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Features of the global cycle of fixed nitrogen are reviewed with an emphasis on perturbations due to man. It is argued that agricultural practices and combustion may lead to an increase in the concentration of atmospheric N 2 O with consequent effects on 0 3 . The level of 0 3 may drop by about 20 % over the next 100 years if world population and the demand for food should continue to grow at anything like rates which prevailed in the recent past. Uncertainties in the model are highlighted and note is taken of areas where there is need for additional data.

A non-resonance charge-transfer reaction for x -particles in hydrogen is investigated by the use of an expansion based on atomic eigenfunctions as in Bates (1958), full account being taken both of the non-orthogonality of the eigenfunctions of the initial and final atomic states and of the effect of distortion. Cross-section for the reaction have been computed for a range of incident particle energies from 50 to 1600 keV. The results indicate that the effect of distortion is considerable up to energies of 800 keV. Comparison is made with the results obtained w hen the effect of the non-orthogonality is ignored (with and without the inclusion of the nuclear-nuclear term in the interaction potential).

Cross-sections are calculated for the accidental resonance reaction, He 2+ + H(ls) -> He + (2s or 2p) + H + , and the non-resonance reaction, H + + H (ls)-> H(2s or 2p) + H + by means of the method due to Bates in which account is taken of the non-orthogonality of the wave functions describing the initial and final states. Proper allowance is made for the effects of distortion and of momentum transfer. The calculations are carried out for incident ion energies in the range 25 to 800 keV. In the accidental resonance case, the cross-section is small at low velocities of relative motion, and tends rapidly towards zero as the velocity is decreased in accordance with the prediction of Bates & Lynn. In all processes investigated the effect of distortion is considerable. Using the results of McCarroll & McElroy and of McCarroll for capture into the ground states of He + and H, the cross-sections for capture into all states are estimated. Comparisons are made with the experimental data of Fite, Smith & Stebbings for the incident alpha particle case and with that of Fite, Stebbings, Hummer & Brackman for the incident proton case. The highest energy for which cross-sections are measured in either case is however only 40 keV.