Gum, a concrete vegetable juice which possesses no peculiar smell, or taste ; it forms a viscid or mucilaginous solution in water, but is not acted upon either by spirits, or oils : it burns in fire without melting or inflaming, and is not dissipated by evaporation.
Gums are divided into two classes, genuine and impure. In the former class are Gum Arabic, Gum Senega, and Gum Traga-canth: the gums of plum and cherry trees, etc. The latter are such as contain a greater or less proportion of resin.
Gum Arabic exudes from the Mimosa Nilotica, or Egyptian Acacia, which abounds in Africa; but according-to Dr. Swediaur, it is chiefly obtained by boiling the roots of certain trees growing in Egypt. The best gum used in this country is of a pale yellowish colour. On account of its glutinous properties, it is preferred as a demulcent in coughs, hoarseness, and other cattarrhal affections, in order to obtund or mitigate irritating acrimonious humours, and to supply the loss of abraded mucus. It has been very generally employed in stranguries, and other urinary complaints.
Gum Elemi exudes from the Amyris elemifera, a native of South America, whence it is imported, and sometimes also brought from the East Indies. The best sort is rather soft and transparent, of a pale yellowish colour, inclining to green ; and of a strong but pleasing smell. It was formerly employed more than it is at present, in the compound ointment of Elemt, which has long been used for digesting and cleansing ulcers.
Besides its utility in medicine, gum is of considerable service in the Arts $ and, as sufficient quantities of it cannot always be procured genuine, different persons have endeavoured to contrive such substitutes, as would effectually answer the same purposes. - From these, we have selected the following, as being most easily prepared, and chiefly from substances produced in this country:
The first is the invention of Mr. Albert ANgell, of Bethnal-green, Middlesex, to whom a patent was granted in January 178I, for hisBritannic Elastic Gum. This preparation consists of one gallon of linseed, or nut-oil, 1lb. of beeswax, 61bs. of glue or size, 1/4 lb. of verdigrease, a similar quantity of litharge, and two quarts of spring or rain-water. These ingredients are to be melted in an iron kettle, till they acquire the consistence of gum. - The patentee observes, that such composition is particularly serviceable in the various branches of portrait and house-painting, as it renders the colours durable, and free from peeling 5 it is also said to be of great utility in the gilding, painting, etc. of silks, calicoes, etc. and in dressing silk, linen, or cotton, in the loom, instead of gum or paste, so as to strengthen the . threads of the finest cottons. He states a variety of other uses, a minute account of which the curious reader will find in the 3d vol. of the "Repertory of Arts and Manufactures."
Another patent was obtained in June 1788, by Mr. Francis BLAI-kie, of Glasgow, merchant, for the invention (discovered to him) of a substitute for gum, in thickening colours fur printing, which fully answers the purpose, and at a more reasonable rate. - This article is prepared by boiling, flax-seed in water, till the whole substance is completely extracted ; the liquor is next to be strained through a linen or woollen cloth, and boiled down to the consistence of a jelly. It is then to be put into a close vessel, and a small quantity of spirits, or sweet oil, poured on the top, in order to preserve it in a fresh state. - In using this substitute, the printer is directed to put a certain portion into a gallon of colour, according to the nature of the latter, and the particular kind of work; while he should regulate himself by trial, in the same manner as is practised in employing common gum.