Chemical Kinetics in the Past Few Decades.

The task before me is to speak of the evolution during the last few decades of the branch of physical chemistry which treats of the intimate mechanism of chemical reactions. The nature of the present occasion seems to call for a survey taken from a rather more personal point of view than is normally desirable or proper. Since the Swedish Academy of Sciences has done me the great honour of bringing me to Stockholm to receive their high award, it is perhaps suitable that I should give some account of the lines of thought and of the accidents which have determined the course of the work in which, with others, I have taken part. Nobody, I suppose, could devote many years to the study of chemical kinetics without being deeply conscious of the fascination of time and change: this is something that goes outside science into poetry; but science, subject to the rigid necessity of always seeking closer approximations to the truth, itself contains many poetical elements. There are many ways in which the mysteries of chemical change might begin to impress themselves on a young chemist. I will mention one: it belongs to practical life and is a real example. Explosives have to be tested for stability and this is done by measuring their decomposition at a suitably high temperature. Phenomena of great interest appear, revealing a remarkably large influence of temperature, a great acceleration of the rate if the solid liquefies, and various striking catalytic effects. Many major problems present themselves, which can be roughly summed up as those involving the dependence of chemical change upon the energy and the environment of the molecules. It was obvious that these problems offered many mysterious and fascinating aspects. In fact they prove to contain more than enough for a lifetime's study. In the early twenties one of the central questions in physical chemistry was that of unimolecular reactions, in which interest had been newly roused by the idea that such reactions were provoked by the absorption of radiation. A curious circumstance is that much theory was centred on the category of unimolecular reactions in the gas phase, of which in 1920 no authentic ex