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In a former paper (‘Phil. Trans.,' A, 1895, vol. 186, p. 657-693) we gave an account of some experiments on the occlusion of oxygen and hydrogen by platinum black, and pointed out that freshly-made platinum black invariably contains a considerable quantity of oxygen. Most of the specimens which we examined contained approximately 100 volumes, or 0.66 per cent, of oxygen, which could only be completely removed by heatingin vacuo at a dull red heat, and although a large fraction of the total oxygen can be extracted at about 400° C., the substance which remains behind is no longer platinum black but platinum sponge which has a much lower absorptive power for gases. When hydrogen is admitted to platinum black containingx volumes of oxygen, 2x volumes are converted into water, and the remainder of the gas absorbed, which, in general, is about 100 volumes, is really occluded. The heat evolved on the occlusion of hydrogen by platinum black has been measured by Berthelot (‘Ann. de Chim. et de Phys.,’ 1883, vol. 30, p. 519), and by Favre (‘Comptes Rend.,' vol. 77, p. 649, and vol. 78, p. 1257), but we have already (loc. cit ., p. 693) expressed ourselves as dissatisfied with the results they obtained, since the heat due to the combination of the oxygen pre-existing in the platinum black with the hydrogen is also included in these measurements. During our attempts to solve the problem whether the occlusion of gases by metals is a chemical or physical phenomenon, we have investigated the thermal changes which take place on the occlusion of hydrogen and oxygen by platinum black. Independent of the object for which the investigation was undertaken, the results we have obtained, which we now beg to lay before the Society, are of interest in connection with many electrical experiments where platinum or platinised electrodes are employed. The present communication also contains an account of the behaviour of several other gases towards platinum black, together with some speculations regarding the occlusion of oxygen.

VI. On the occlusion of oxygen and hydrogen by platinum black