The nature of the Arrhenius activation energy and frequency factor is reexamined in terms of information now becoming available on the microscopic aspects of collisional reactions. It is pointed out that the activation energy is not generally equal to the threshold for reaction, and its correct conceptual meaning is discussed. The temperature dependence of this quantity and its relation to the threshold energy is developed for a number of representative forms of the energy dependence of the reaction cross-section (excitation function). The uses and limitations of the activation energy as a means of evaluating thresholds, excitation functions, and the presence of tunneling processes are discussed.

Detailed studies with C11 atoms of high kinetic energy confirm that natural radio‐carbon initially reacts in the atmosphere to form CO. In accord with this, several samples of atmospheric CO show a specific activity much higher than would be expected from the predominantly fossil fuel origin of this gas.

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