694 CAOUTCHOUC 



^ Sharp and clean casts wero taken with this liquid, and as it is susceptible of being 

 tinted with delicate colours, it might bo used for beautiful ornamental purposes ; when 

 the solid rubber separates, it is white, but in small pieces or thin sheets, it is semi- 

 transparent. 



The specific gravity of caoutchouc is 0-925, and it is not permanently increased by 

 any degree^ of pressure. By cold or long quiescence it becomes hard and stiff. When 

 the milky juice has become once coherent, no means hitherto known can restore it to 

 the emulsive state. By long boiling in water it softens, swells, and becomes more 

 readily soluble in its peculiar menstrua ; but when exposed to the air it speedily resumes 

 its former consistence and volume. It is quite insoluble in alcohol ; but in ether, de- 

 prived 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, distilled 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 varnish, which 

 being applied by a flat edge of metal or wood to cloth, prepares it for forming the 

 patent water-proof cloth of Macintosh. 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 condense and smoothe them 

 together. This double cloth is afterwards suspended in a stove-room to dry, and to 

 dispel the disagreeable odour of the naphtha. 



Caoutchouc dissolves in the fixed oik, such as linseed oil, but the varnish has not 

 the property of becoming concrete upon exposure to the air. 



It has lately been asserted that caoutchouc is soluble in the oils of lavender and 

 sassafras. Koxburgh found it perfectly soluble in the oil of cajeput. 



It melts at 248 F., and stands afterwards a much higher heat without undergoing 

 any further change. When the melted caoutchouc 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, muriatic acid gas, ammonia, nor fluosilicic 

 acid gas, affect it, whence it forms very valuable flexible tubes for pneumatic che- 

 mistry. Cold sulphuric acid does not readily decompose it, nor does nitric acid, unless 

 it be somewhat strong. The strongest caustic potash-lye does not dissolve it oven at 

 a boiling heat. 



According to Dr. Faraday, the pure caoutchouc, obtained from the sap, had a 

 specific gravity of 0'925, and no reduplication of it in a Bramah's press was found to 

 effect permanent alteration. 



The specific gravity of the best compact Para caoutchouc, 



taken in-dilute alcohol, is 0-941567 



The specific gravity of the best Assam is v . . . . 0-942972 



Singapore .... 0-936650 



Penang .... 0-919178 



Having been favoured by Mr. Sievier, formerly managing director of the Joint-Stock 

 Caoutchouc Company, and by Mr. Beale, engineer, with two different samples of 

 caoutchouc juice, Dr. tire subjected each to chemical examination. 



' That of Mr. Sievier, is greyish brown, that of Mr. Beale is of a milky grey colour ; 

 the deviation from whiteness in each case being due to the presence of aloetic matter, 

 which accompanies the caoutchouc in the secretion by the tree. The former juice is 

 of the consistence of thin cream, has a specific gravity of 1-04125, and yields, by 

 exposure upon a porcelain capsule, in a thin layer, for a few days, or by boiling for a 

 few minutes with a little water, 20 per cent, of solid caoutchouc. The latter, though 

 it has the consistence of pretty rich cream, has a specific gravity of only 1-0175. It 

 yields no less than 37 per cent, of white, solid, and very elastic caoutchouc. 



' It is interesting to observe how readily and compactly the separate little clots or 

 threads of caoutchouc coalesce into one spongy mass in the progress of the ebullition, 

 particularly if the emulsive mixture bo stirred ; but the addition of water is necessary 

 to prevent the coagulated caoutchouc from sticking to the sides or bottom of the vessel 

 and becoming burnt. In order to convert the spongy mass thus formed into good 

 caoutchouc, nothing more is requisite than to expose it to a moderate pressure between 

 the folds of a towel. By this process the whole of the alootic extract, and other 

 vegetable matters, which concrete into the substance of the balls and junks of caout- 

 chouc prepared in Assam and Java, and contaminate it, are entirely separated, and an 

 article nearly white and inodorous is obtained. Some of the cakes of American 

 caoutchouc when cut exhale the fcetor of rotten cheese ; a smell which adheres to the 

 threads made of it after every process of purification. 



' In the interior of many of the balls which come from both the Brazils and East 



