The first part of the paper deals with the quantitative separation of gases by fractional adsorption, the theoretical conditions of which had to be investigated before a really accurate helium analysis could be undertaken. The second part describes the micro-method and apparatus used for the determination of the helium and neon contents of atmospheric air, and discusses questions affecting the accuracy of the results, especially with respect to the samples obtained from the stratosphere (see

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... ceding paper). Figures are given for the most reliable absolute values of the helium and neon contents of atmospheric air: 5-239 + 0-004 and 18-21 + 0-04 parts per million respectively. = I W - As long as the adsorbent is far from being saturated, the adsorption coefficients a and /? vary as a function of the temperature only, their ratio becoming more different from unity at lower temperatures. Therefore, if for experimental reasons a suitable 7-2 on July 20, 2018 http://rspa.royalsocietypublishing.org/ Downloaded from * For equal quantities A 0 and B 0 this distribution has the sm a states perm itted by equations (3a) and (36). a S---log ! A x ! B j !. D ifferentiation leads to equation (4).