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Resins and Gums include a great number of substances, such as resin or hard pitch, which is distilled from pine-trees; shellac, mastic, elemi, copal, kauri gum, amber, gum arabic, dextrine made from flour, the gum of the peach and cherry, and of many other trees. To these may be added frankincense and tragacanth, which is less an adhesive than a stiffener and dresser. Gums are generally rather brittle; this is remedied by combination with oily substances, volatile oils, or caoutchouc. With these gums Lehner includes asphaltum. The defect of such adhesives is, as he also remarks, that they will not resist high temperatures. This, however, will apply to most objects.

Varnish.—This belongs properly to the gums, but is technically regarded as a separate material. It is gum in solution in turpentine or spirits. For details vide Die Fabrikation der Copal- Terpentinöl und Spiritus-Lacke, by L. E. Andés; Leipzig, price 5 m. 40 pf.

Caoutchouc and Gutta-Percha are gums which when hard are still elastic, and resist the action of water. I have read that a perfect imitation or substitute for them has been made of turpentine, but have not seen it, though I have met with glue made with oil and turpentine, which very much resembled them in elasticity or flexibility. Reduced to a liquid form with ether, benzine, &c., these gums can be kept in a liquid state for a long time, and then hardened in any form by exposure to the air. They enter into a very great variety of cements, such as are meant to be tough or waterproof. Indiarubber is, on the whole, the best, and gutta-percha the cheapest, for cements.