From the agricultural point of view the amount of oxygen carried into the soil as gas dissolved in rain has considerable interest. Since all the oxygen required either for the bacterial activities of the soil or for the root aeration of growing crops must first pass into the dissolved state, the supply of this essential element in a form immediately available must exercise a proportionately beneficial effect on the manufacture of plant food or on the growing crop. Recent work on the decomposition of organic matter in the soil has shown that rainfall is one of the chief factors controlling bacterial activity, but that its influence is due to something more than the supply of water. The oxygen dissolved in the rain appears to be the missing factor. As very little systematic work seems to have been done in this direction, estimations of dissolved oxygen in rain at Rothamsted were made from time to time during the year 1915.