Cut sheets, or articles made from them, may be saturated by being laid in powdered sulphur maintained for some hours at about 110º C. Sheets sulphured in this way can be made up into articles and joined together either by warming the parts to be united, or by means of india-rubber solution ; after which the true vulcanization, or "curing" as it is termed, can be brought about in the usual way. Another method of vulcanizing articles made from cut sheet rubber consists in exposing them to the action of chloride of sulphur. Either they are placed in a leaden cupboard into which the vapour is introduced, or they are dipped for a few seconds in a mixture of one part of chloride of sulphur and forty parts of carbon disulphide or purified light petroleum. Vulcanization takes place in this instance without the action of heat; but it is usual to subject the goods for a short time to a temperature of 40º C. after their removal from the solution, in order to drive off the liquid which has been absorbed, and to ensure a sufficient action of the chloride of sulphur. Treatment with a warm alkaline solution is afterwards advisable, in order to remove traces of hydrochloric acid generated during the process. Another very excellent method of vulcanizing cut sheet goods consists in placing them in a solution of the poly-sulphides of calcium at a temperature of 140º C. Rubber employed for the manufacture of cut sheets is often coloured by such pigments as vermilion, oxide of chromium, ultramarine, orpiment, antimony, lamp black, or oxide of zinc, incorporation being effected either by means of the masticator or by a pair of rollers heated internally by steam, and so geared as to move in contrary directions at unequal speed (fig. 8). Most of the rubber now manufactured is not combined with sulphur when in the form of sheets, but is mechanically incorporated with about one-tenth of its weight of that substance by means of the mixing rollers,—any required pigment or other matter, such as whiting or barium sulphate, being added. The mixed rubber thus obtained is readily softened by heat and can be very easily worked into any desired form or rolled into sheets by an apparatus known as the calendering machine. Vulcaniza-tion is then ensured by exposure for half an hour or more to a temperature of 135º-150º C., usually in closed iron vessels into which high-pressure steam is admitted (fig. 9). Tubes are generally made up around mandrels, and allowed throughout the curing to remain imbedded in pulverized French chalk, which affords a useful support for many articles that tend to lose their shape during the process. Of late years a considerable amount of seamless tubing has been made, much in the same way as lead piping, by forcing the mixed rubber through a die, and curing as above. The calendered sheets are generally cured between folds of wet cloth, the markings of which they retain; and hollow articles, such as playing balls or injection bottles, are vulcanized in iron or brass moulds, tinned inside and very slightly greased. Before it is put in, the article is roughly put together, and the expansion of the included air forces the rubber into contact with the internal sur-face of the mould, or a little carbonate of ammonia is enclosed. Belting intended for driving machinery is built up of canvas which has been thoroughly frictioned with the soft mixed rubber, and is cured by placing it in a kind of press kept. by means of steam at a dry heat of about 140º C. Packing for the stuffing boxes of steam engines is similarly prepared from strips of rubber and frictioned canvas, as also are the so-called insertion sheets, in which layers of rubber alternate with canvas or even wire gauze. India--rubber stereotypes are now extensively made use of as hand stamps, and attempts have been made to introduce them for press and machine printing. A plaster cast of the type is, when dry, saturated with shellac varnish and redried. Rubber mixed in the usual way with about 10 per cent. of sulphur is now softened by heat, forced into the mould, and retained there by pressure during the operation of curing, which is usually effected in an iron box heated over a gas burner to 140º C.
The ordinary macintosh or waterproof cloth is prepared by spreading on the textile fabric layer after layer of india--rubber paste or solution made with benzol or coal-naphtha. If cotton or linen is used, it is usual to incorporate sulphur with the paste, and to effect vulcanization by steam heat ; but, when silk or wool is employed, no sulphur is added to the paste, the dried coating of rubber being in merely brought into momentary contact with the mixture of chloride of sulphur and carbon disulphide already mentioned. Double texture goods are made by uniting the rubber surfaces of two pieces of the coated material. Air goods, such as cushions, beds, gas bags, and so forth, are made of textile fabrics which have been coated with mixed rubber either by the spreading process above described, or by means of heated rollers, the curing being then effected by steam heat. The manufacture of overshoes and fishing boots is an analogous process, only the canvas base is more thickly coated with a highly pigmented rubber of low quality. The articles are first fashioned by joining the soft material; they are then varnished, and afterwards cured in ovens heated to about 135º C. The fine vulcanized "spread sheets" are made by spreading layers of india-rubber solu-tion, already charged with the requisite proportion of sulphur, on a textile base previously prepared with a mixture of paste, glue, and treacle. Vulcanization is then effected by steam heat, and the preparation on the cloth being softened by water, the sheet of rubber is readily re-moved. The required thickness of the spread sheet is very often secured by the rubber-faced surfaces of two cloths being united before curing. The threads used in making elastic webbing are usually cut from spread sheets. The manufacture of springs, valves, and washers does not re-quire any very special notice, these articles being generally fashioned out of mixed rubber, and vulcanized either in moulds or in powdered French chalk. Rollers are made to adhere to their metal spindles by the intervention of a layer of ebonite, and after vulcanization they are turned. In order to make spongy or porous rubber, some material is incorporated which will give off gas or vapour at the vulcanizing temperature,—such as carbonate of ammonia, crystallized alum, and finely ground damp sawdust. Uncombined sulphur is injurious, and often leads to the decay of vulcanized goods ; but an excess of sulphur is generally required in order to ensure perfect vulcanization. Sometimes the excess is partially removed by boiling the finished goods with a solution of caustic soda, or some. other solvent of sulphur. In other cases the injurious effects of free sulphur are obviated by using instead of it a metallic sulphide,—generally~ the orange sulphide of antimony ; but, for the best results, it is necessary that this should contain from 20 to 30 per cent. of uncombined sulphur.
When the vulcanization of rubber is carried too far—say from the presence of a very large proportion of sulphur and an unduly long action of heat, the caoutchouc becomes hard, horn-like, and often black. Rubber hardened by over-vulcanization is largely manufactured under the name of ebonite or vulcanite. It is usually made by incorporat-ing about 40 per cent. of sulphur with purified Borneo rubber by means of the usual mixing rollers, shaping the required articles out of the mass thus obtained, and heating for six, eight, or ten hours to from 135º to 150º. Ebonite takes a fine polish, and is valuable to the electrician on account of its insulating properties, and to the chemist and photographer because vessels made of it are unaftected by most chemical reagents. A kind of vulcanite which contains a very large proportion of vermilion is used, under the name of dental rubber, for making artificial gums.
The following list of works and papers on the rubber industry enumerates the writings which are calculated to be especially useful to enquirer:—Charles Goodyear, Gum. Elastic and its Varieties, New Haven, U.S.A., 1853; Friedrich Harzer, Gutta-Percha und Kautschuk, ihr Vorkommen, &c., Weimar, 1853; Paulin Desor-meaux, Nouveau manuel complet du fabricant d’objets en caoutchouc, en gutta-percha, et en gomme factice, 424 pp., Paris, 1855; C. H. Schmidt, Der Fabricant von Kautschuk und Gutta-Percha, Waaren, 207 pp., Weimar, 1856; Thomas Hancock, Origin and Progress of the Indian-Rubber. Manuufacture in England, London, 1857; Heinrich Keysserling’s edition of Friedrich Harzer’s Gutta-Percha und Kautschuk, 273 pp. and atlas, Weimar, 1864; Abridgments of Specifications relating to the Preparation of India-Rubber and Gutta-Percha, 1791-1866, 262 pp., printed by order of the Commissioners of Patents, London, 1875; "India-Rubber and Gutta-Percha," a series of articles in the Universal Engineer, vol. ii., Manchester, 1879; Franz Clouth, Die Kautschuk Industrie, 76 pp., Weimar, 1879; T. Bolas, Cantor Lectures on the India-Rubber and Gutta-Percha Industries, London, 1880; M. Maigne, Nouveau manuel complet du fabricant d’objets en caoutchouc, &c., 2 vols., 506 pp., Paris, 1880. (T. B.)
Footnotes
FOOTNOTE (page 840)
1 Compt. Rend., lxvii. p. 820, and Zeitschrift für Chem., 1869, p. 66.
The above article was written by two authors:
(a) Introduction. Botanical Sources, Modes of Prepartation, &c.
E. M. Holmes
(b) Chemistry, Manufacture and Industrial Uses
Thomas Bolas.