Every Pea Helps to Fill the Peck.
2487. Cements
The term "cement" includes all those substances employed for the purpose of causing the adhesion of two or more bodies, whether originally separate, or divided by an accidental fracture. As the various substances that may require cementing differ very much in texture, &c., a number of cements possessed of very different properties are required, because a cement that answers admirably under one set of circumstances may be perfectly useless in others. The general principles upon which the success or failure of cementing usually depends are:
The different parts of a solid are held together by an attraction between their several particles, which is termed the attraction of cohesion. 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 a contact as before, and perfectly exclude 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.
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. To unite broken substances with a thick cement is disadvantageous, the object being to bring the surfaces as closely together as possible. The general principles that ought always to be borne in mind having been mentioned, the manufacture and uses of some of the more useful cements may be described.
2488. Mouth Glue
The very useful preparation sold under this title is merely a thin cake of soluble glue, which, when moistened with the tongue, furnishes a ready means of uniting papers, &c. It is made by dissolving one pound of fine glue or gelatine in water, and adding half a pound of brown sugar, boiling the whole until it is sufficiently thick to become solid on cooling; it is then poured into moulds, or on a slab slightly greased, and cut into the required shape when cool. (
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