THE thirty-sixth Bedson Lecture was delivered by Prof. M. Polanyi, of the University of Manchester, on February 1, in Armstrong College, Newcastle-on-Tyne. Prof. Polanyi took as his subject, “The Transition State in Chemical Reactions”, and illustrated his remarks with particular reference to the ozone-oxygen reaction. Molecules are closely-packed groups linked by chemical bonds and separated by wider gaps from other molecules. The course of a chemical reaction between the molecules involves the breaking of these original chemical bonds, with the formation of new ones. During this change, the gaps between the initial molecules disappear and new ones appear between the final molecules, and between these two states there must be an infinite number of atomic configurations. At some point between the initial and final gaps separating the molecules, there is an intermediate state when no distinction can be drawn between the initial and final states. This is defined as the ‘transition state of the reaction’. Turning from the purely geometric to the energy conception of the transition state, Prof. Polanyi demonstrated how the potential energy in the reaction between the molecules rises to a maximum. The position of the maximum indicates the transition state, and further, the energy of the reaction must be sufficient to surmount this energy barrier. Hence the plotting of the energy changes occurring in the reaction of two particles will indicate the transition state and the energy of activation of the reaction. Prof. Polanyi traced the evaluation of the energy changes and showed how it was possible to determine the heat of activation by application of wave mechanics and molecular constants. He also indicated how various properties of the transition state determine the changes in reaction rate caused by varying conditions such as hydrostatic pressure, change of solvent, etc.