All matter is composed of countless separate molecules which exert their own powerful electro-magnetic forces, and thus adhere together by mutual attraction. In theory, therefore, any two smooth surfaces should 'also adhere, and in fact will do so providing the surfaces are in reality perfectly smooth. However, these inter-molecular forces have only a very short range, and any roughness of the surfaces will prevent the molecules adhering except at local high spots. Where the surfaces are rough—and a steel plate polished to mirror smoothness will still be extremely rough by molecular standards—a bonding agent is necessary whose own molecules will flow out over the rough surfaces, and render them smooth enough to adhere together. Simple liquids such as water, which exerts considerable forces of attraction (one ton per square inch), are in effect powerful adhesives, but they have low viscosity and high volatility and are, therefore, totally unsuitable. Any good adhesive, therefore, must be not only highly mobile and free flowing, but also non-volatile, with adequate tensile strength, and with little if any propensity to shrink back as it solidifies; moreover, certain additional characteristics are necessary according to the type of material for which it is to be used. Wood, for instance, will require cheapness, ease of preparation, a measure of flexibility to accommodate residual strains and stresses, resistance to fungoid and
insect attack, water-resistance, acceleration of the set by heat, and absence of staining.
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