Of the use of COMMON GLUE, very little need be said; it should always be prepared in a glue-pot or double vessel, to prevent its being burned; which injures it very materially; the objection to the use of this contrivance is, that it renders it impossible to heat the glue in the inner vessel to the boiling point; this inconvenience can be obviated by employing in the outer vessel some liquid, which boils at a higher temperature than pure water, such as saturated solution of salt (made by adding one-third as much salt as water). This boils at 224° Fahr., 12° above the heat of boiling water, and enables the glue in the inner vessel to be heated to a much higher temperature than when pure water is employed. If a saturated solution of nitre is used, the temperature rises still higher. WATERPROOF CEMENTS are very numerous; a very good one for uniting china and glass will be found in another page. It should be stated, however, that the gum ammoniac should be also dissolved in a small quantity of spirit. Mastic, used instead of ammoniac, makes a clearer cement. This mixture, under various fanciful titles, is usually sold at a most exorbitant rate. LIME AND EGG CEMENT is frequently made by moistening the edges to be united, with white of egg, dusting on some lime from a piece of muslin, and bringing the edges into contact. A much better mode is to slake some freshly burned lime with a small quantity of _boiling_ water; this occasions it to fall into a very fine dry powder, if excess of water has not been added. The white of egg used should be intimately and thoroughly mixed, by beating, with an equal bulk of water, and the slaked lime added to the mixture, so as to form a thin paste, which should be used speedily, as it soon sets. This is a valuable cement, possessed of great strength, and capable of withstanding boiling water. Cements made with lime and blood, scraped cheese, or curd, may be regarded as inferior varieties of it. Cracked vessels, of earthenware and glass, may often be usefully, though not ornamentally repaired by white lead spread on strips of calico, and secured with bands of twine. But in point of strength, all ordinary cements yield the palm to Jefferey’s Patented Marine Glue, a compound of India-rubber, shell-lac, and coal-tar naphtha. Small quantities can be purchased at most of the tool warehouses, at cheaper rates than it can be made. When applied to china or glass, the substance should be cautiously made hot enough to cement the glue, which should be then rubbed on the edges so as to become fluid, and the parts brought into contact immediately. When well applied, the mended stem of a common tobacco-pipe will break at any other part in preference to the junction. The colour of the glue unfortunately prevents its being used.