Cements and Uniting Bodies. In the preparation of cements and all substances intended to produce close adhesion, whether in a semi-fluid or pasty state, freedom from dirt and grease is a most essential and necessary condition. Quite as much depends upon the manner in which a cement is applied as upon the cement itself. The best cement that ever was compounded would prove entirely worthless if improperly applied. The preparations given below will be found to answer every reasonable demand; and if properly prepared and used strictly according to the directions laid down, will seldom fail to form a union as strong, if not stronger than the substances joined. The first point that demands attention, is to bring the cement itself into intimate contact with the surface to be united. This end is best reached, when using hot cements, by making the edges to be joined at least as hot as the cement when applied, or as nearly so as can be done without injury to the substance; in some cases it is even preferable to melt the cement on the heated edges. Another very important point is to use as little cement as possible. "When the surfaces are separated by a large mass of cement, we have to depend upon the strength of the cement itself, and not upon its adhesion to the surfaces which it is used to join; and, in general, cements are comparatively brittle. Sealing- wax is a very good agent for uniting metal to glass or stone, provided the masses to be united are made so hot as to fuse the cement; but if the cement is applied to them while they are cold, it will not stick at all. This fact is well known to venders of cement for uniting earthenware. By heating two pieces, so that they will fuse shellac, they are able to join them so that they will rather break at any other part than along the line of union. But although people constantly see the operation performed, and buy liberally of the cement, it will be found in nine cases out of ten the cement proves worthless in their hands, simply because they do not know how to use it. They are afraid to heat a delicate glass or porcelain vessel to a sufficient degree, and they are apt to use too much of the material, and the result is a failure ; the cement is consequently deemed good for nothing. The great obstacles to the junction of any two surfaces are air and dirt. The former is universally present, the latter is due to accident or carelessness. All surfaces are covered with a thin adhering layer of air which it is difficult to remove, and unless this is displaced, the cement cannot adhere to the surface to which it is applied, simply because it cannot come into contact with it. The most efficient agent in displacing this adhering air is heat. Metals warmed to a point a little above 200° become instantly and completely wet when immersed in water. Hence, for cements that are used in a fused condition, heat is the most efficient means of bringing them in contact with the surfaces to which they are to be applied. In the case of glue, the adhesion is best attained by moderate pressure and friction.