Glue , (Lat. gluere, to draw together), an impure variety of gelatine, used in the arts for uniting substances through its adhesive quality. It is obtained much in the same manner from the same substances as gelatine, but usually from the more refuse portions, as damaged hides and other tissues undergoing putrefaction. Glue obtained from bones by the use of acids is preferred to that which is obtained by steam, the latter being more soluble in cold water. The strongest glue is made from the parings of ox hides, which yield over 50 per cent. They are steeped for several days in milk of lime to remove the hair, blood, and other impurities; then washed in cold water, drained on an inclined plane, and again washed. Exposure to the air converts the lime into carbonate, so that in boiling the caustic action of the lime is prevented. The material is then enclosed in a coarse cloth and put into a copper boiler, which is two thirds filled with rain water, and the whole is boiled. The dissolved glue mingles with the water outside of the cloth, and when the liquid sets into a firm jelly on cooling it is run into a deep vessel or settling back and kept warm for impurities to subside.

Water is again added to the boiler, and the material in the cloth subjected to a second boiling, by which an inferior glue is obtained. The liquid in the settling back is drawn into coolers, where it solidifies, and is then cut into slices with a wire frame. The slices are laid upon netting in a drying room, in which there is a free circulation of air. The operation of drying is a critical one. Too much heat will cause liquefaction; a fog may cause mouldiness, and frost will split the slices. Good glue is of a pale brown color, hard and brittle, and breaks with a glassy fracture. Its other chemical and physical properties are like those of gelatine. The quality of glue may be judged of by the quantity of water which the dry glue will absorb in 24 hours. The best glue kept immersed in water of the temperature of 60° F. has absorbed 12 times its weight. Other qualities, it is said, take up a proportionally less quantity. Besides its use for cementing wood and hard substances, glue is employed in preparing the felt bodies of hats, and as an ingredient in the composition of inking rollers, to give them flexibility. - Several varieties of glue are employed in the arts, some of which may properly be noticed here, although they are not all preparations of gelatine.

If glue is treated with a small proportion of nitric acid, it loses its property of gelatinizing when cold, though not that of causing substances to adhere together. With acetic acid a similar effect is produced. What is called liquid glue is made by slowly adding nitric acid to the ordinary preparation of glue in the proportion of 10 oz. of strong acid to 2 lbs. of dry glue dissolved in a quart of water. The product is. a strong glue, which remains in a liquid state, and may be thus kept for years always ready for use. Marine glue is a preparation of caoutchouc dissolved in naphtha or oil of turpentine, with the addition of shell lac after the solution has by standing several days acquired the consistency of cream; two or three parts by weight of shell lac are used for one of the solution.

The composition is then heated and run into plates, and when used it is heated to the temperature of about 250° F. It possesses extraordinary adhesive properties, and being quite insoluble in water, it has been recommended as a material for fastening together the timbers of ships; so securely are these held by its application that it is said they will sooner break across the fibres than separate at the joint.