The different parts of a solid are held together by an attraction between their several particles, which is termed the attraction of cohesion, or cohesive attraction. The amount of this varies with the substance; thus, the cohesion of the particles of iron to one another is enormously great, whilst that between those of chalk is but small. This attraction acts only when the particles are in the closest possible contact; even air must not be between them. If, after breaking any substance, we could bring the particles into as close contact as before, and remove the air, they would re-unite, and be as strongly connected as ever. But, in general, this is impossible; small particles of grit and dust get between them; the film of interposed air cannot be removed; and thus, however firmly we press the edges of a broken cup together, it remains cracked china still. _Perfectly_ flat, clean surfaces, like those of freshly ground plate-glass, may sometimes be made to cohere, so that the two pieces become one, and cannot be separated without breaking. The attraction of cohesion takes place between the parts of the same substance, and must not be confounded with that of adhesion, which is the attraction of different substances to one another; for example, the particles of a piece of wood are united by cohesive attraction, whilst the union of glue and wood to each other depends on adhesive attraction. And it is important that this distinction be borne in mind, for, in almost all cases, the cohesion between the particles of the cement is very much less than the adhesion of the cement to other bodies; and if torn apart, the connected joint gives way—not by the loosening of the adhesion—but by the layer of cement splitting down the centre. Hence the important rule, that the _less_ cement in a joint, the stronger it is. Domestic manipulators usually reverse this, by letting as much cement as possible remain in the joint, which is, therefore, necessarily a weak one. A thick, nearly solid cement, which cannot be pressed out of the joint, is always inferior to a thinner one, of which merely a connecting film remains between the united surfaces. Having thus mentioned the general principles that ought always to be borne in mind, we will now proceed to describe the manufacture of some of the more useful cements, and their mode of use.