INDIAN RUBBER. 



parts of Asia Minor, and has been introduced 

 into Italy, Spain, Switzerland, and some parts 

 of Germany; also into China, Cochin-China, 

 and the West Indies, where it grows commonly 

 five or six feet high or more, and, being es- 

 teemed a hearty food for labourers, is called 

 Negro Guinea corn. Its long awns or bristles 

 defend it from the birds. In England, the au- 

 tumns are seldom dry and warm enough to 

 ripen the seeds well in the field. In Arabia it 

 is called dora or durra. The flour is very white, 

 and they make good bread of it, or rather cakes, 

 about two inches in thickness. The bread 

 which they make of it in some parts of Italy is 

 dark and coarse. In Tuscany it is used chiefly 

 for feeding poultry and pigeons ; sometimes 

 for kine, swine, and horses. Brooms are made 

 of the spikes, which are also sent to England 

 for the same purpose. The Indian millet, as 

 well as the common sort (/><?/iin<w) is cultivated 

 in some parts of North America, and has been 

 tried in England, but it is only in the warmest 

 autumns that it ripens its seeds. (Laudon'$ En- 

 tychptedia of Phnt*.') See GUINEA COBW. 



INDIAN RUBBER, Gum Elastir, or Caout- 

 chouc, is a well-known tough and yielding sub- 

 stance, obtained in South America and Java 

 from the sap of a tree called the Siplmniii cahum. 

 To procure the sap, incisions are made through 

 the bark in many places, and the milky juice 

 which exudes is spread over clay moulds, and 

 drii-il 111 the sun, or by a fire, the smoke from 

 which last blackens it. 



The juice itself has been of late years im- 

 ported. It is of a pale-yellow colour, and has 

 the consistence of cream. It becomes covered, 

 in the bottles containing it, with a pellicle of 

 concrete caoutchouc. Its specific gravity is 

 1*012. When it is dried, it loses 55 per cent. 

 of its weight: the residuary 45 is elastic gum. 

 When the juice is heated, it immediately coagu- 

 lates, in virtue of its albumen, and the elastic 

 gum rises to the surface. It mixes with wa- 

 ter in any proportion ; and, when thus diluted, 

 it coagulates with heat and alcohol as be- 

 fore. 



The specific gravity of caoutchouc is 0-925, 

 and it is not permanently increased by any de- 

 gree of pressure. By cold or long quiescence, 

 it becomes hard and stiff. By long boiling in wa- 

 ter it softens, swells, and becomes more readily 

 soluble in its "peculiar menstrua; but when ex- 

 posed to the air, it speedily resumes its pristine 

 consistence and volume. It is quite insoluble 

 in alcohol ; but in ether, deprived of alcohol 

 by washing with water, it readily dissolves, 

 and affords a colourless solution. When the 

 ether is evaporated, the caoutchouc becomes 

 again solid, but is somewhat clammy for a 

 while. When treated with hot naphtha, dis- 

 tilled from native petroleum, or from coal-tar, 

 it swells to 30 times its former bulk ; and if 

 then triturated with a pestle, and pressed 

 through a sieve, it affords a homogeneous var- 

 nish, which being applied by a flat edge of 

 metal or wood to cloth, prepares it for forming 

 the patent water-proof cloth of Mackintosh. 

 Two surfaces of cloth, to which several coats 

 of the above varnish have been applied, are, 

 when partially dried, brought evenly in contact, 

 and then passed between rollers, in order to 



INDIAN RUBBER. 



\ condense and smooth them together. Thi 

 I double cloth is afterwards suspended iri a 

 | stove-room to dry, and to discharge the disa- 

 greeable odour of the naphtha. 



Caoutchouc dissolves in the fixed oils, such 

 as linseed oil, but the varnish has not the pro- 

 perty of becoming concrete upon exposure to air. 



It is more or less soluble in the oils of la- 

 vender and sassafras. 



It melts at 248 Fahrenheit, and stands after- 

 wards a much higher heat without undergoing 

 any further change. When the melted caout- 

 chouc is exposed to the air, it becomes hard 

 on the surface in the course of a year. When 

 kindled, it burns with a bright flame and a 

 great deal of smoke. 



Neither chlorine, sulphurous acid gas, muri- 

 atic acid gas, ammonia, nor fluosilicic acid gas 

 affects it, whence it forms very valuable flexi- 

 ble tubes for pneumatic chemistry. Cold sul 

 phuric acid does not readily decompose it, nor 

 does nitric acid, unless it be somewhat strong. 

 The strongest caustic potash ley does not dis- 

 solve it, even at a boiling heat. 



Mr. William Henry Barnard, in the course 

 of some experiments upon the impregnation 

 of ropes with caoutchouc, at the factory of 

 Messrs. Enderby, at Greenwich, discovered 

 that when this substance was exposed to a heat 

 of about 600 Fahrenheit, it resolved itself into 

 a vapour, which, by proper refrigeratory me- 

 thods, was condensable into a liquid possessing 

 very remarkable properties, to which the name 

 caoutchovcine has been given. For this in- 

 vention "of a solvent not hitherto used in the 

 arts," Mr. Barnard obtained a patent, in Au- 

 gust, 1833. His process for preparing it is 

 described in his specification as follows : 



" I take a mass of the said caoutchouc, or 

 Indian rubber, as imported, and having cut it 

 into small lumps, containing about two cubic 

 inches each (which I prefer), I throw these 

 lumps into a cast-iron still. I then apply heat 

 to the still in the usual manner, which heat is 

 increased until the thermometer ranges at 600 

 degrees of Fahrenheit, or thereabouts. And 

 as the thermometer ranges progressively up- 

 wards to 600 degrees of Fahrenheit, a dark- 

 coloured oil or liquid is distilled over, which I 

 claim as my said invention, such liquid being 

 a solvent of caoutchouc, and other resinous 

 and oleaginous substances. When the ther- 

 mometer reaches 600 degrees, or thereabouts, 

 nothing is left in the still but dirt and charcoal. 



"I aAerwards subject the dark-coloured li- 

 quid thus distilled to the ordinary process of 

 rectification, and thereby obtain fluids varying 

 in specific gravity, of which the lightest hitherto 

 has not been under 670, taking distilled water 

 at 1000, which fluids I also claim as my said 

 invention. 



"At each rectification the colour of the liquid 

 becomes more bright and transparent, until, at 

 the specific gravity of 680, or thereabouts, it is 

 colourless and highly volatile. 



" In the process of rectification (for the pur 

 pose of obtaining a larger product of the oil 

 colourless) I put about one-third of water into 

 the still. In each and every state the liquid is 

 a solvent of caoutchouc, and several resinous 

 and oleaginous substances, and also of other 



659 



