The milky sap of indiarubber-yielding plants lies chiefly in the middle layer dale bark, and is contained in a network of minute tubes (" laticifernus vessels"), which, in the Apocyncwor, are found also in the inner bark or bast layer. Examinations of this sap are less complete than they might bo, and helve mostly been made upon samples transported from S. America in closed vessels. A specimen analysed by Faraday gave 31.70 per cent. caoutchouc, 7.13 wax and bitter principle, 2.90 rnatter soluble in water but insoluble iu alcohol (? gum), 1.90 albumen, 56.37 water, acetic acid, and salts. The acidity was clearly due to fermentation. The fresh milks of some European upliorhias (Euphorbia Cyparissias and E. p'atyphylla) reveal the presence of water, resin, caoutchoue, essential oil, albumen, gum, yellowish-brown extractive matter, sugar, starch, fatty oil, tartaric acid, malio acid, &c. The chief ingredients are :— Their respective sp. grs. at ordinary temperatures aro 1.0449 and 1.046S.
According to Adriani, the fresh milk of itidiarubber and guttapercha plants appears under the microscope as a kind of emulsion, a clear liquid having suspended in it nainute (-/-,±„, in. diam.) globules of eaoutchoue. Raw balata exhibits the same appearance. The milk of the above-named ouphorbias coagulates on exposure to the air. The same is the case with indiarnbber. That the eaoutcholic is held in suspension in the juice by the agency of ammonia, gains probability from the fact that many of the fresh milks have au ammouiacal odour, and that the addition of liquid ammonia is resorted to as a preventive of coagulation. Exposure of the milk to the air causes a change (usually reddening) of colour. The presence of an indiarubber in a plant may always be detected by making an incision, and testing whether the exuding milk will coagulate into an elastic fibre when rubbed between the fingers. An incision in the dry bark of such plants will disclose parallel elastio threads. The main distiuctive features of all the meiubers of this group are their elasticity, and their insolubility in water, alcohol, alkalies, aud organic aoids. Their composition is some what complex, and their market value depeuds upon a proportionate abundance of the elastic substance, with a relative absence of a certain oxidized, viscid, resinous body soluble in alcohol, and whose formation is in great measure prevented by rapid evaporation of the milk, and other means of avoiding oxidation.
In quantity, iudisrubber and its allies are found in numerous tropical and subtropical plants of the families Euphorbiaccw, Apocynacew, Asclepiadacece, gapotacece, Lobeliacece, Artocarpacece, and Moracew. The following list embraces the chief plants yielding indiarubber-like products :—
Euphorbitteers.—Hevea [Siphonia] spp. (Para lndiarubber); Manihot Glaziovii (Ceani Indiarubber); Mabea spp. ; Omphalet cordata ; Sapium spp. ; Euphorbia spp.
Apoc,ynaccce.—Urceola elastica, Walughbeia spp. (Borneo Indiarubber); Trahea spp. (Mozambique or Madagascar Indiarubber); Hancornia speciosa (Mangabeira or Pernambuco Indiarubber); Landolphia spp., Tabcrnomontana sp. (African Iudiarubber).
Asclepiadacece.—Calotropis gigantca, Cryptostegia spp., Alstonia spp. (Indiarubber). Saputacew.—Isonandra [Dichopsis] spp. (Guttapercha); Sideroxylon attenuatum ; Mimusops spp. Palate).
Lobeliacco.—Siphocampylus sp.
Artocarpacece.—Castilloa spp. (Central American Indiarubbers); Artocarpus integrzfolia ; Cccropia pcltata.
ilforaccce.—Ficus spp. (Assam and Java Indiarubber) ; Urostigrna spp. (African Indiarubber).
It will have been observed that the preceding classifications necessarily exclude an artificial product such as dextrine or British gum, and the fossilized resins of which amber is the chief, as well as one or two bodies, which, though really astringent extracts, are commonly included among gums,--the kinos and mochurrus. This fact, combined with the want of uniformity in different systems of classification, and the disregard for all classification exhibited by the mercantile classes, renders it inadviaable to continue a grouped arrangement in dealing with each product in par ticular. The following sections will therefore be arranged in alphabetic order according to the commercial names of the most important.—Amber, Ammoniacurn, Arabic, Asafoetida, Balate, Balm of Gilead, Bdellium, Benzoin, Cadjii, Chag-ual, Cherry, Chicle, Chironji, Coco-nut, Copaiba, Copal and Animi, Dammar, Dextrine, Dhoura, Dika-mali, Dragon's-blood, Elemi, Euphorbium, Frankincense, Galbanum, Gamboge, Ging and Kunnee, Guaiacum, Gurjun, Guttapercha, Guttashea, Indiarubber, Jumrasi, Jutahy-seca, Kauri, Kino, Kos, Lac, Mahogany, Mango, Mastic, Mezquite, Mocburrus, Moringa, Myrrh, Nagdana, Olibanum, Opoponax, Orange, Peru balsam, Phormium, Piney, Pitches, Retinite, Rimu, Rosin, Sagapenum, Sandarach, Sarcocolla, Satin wood, Schraufite, Storax, Tamanu and Tacamahaca, Tars, Tendoo, Thus, Tolu balsam, Tragacauth, Turpentines, Varnishes, Wood-apple, Xanthorrhcea. To these, will be appended an alphabetic list (under botanical names) of the less important plants known to afford resinous, gummy, or balsamic exudations.