Popular Science Monthly/Volume 50/March 1897/India Rubber and Gutta-Percha
|INDIA RUBBER AND GUTTA-PERCHA.|
WAS India rubber known to the ancients? Early writers do not mention it. We need not necessarily conclude, however, that the primitive peoples established on the shores of the Mediterranean were ignorant of the existence of this substance. The game of tennis is one of great antiquity, Herodotus attributing its invention to the Lydians. It is believed they got it from Egypt, which may have received it from Ethiopia. It is known that India-rubber trees are found in Abyssinia to-day. Hence it is reasonable to suppose they were indigenous there in earlier times, and that the inhabitants knew how to prepare resilient balls from their milky product. The Chinese have laid claim to the discovery of rubber, but have so far been unable to prove that they were the first to employ it. Modern Europe had no knowledge of it until the discovery of America. The Spaniards were much surprised to find the Indians playing tennis with balls made of a strange substance which excited their attention, as mentioned by Fernandez de Oviedo at the beginning of the sixteenth century.
This remarkable substance is obtained from the milky juice of certain trees and different varieties of climbers. South America is the principal source of supply — Brazil, of the many states producing it, leading in quantity and quality, and having in its great forests sufficient to meet twice the wants of the world. The best is Pará (fine, medium, and sernamby), from the great basin of the Amazon, where more than eighty thousand seringueiros (gatherers) are engaged in the dry season in collecting gum. White Para, "virgin sheets," a new variety in three grades, comes from Matto Grosso. Since its importance first began to be felt, this gum has exerted an increasing influence upon the spread of civilization, especially along the Amazon and Orinoco and their tributaries and the great streams which pour out from the interior of the Dark Continent. Pará, formerly an insignificant village, has grown to be a city of a hundred thousand inhabitants, with modern features, and Manáos, up the river, is fast following it. India rubber is the mainstay of the northern Brazilian states, Bolivia, and eastern Peru. Brazil has a great advantage in its immense waterway; ocean-going steamers run twelve hundred miles up the Amazon, whereas every African river except the Congo has a bar at its mouth and cataracts not far distant from the coast line. It is, besides ivory, about the only commodity produced in the interior of a tropical country that will bear the expense of transportation, often on the heads of natives along tangled man-paths, to the seaboard. So in many places it has been the basis of first commerce. The principal trees in South America are the Manihot Glaziovii, the Ficus gameleira, and varieties of the Castilloa, the Hevea, and the Hancornia. "The production of Pará rubber," says the Scientific American, December 5, 1896, "increased from 8,243,000 pounds in 1865, to 15,144,000 in 1875, 29,310,000 in 1885, and 46,363,000 pounds in 1895; the great advance in the decade between 1885 and 1895 being the direct result of the increased demand produced by the tire-makers. Last year 37,456,000 pounds were delivered to manufacturers in the United States, against 31,062,000 pounds in 1894 and 35,583,000 pounds in 1893. The highest price paid in this country last year for fine Para rubber was eighty-one cents and a half in November." The United States has been from the first the largest consumer, and an American syndicate, it is said, is now seeking capital to develop ten million acres in the Orinoco Valley, chiefly with a view to profits from the great virgin rubber forests known to exist there.
Enormous supplies are stored up in Africa and her adjacent islands, where a variety of Ficus and great climbing shrubs, the Landolphia and Vahea, produce it. Stanley alludes to great numbers of these climbers entwining the trees, so as to make passage exceedingly troublesome. Attempts to force the price to unreasonable limits are therefore not likely to meet with permanent success, and we may banish fears of approaching exhaustion. The gum fully thrives, seemingly, nowhere but in the tropics. "It is the one jungle product which society finds indispensable," said The Spectator recently, and, further: "Everybody knows that in the last five years the use of pneumatic tires for cycles and solid rubber tires for horse- vehicles has enormously increased our consumption of this article; but, quite apart from that more obvious fact, India rubber is daily being introduced more and more into all sorts of machinery. Highly competent judges say that if the output could be doubled within a year, so many applications would instantly arise that the price would not fall appreciably." The negroes have no regard for the climbers, and cut them down in order to extract the utmost possible amount of sap.
As a result of this practice collectors have to go farther and farther inland for a supply, so that it often does not pay to transport it to the coast. There is a vast need of better management in collecting and curing the gum, as the African product is of lower grade and brings less than many other sorts in the London market. It seems a certain source of wealth, and is easy of cultivation, so proper steps are almost sure to be taken for its encouragement by the nations engaged in civilizing the continent. Traffic in India rubber is one great incentive for the building of the Congo Railroad.
Asia was the second country to furnish Europe with India rubber. The supply has of late years decreased in importance in consequence of the destruction of the trees. American varieties have been introduced with some success by the Indian Government. The principal native trees are the Urceola elastica, the Ficus elastica (the well-known window plant) and a species of fig, the Ficus religiosa, which is one of the most beautiful trees in the world. Its branches bend down, take root, and form new trunks. The great fig of Narbuddah has three hundred and fifty large and three thousand medium-sized stems, thus constituting in itself a veritable forest. The principal rubbers from India are the Assam and Rangoon. India rubber is also obtained in Oceania, notably in Sumatra, Java, and Borneo, but very little comes from Australia. It was formerly thought that the rubber tree only grew in moist ground, under tropical suns, but explorers have found them in hard soils on high plateaus. A beginning is being made with cultivation of the trees. The Indian Government has a nursery of Pará trees in Assam extending over two hundred square miles, and has shown that they may be productively raised from foreign seed with little care. The cultivation is also attracting attention in Central America and Mexico. According to estimates, it is very profitable. The long waiting of fifteen to twenty years, however, till the tree attains its full vigor, is apt to make individual capital cautious.
France was the cradle of the rubber industry, and French researches permitted the anticipation of many applications of the substance afterward carried out by the English and Americans. La Condamine, who was with the expedition sent to the equator by the French Academy, found the novel article at Quito, where it was known as caoutchouc (from cahuchu of the Maïnas Indians), and sent the first accurate knowledge of it to Europe in 1736. The natives called it hhévé (hence hevea). The Omaguas made water bottles of it, provided with a cannula, which were presented to guests before the repast. They were primitive syringes, and gave the name to the tree in some localities. Hérissant and Macquer, in France, soon attracted attention to the gum by their investigations. Some confusion prevailing in regard to the expression "India rubber," what follows may justify the English name of the substance: In 1765 Aublet announced his discovery of the tree in French Guiana. In 1772 Magellan, descendant of the great navigator (Le Caǫutchouc et la Gutta-Percha, E. Chapel), proposed caoutchouc, or "résine élastique de Cayenne," as a substitute for bread crumb in erasing pencil marks. It was known in France as peau de négre. In England, where the discovery was attributed to the celebrated Priestley, who only propagated a foreign idea, designers soon came to call the article "Indian rubber." It was not known as a product of Asia until 1798. Hence the thought of the time probably connected it with its South American habitat and thus with its Indian gatherers. The appearance of the little cube in the shops about 1775 was the beginning of the great part now played by India rubber in the arts, sciences, and industry. In 1780 Berniard, a Frenchman, experimented in a line with Hérissant and Macquer, and found oil of turpentine to be the best solvent. He also succeeded in imparting various colors to the gum.
By 1791 syringes, sounds, bougies (1779), and elastic bands had been made of caoutchouc. The manufacture of impermeable tissues had been tried in France in 1791 by Besson. The idea was borrowed from the Indians, from whom the early Spaniards had learned to gum their hempen cloaks, which when thus treated were impervious to rain, but degenerated in the sun. Hancock and Mackintosh, in England, were the first to make waterproof garments, later than 1818, the rubber cloth being used as lining (E. Chapel). Hancock devised important processes for treating rubber, and in 1838 invented ink erasers; but it remained for Mackintosh alone, by employing benzene as a solvent, to produce, in 1823, the first successful garments, which at once came into great favor, in spite of their disagreeable odor. At present, three tissues are made: "simple tissues" (having one rubber face), "double-faced tissues" (rubber on both sides), and "double tissues" (two stuffs with one rubber coating between). "Mackintosh" is "double tissue," and the method of manufacturing it is substantially the same as at first, only machinery is used. In the calender machine the fabric is spread with rubber solution, is drawn by a roller under a scraper to remove excess, and passes upon a steam table, where the solvent evaporates, leaving a thin pellicle of rubber on the stuff, which thoroughly dries in passing around a drum and is wound upon a mandrel. It may then be taken to the front of the machine and the process repeated. Ten coats are sometimes applied. The product must afterward be vulcanized, to render it less sensitive to heat and cold.
Many do not understand how much we owe to vulcanization and to the American, Charles Goodyear, its real discoverer, who indefatigably pursued it many years through prosperity and want, encouragement and discouragement, now with friends around him, and, again, lying in a debtor's prison. La Condamine found India-rubber boots among the South American Indians in 1736. Up to 1820 the seringueiros had sent rubber to European markets in the form of "pears," or bottles, and rude shoes. They were termed "shoemakers" in consequence, but the appellation has fallen into disuse. The gum did not begin to be known in the United States until 1820. Three years later, five hundred pairs of Brazilian shoes of direct importation had been sold in Boston. Between 1845 and 1850, flat balls, "biscuits," as at present, began to be sent. Rubber threads, making elastic suspenders, garters, etc., possible, were first made, in 1830, in France, and speedily became an important manufacture. About this time the child's ball was doing more to popularize the novelty than anything else. Soon after, the manufacture of rubber became so important in New England that from 1834 to 1836 new factories rose on all sides. But the popularity of, the new substance declined when it was found that rubber lost its elasticity at low temperatures and was deteriorated and stuck together at high ones. Then, when many factories in America had closed, and English and French manufacturers were menaced with ruin, Goodyear succeeded in producing rubber unalterable by cold or ordinary heat or solvents, and some of his dark-yellow shoes and bands of perfect elasticity met with a glad reception in Europe in 1841. In 1839, Nathaniel Hayward, an American, had patented a process for powdering the sheets of rubber with sulphur. The discovery was simultaneously made in Germany by Dr. Luedersdorf. But neither thought of applying heat. Goodyear, who had been experimenting with India rubber for four years, bought Hayward's patent, and after long investigation accidentally found, in 1839, that heat caused the sulphur and rubber to combine so as to change the nature of the latter. He afterward conceived the idea of plunging the rubber into a bath of sulphur. The process is called "vulcanization." Goodyear's experiments also laid the basis of hard-rubber manufacture. Hancock, in England, scraped the new article, and was led to make and patent the same discovery in 1843, while Goodyear, through negligence, did not obtain a patent until 1844. Alexander Parkes patented a process in 1846, when molding was invented by Hancock, in which rubber is vulcanized almost instantly by dipping in a mixture of chloride of sulphur and sulphide of carbon; so we now have three processes: "The Goodyear" by steam (improperly), "the Hancock" in the sulphur bath, and "the Parkes" or dipping. They are all reliable and inexpensive. No important discoveries to change the method of treatment have been made since Goodyear, who died in 1860.
The problem of regenerating vulcanized waste (old shoes, etc.) has not yet been satisfactorily solved, no means having been found to remove the sulphur completely, and to restore to the gum its original properties; yet there is an increasing tendency to use partly reclaimed rubber and foreign mixtures. From 1881 to 1883 the waste followed the rise in the price of the gum, and this led to the use of imitations made from linseed oil (first announced about 1846), from arachis, and from colza oil, and they still continue to be considerably employed in the fabrication of cheap articles.
Pure India rubber is whitish. It is rarely used, the vulcanized being preferred. Crude rubber is often mixed with pieces of bark, stone, clay, etc. The lumps are softened in hot water, cut into slices, generally by hand, and passed through washing rollers to remove foreign substances. When dried, they are ready for mixing with sulphur, etc., or for the "masticating machine," which kneads the stuff into a solid mass. The machine gets very hot and has to be cooled with water. The gum is then heated, molded, and cut into sheets by a rapidly moving knife. Balls, etc., which are made of these sheets, have to be cemented. It was not until about 1850 that manufacturers of "balloons" (hollow articles) began to make the endless variety of playthings with which the child of the present is familiar. India rubber grinding stones are made of the waste by an admixture of glass, pumice stone, or emery. Kamptulicon, of English origin, invented about 1843, is a mixture of rubber and pulverized cork applied to coarse cloth and covered with several layers of linseed oil. It is now largely superseded by linoleum (Walton's patent, 1860). Imitation leather and ivory (the latter not with complete success), hevenoide (for billiard balls, piano keys, etc.), baleinite, plastite (for gun rammers, canes, whip handles, etc.), and similar products are also made; even sponges. Stamps were made early in the history of India rubber, and by an American, James Peck, in 1862. They were ruined by the ink, and had to be abandoned until inks with an aniline base came, when they were able to supplant almost all their rivals. Hard-rubber dental plates are said to have been the invention of Dr. Evans, an American dentist in Paris, in 1854. He made several pieces for Goodyear's use in 1855. The latter showed them to Dr. Putnam in this country, who, with the assistance of a chemist and Goodyear, finally succeeded in making an article which has now obtained a high degree of perfection. Street-paving has been tried with success in London and Hanover. It deadens the rolling of vehicles, but the cost bids fair to prevent its general introduction. India-rubber horseshoes, an American patent, are announced, for special use on asphalt pavements.
It is impossible to mention all the uses to which India rubber is applied, and reference can only be made, in concluding, to two more, very important ones. It is at present finding increasing favor in tires for vehicles, the solid kind being most frequent, though occasionally pneumatic-tired equipages are seen. Hancock claims to have made the solid kind for her Majesty in 1846. The recent tire of American invention with wires running through it is thought, in this country, to be the best, as double the quantity of rubber of better quality is used, which secures greater elasticity, and vastly cheaper, from better rim- construction and because worn spots may be cut out and renewed. The bicycle industry, it is estimated, is turning out, in 1896, in the United States alone, six hundred thousand bicycles and one million and a half pairs of pneumatic tires, which will require about one thousand tons of rubber. The output in England is about the same, that country and the United States producing seventy- five per cent of the wheels manufactured. A writer on this subject (Hawthorne Hill) recently said that probably not more than four per cent of the output of rubber is used in the bicycle trade.
The discovery of gutta-percha, which seems to unite all the advantages of India rubber, excepting elasticity, without its disadvantages, has sometimes been attributed to the traveler Tradescant, who brought it to England, where it was known as mazerwood. It was neglected and soon forgotten. To an Englishman, Dr. Montgomery, is due the merit of having brought the importance of the new article to the attention of the world of science and industry. Hearing of it at Singapore, in 1822, he procured specimens from the natives, who collected it in the neighboring forests, and formed it chiefly into axe handles by malaxation in boiling water. He found it differed materially from elastic gum. Having proved that it retained the shape on cooling, imparted to it in boiling water, while recovering its hardness and primitive tenacity. Dr. Montgomery thought such a substance could serve better than rubber for certain instruments of surgery, and communicated his views to the Medical Board of Calcutta in 1843, which warmly indorsed the idea. Ho also sent specimens and a study of the product to the London Society of Arts, and received from it a gold medal in recognition of his important discovery. According to Dr. Gützlaff, the celebrated missionary, gutta-percha was known in China long before it appeared in Europe, though certainly not gathered there.
Gutta-percha, like India rubber, is obtained from the juices of certain trees and climbers. The best is produced by a tree, the Isonandra gutta, of the order Sapotaceœ, which formerly abounded at Singapore and in all Malaysia, but which now tends to disappear under the ravages committed by gatherers. Gutta, in Malay, signifies gum or lime; percha signifies scrap. Incisions are made in the bark, as on rubber trees, and the liquor flows of perfect whiteness, darkening at contact of air. Coagulation takes place spontaneously in a short time. Like rubber, the liquid forms a film on top. This cream is removed, kneaded into a large lump and plumped into boiling water. Under the action of a high temperature it softens and forms the cake usually found in commerce. Other trees in Malaysia and Farther India, in Cambodia and Cochin China, produce good gutta. In Hindustan different grades are mixed by the natives. Chinese merchants, in their depots, mix and manipulate to give a good superficial appearance to the product, as the price is constantly advancing. As the gatherers also do not scruple to add vegetable débris, earth, or sand, it has become difficult to secure a pure article. An inferior quality is obtained from trees and climbers in Africa and Madagascar, and, with the development of those countries, more may be expected. For the present, the same ravaging system of gathering seems bound to prevail in these countries as that which the now indispensable gutta-percha and India-rubber growths are suffering in other countries. The valuable discovery of enormous supplies of gutta trees in the vast forests of Guiana was made about 1860. As might be expected, the rich flora of Brazil furnishes many varieties of the tree; one of which, the Mimusops elata, exudes a white liquid of an agreeable flavor, which is often used with tea or coffee. Other countries to be mentioned are Venezuela, Costa Rica, and Australia. (For the botany of India rubber and gutta-percha, the reader is referred to E. Chapel's comprehensive work, Le Caoutchouc et la Gutta-Percha.) Some growths produce caoutchouc gutteux, a substance of inferior quality, having the character of gutta-percha and India rubber.
The largest quantities of gutta-percha come from the Sunda Islands, Cochin China, Cambodia, and Hindustan. At first it was exported exclusively from Singapore, but now some shipments are made direct from the above-mentioned regions, from the island of Celebes and the Philippines; some going to the United States and the ports of Amsterdam and Rotterdam, but the greatest portion to England.
The different varieties in European markets are designated after their place of origin, the best being the Macassar, Java, Sumatra, Borneo, etc. The gutta-percha of the Orient is of various colors. The best grades are white or grayish, slightly rose-tinted, and generally contain very little foreign substance; inferior grades are dark and mixed with impurities. The former are tenacious, the latter often very friable. Macassar and Sarawak, the finest grades of gutta, are light-brown verging on yellow, while Balata is rose-brown.
On being refined and drawn thin, gutta-percha is translucent; drawn very thin, it is transparent; but placed on a white surface, it is rose or gray. At ordinary temperatures it is supple, flexible, and very tenacious and extensible, so that it may be drawn to three times its length, when it retains almost all of the extension. If a dent be made with the finger nail, a trace will remain. It softens above 50° C. and becomes adhesive above 100° C. Two pieces may be permanently joined by applying a hot iron to the surfaces and using light pressure. It is a bad conductor of heat and electricity, but may be electrified by friction; so it is sometimes employed for the disks of electric machines. Exposed to the air, it undergoes, at length, a great change, losing its fibrous structure and becoming granular and friable; more quickly in hot countries. This is a result of the action of oxygen under the influence of light. It is insoluble in water, softens in boiling water and steam; resists alkalies, hydrofluoric acid (being used for a receptacle for this acid), and ordinary dilute acids, but, when strong, they attack it.
The property of gutta-percha of greatest value to mankind is its dielectric or nonconducting property. This is not lessened by atmospheric conditions, nor is it destroyed by plunging under water or burying in the ground or subjection to other deteriorating influences. Hence its fitness for cables, telephone wires, etc. Its power of insulation decreases as the temperature increases. Sea water is a medium in which gutta-percha undergoes no alteration, and the enormous pressure at great depths exerts a favorable action on it by closing up accidental splits. Wrappings are put upon electric cables to protect the insulation from abrasion and the attacks of marine animals, as well as to strengthen the cable. Gutta insulation is preferred for telegraphy, telephony, bell service, etc.; rubber insulation for lighting and power, as intense currents are liable to lead to accidents by fusion of gutta insulation. What the world owes to gutta-percha may be most easily illustrated by more than one hundred and thirty-nine thousand miles of ocean cables, not to speak of the myriads of wires on land, under it, and in buildings for every conceivable purpose.
As soon as this valuable substance became known, industrial enterprise at once sought to make use of it. From the year 1844 the new product received numerous applications and gave rise to many patents. It was used for making stoppers, glues, and wires; then shoes, surgical instruments, and clothing. The most fortunate application was as coating for telegraph cables (patents to W. H. Barlow and Theodore Forster, April 27, 1848, and to E. W. Siemens, April 23, 1850). Efforts had been made to solve the problem of submarine telegraphy for some years. The Count de Moncel, in his Traité de Télégraphie Électrique, gives to Mr. Wheatstone, London, the palm as its inventor, but all do not agree in this. The first mention (The Atlantic Telegraph, W. H. Russell) we are able to find of a current being transmitted a distance under water refers to Sir W. O'Shaughnessy, Superintendent of Electric Telegraphs in India, who hauled an insulated wire across the Hoogly, at Calcutta, and produced electric phenomena on the other side of the river, in 1839. Wheatstone, who is said to have been thinking of binding England and the Continent in electric connection as early as 1837, laid a plan before the House of Commons in 1840 for a cable between Dover and Calais. He seems to have had no definite idea of the kind of insulator to be employed, and, as the insulating quality of gutta-percha was not yet known, his project was not carried out. Morse, in 1812, had succeeded in telegraphing with a copper cable in New York harbor from Castle Garden to Governor's Island, and said, in a letter to the Secretary of the Treasury in 1813, that electric communication "may with certainty be established across the Atlantic Ocean." Three years later Ezra Cornell had successfully employed a cable insulated with rubber in the Hudson at Fort Lee; and in 1846 Colonel Colt, the patentee of the revolver, and Mr. Robinson, of New York, laid a wire across the river from New York to Brooklyn and from Long Island to Coney Island. So this great fact of instantaneous exchange of intelligence with nations beyond seas was in a promising embryonic stage. Yet a satisfactory insulation still was wanting. Then, at the very time when science was earnestly seeking to find a suitable insulating material, Montgomery was studying the properties and supply of gutta-percha. The late distinguished German inventor, E. W. Siemens, recognized the superior insulating power of this substance in 1846, and constructed the first subterranean line in Germany in 1847. Thence to the submarine cable was but a step.
It is now interesting to Americans to note that S. T. Armstrong, of New York (The Story of the Telegraph, by Briggs and Maverick; The Telegraph Manual, by Schaffner), who had been invited to England in 1847 to inspect the products of the new industry, established this industry the same year in Brooklyn (as the president of a company), and made highly favorable experiments across the Hudson in the autumn in 1848. He was so sanguine of the success of gutta-percha insulation that he offered in The Journal of Commerce, the same year, to lay a line across the Atlantic for $3,500,000. (We have been careful to allude to this, as there is usually no reference to Armstrong's experiments works upon the subject.) Nor was it until January 10, 1849, some months later, that Walker, one of the first savants that busied themselves with electricity, and who first experimented with the railway block-system, successfully telegraphed in the harbor of Folkestone on what is generally considered the first submarine cable. It was a gutta-covered cable extending from a boat two miles to the shore. The favorable results at Folkestone encouraged John W. Brett, who is known in England as the father of submarine telegraphy, to construct the first long line from Dover to Calais, with the encouragement of the French Government, in 1850, when the successful application of gutta-percha to real submarine telegraphy was demonstrated and Wheatstone's hopes were completely realized. Brett labored with great energy for the success of long-distance submarine communication, and his faith and advice, as well as capital, were of the very greatest value to Field and his fellow-promoters of the first Atlantic cables. In 1851 the fire-hose at the exposition in London was of this substance, and on the last voyage in search of Franklin a light and portable gutta-percha boat did good service, showing that the material was suitable for the sheathing of boats. Advantageous use was also made of it in making molds for reproducing delicate impresions in the galvanoplastic process. But, except in telegraphy and galvanoplasty, its employment soon diminished. Shoes of gutta-percha softened before the fire and stuck; clothing made of it in turn fell into ill favor. Vulcanizing and mixing with rubber were thought of, but still satisfactory products were not obtained. They were hard and were soon abandoned.
The failure of gutta-percha for some purposes is owing to its greater suppleness at a slightly elevated temperature. It may be vulcanized by adding sulphur and heating to 135° to 150° C, but this process, owing to inconvenient results obtained, is now very rarely employed. Its resistance to acids led to its being put to use for vessels and tubes in factories of chemical products and laboratories, while medical science, as seemed promised at its coming, has found in it a valuable auxiliary.
The gutta-percha of commerce contains impurities — wood, earth, sand, etc. — which must be eliminated. It may be dissolved in sulphide of carbon or benzene. Then decant and allow to evaporate. A cheaper and preferred method is by mechanical means. The cakes are cut into chips, which are plunged into a hot-water vat and stirred by an agitator; the heavy substances go to the bottom, the gutta and wood remaining on top. A series of rasps removes the wood, and the gutta is taken and dried. The process is often accomplished by still other machinery for removing the foreign substances, care being taken to squeeze out all the air bubbles. The gutta is now pure, but may be mixed with other substances to harden it or diminish the price. Caoutchouc gutteux is sometimes added to make it more supple, and a certain degree of elasticity may be imparted to it by adding India rubber of good quality. With the perfect machinery now employed it is made into sheets of almost any desired thickness. Many articles are molded and the seams are finished with a hot iron. Cords and tubes are made by a machine employed for similar articles in the manufacture of India rubber, and which is on the principle of one used in making macaroni. Belting is made for use in moist air or where acid vapors are given off. Such belts have less resistance than those made of rubber or leather, and are only used for small powers. According to Meyer's Lexikon, there is a transparent gutta-percha varnish which can be used for covering documents. It does not change the paper; the document is perfectly protected against water, acids, and alkalies; and the writing can not be erased. Gutta-percha is also used in dentistry.
At the International Congress of Electricity at Paris, in 1881, the alarm was sounded in regard to the decreasing supply of this substance, and England, France, and Holland caused investigations to be made with a view to gutta-percha planting, but they do not seem to have led, as yet, to any specially practical results. Land of volcanic origin has been observed to be favorable, and heat and light and constant humidity the conditions essential for the growth of the tree. There are vast regions in Cochin China and Cambodia where, it is said, the Isonandra could be grown at slight expense; while all Malaysia, and probably other regions where it grows native, would be found to lend themselves to the same purpose. As the tree requires thirty years to reach maturity, little can be expected from private enterprise, and national aid should be extended. Meanwhile, the increasing demands of submarine telegraphy, etc., and the ruthlessness of gatherers are making it scarcer, and manufacturers must speculate with very variable prices.