ADHESIVES Adhesives.—Adhesives are a class of substances capable of attaching themselves to the surface of solid bod ies, and thus, when interposed between them, of uniting such bodies. Vari ous substances and compounds have adhesive or sticky properties. Among these are the gums arabic, tragacanth, and senegal; dextrin, gelatin or glue, isinglass or fish glue; various resins, as shellac, rosin, etc.; casein, from the curd of milk and cheese; India rubber, gutta percha, litharge, and other substances too numerous to men tion.
Various preparations of these sub stances may be loosely classed accord ing to their composition in the order of their adhesive power, as paste, mu cilage, glue (either solid or liquid), and cement. But these terms hardly have a definite meaning, and are often used interchangeably. Some knowledge of the origin and properties of the different substances used in adhesive compounds will be of much assistance in preparing and using them.
Dextrin.—A substance formed from starch, rice flour, or cornstarch, also known as British gum or starch gum. It is used as a substitute for gum arabic, as a size for mucilage, and especially for the backs of postage stamps and sealing envelopes.
Gluten.—A substance believed to be produced by the action of a ferment. It is formed in flour by uniting or mixing it with cold water. Gluten is the substance which retains the car bonic-acid gas in bread making, and thus assists in the process of raising bread.
Gelatin.—Gelatin is produced from certain animal membranes by the ac tion of hot water. Isinglass, calf's foot jelly, and glue are chiefly com posed of gelatin. It absorbs water, which causes it to swell, and may be dissolved in hot water or acetic and other acids. The addition of alcohol, corrosive sublimate, or tannic acid to a solution of gelatin in water causes the gelatin to be thrown down.
Glue.—The glue of commerce is dry gelatin having a more or less brown ish color according to its purity. White or pale glue is the best. It is a hard, brittle, glossy substance which usually comes in thin sheets. Glue is obtained by cleansing scraps of hides, hoofs, and horns with lime, and boil ing them until changed into gelatin.
Glue can also be made from bones, but this is of inferior quality. Pre pared or liquid glue is ordinary glue dissolved in water with the addition of acids that have the power to dis solve glue without heat and hold it in a state of solution.
Isinglass.—Dry gelatin is prepared from the air bladder of sturgeon and other fish, such as cod, weakfish, hake, etc. It is used in preparing jellies, blancmange, gum drops, etc.; in mak ing court-plaster, as a size for deli cate fabrics, and as an adhesive.
Fish Glue is an inferior isinglass made from the offal of fisheries.
To melt isinglass, beat up tea spoonful of white of egg in 1 pint of water. Add 4 ounces of isinglass, and melt over a slow fire.
To detect adulteration by gelatin, drop a sample of the suspected isin glass into vinegar. Pure isinglass will swell like jelly, while gelatin will IN come hard.
Or put a sample in cold water. Pure isinglass becomes cloudy and white, and the adulteration becomes jellylike and clear.
Resins.—For the various resins hav ing adhesive qualities including rosin, shellac, and the like, see under " Var nish." Gutta Percha.—The hardened milky juice of a large tree growing in the East Indies. It is insoluble in water, slightly soluble in alcohol and ether, and readily dissolved in bisulphide of carbon, benzol, chloroform, and oil of turpentine. It deteriorates rapidly when exposed to the air, and becomes brittle and useless. It is chiefly used for coating submarine telegraph wires and other metallic articles under water.
Caoutchoue, gum elastic, or India rubber is the juice or sap of several tropical plants growing in the East Indies and South America. It is ob tained by cutting the bark and drying the juice over smoky fires, which im part its black color. It is elastic and waterproof. When combined with about 25 per cent of sulphur and raised to a temperature of about 270° F., it is converted into soft vulcanized rubber; by the addition of 50 per cent of sulphur and heating to 300° F., it becomes hard vulcanized rubber or ebonite.