_Synthetic resin._—A complex, substantially amorphous, organic semisolid or solid material (usually a mixture of substances) built up by chemical reaction of comparatively simple compounds and, depending upon the temperature at which the examination is made, approximating the natural resins in various physical properties: namely, luster, fracture, comparative brittleness, insolubility in water, fusibility or plasticity when heated or exposed to heat and pressure, and, at a certain more or less narrow temperature range before fusion, showing a degree of rubberlike extensibility; but commonly deviating widely from natural resins in chemical constitution and behavior with reagents. _Synthetic rubber._—Caoutchouc synthesized in the laboratory. The term is a misnomer and most probably represents an impossibility. _Tensile strength._—The greatest internal force per unit of cross section which a material develops before failure under tension.[30] _Thermoplastic._—The property of softening under heat. All molding materials are thermoplastic at the initial application of heat. One class (the so-called thermoplastics) remains soft permanently under heat; the other (thermosetting), after first softening, sets or cures more or less quickly to a more solid form. A practical distinction is that with the first class the mold must be cooled before the molded article is removed, but not with the second. A thermoplastic substance is adequately rigid at normal temperatures and under ordinary conditions of stress but is capable of deformation under heat and pressure. _Thermosetting._—The property of undergoing a chemical change when heated whereby a hardened product is obtained. Property most pronounced in phenol and urea formaldehyde resins and less so with alkyds. A thermosetting substance possesses initially the properties of a thermoplastic but under the influence of heat undergoes chemical change so that it is no longer thermoplastic but becomes permanently infusible. _Viscosity._—Internal friction or resistance to change of form of a liquid. The constant ratio of shearing stress to rate of shear. _Water-absorption._—Amount of water taken up when exposed to humid conditions or when immersed. Both rate of absorption and total absorption are important, also change in dimensions. A certain amount of absorbed water may improve mechanical properties but usually weakens electrical characteristics. FOOTNOTES [1] A glossary of technical terms is included in appendix F. p. 160 of this report. There are, however, certain fundamental terms which it would be advisable to have clearly in mind at this point. [2] Shellac is a substance secreted by lac insects feeding on certain types of hardwood trees. [3] Journal of the Society of Chemical industry, 1901. Vol. 20, p. 1075.