RESINOUS AND GUMMY SUBSTANCES (FR., MatMres Resineuses et Gommeuses ; GER., Harz- und Gummi-arten).
Resinous and gummy substances may be primarily divided into three great classes—gums, resins, and indiarubbers ; each of these classes may be split into a number of sub-sections.
1. GUMS. —The term " gum" is properly restricted to those exudations from the stems, branches, and fruits of plants, which dissolve or soften in water to a slimy liquid state, or at least to a gelatinous consistency ; which refuse to dissolve in alcohol of 60 per cent. ; which yield mucic and oxalic acids when treated with nitric acid ; and which are capable of conversion (by sulphuric acid) first into dextrine and then into sugar. The form, surface, colour, transparency, density, microscopic characteristics, and optical properties scarcely admit of any generalization, and will best be noticed under the individual substances. As to the chemical constitution of gums, all natural vegetable gums (thus excluding dextrine) are substantially cornposed of one or more of the three bodies—bassorine, arabine, and cerasine. Bassorine is a pure hydrocarbon ; arabine and ccrasine are hydrocarbons combined with mineral bases. Baasorine is devoid of colour, odour, and flavour ; it is insoluble in water and alcohol, but heated in the former, assumes a gelatinous character ; dried at 100° (212° F.), its formula is C6H,,,05. Arabiue is a compound of arabic acid with lime and some potash or magnesia ; it is a colourless, odourless body, of acid reaction, forming with water a glutinous, frothy solution; on burning, it leaves an ash consisting chiefly of carbonate of lime, but containing also same carbonate of potash. Arabic acid is a white substance, soluble in water, ant-I.-reddening litmus; its solution dries to a colourless, amorphous body ; dried at 100° (212° F.), its forrnula is 0,21-1.011. Cerasine is a colourless body, insoluble in water and alcohol, but, like bassorine, swelling in the former to a kind of gelatine ; it is a compound of metagummic acid and lime. Natural gums also contain water (12-17 per cent.), dextrine, sugar, tannin,
colouring matter, and rnineral ingredients ; they afford 2-3 per cent. of ash.
Concerning the origin of gums. They were formerly supposed to be secretions of plants ; recent researches, however, have clearly proved that at least some gums are formed from the whole tissue of the cell-walls, by chemical metamorphosis. This is considered certain in the case of tragacanth, cherry, and arabic gums. [The reader is referred to sorne special remarks on traga canth under that head, pp. 1685-6.; Wiesner holds the same view with regard to moringa and Indian tmgacanth (kuteern) gums. Accordingly it happena thut gums are yielded most abundantly whou the plants aro in a sickly state, caused by a fulnesa of sap in the young tissues, whereby tbe 111.W cells are softened and finally decomposed ; the cavities thus formed fill with liquid, which exudes, dries, and constitutes " gum," which, in structure, is quite amorphous, being neither crystallized nor organized. Gum is one of the moat common plant-products. It occurs abundantly in the liviag rind of many plants, and exudes upon the aurface of the bark. In tbe woody structure, it occurs more seldom, and in srnaller quautity. The sources of the most important guins may be seen from the following synopsis :— .11iniascw.—Acacia spp., giving Arabic (includiug Senegal, Suakin or Talc.% Morocco or Barbary, Cape, E. India, aud Australia or Wattle), and Kutteia (Indian Tragacanth) ; Prosopis spp. (Mezquite); Parkia spp.
Papilionacem.—Astragalus spp. (Tragacanth).
Drupaccw.—Prunus spp., and Amygdalus spp. (collectively Cherry gum). Anacardiacem.—Anacardium occidentale (Cadjii); Spondia spp. ; Odina spp. (Ging awl Kunuee); Elms Metopiurn.
Aurantiacete.—Feronia ekphantum (Wood-apple).
Meliacem.—Melia Azedarach.
Ternstromiacece.—Cochlospermum gossypium (Kuteera [Iadian Tragacanthp.