DR. L. R. G. TRELOAR. delivered a discourse at the Royal Institution on December 15, speaking on "Rubbers and their Characteristics: Real and Ideal". He pointed out that although rubbers are diverse in chemical constitution, they all conform to a general type of molecular structure, and it is in this structure that the origin of the remarkable physical properties of rubbers is to be found. All rubbers are built up of enormously long chain-like molecules which are linked together so as to form a loose three-dimensional network. The atoms of the molecular chain are in a state of continuous motion, due to their thermal energy; hence the molecule tends to take up a randomly-kinked, continuously fluctuating form, in which its effective length is only a small fraction of the full chain-length. As a result, the molecule exhibits elasticity. The elasticity of rubbers, like that of gases, is thus kinetic in origin. Rubber-like elasticity is always limited to a certain range of temperature, varying with the chemical constitution of the molecule. At low temperature, rubbers are transformed to a glass-hard condition, whereas at high temperatures they tend to lose strength and to approach the condition of a highly viscous liquid. In some rubbers, also, a crystalline state is possible. Crystallization develops slowly in natural rubber at low temperatures, but it may be produced almost instantaneously by stretching at normal temperatures. Crystallization profoundly affects the physical properties of rubber, and its study has had an important bearing on the elucidation of its molecular structure.