The chemical changes occurring in combustion hold such an important position, not only in the history and advance of chemical science, but also in the applications of science to industry, that special interest is attached to the discovery of the nature and order of the chemical changes involved. The phenomena presented by the oxidation of carbon, sulphur, and phosphorus have been studied by chemists as typical of those which are found in other processes of burning. With the object of determining the conditions necessary for the oxidation of these three substances, I began in 1884 an investigation, the results of which are described in the following paper. That water vapour might play an important part in such actions seemed very probable. The interesting facts brought to light by my former tutor, Professor H. B. Dixon, with regard to the oxidation of carbon monoxide (‘Phil. Trans./ 1884) made it seem likely that water vapour might exert as strong an influence on other combustions as he has shown it does on that of carbon monoxide. It was suspected some years ago that the combustion of carbon is affected by the presence or absence of moisture. In 1871 M. Dubrunfaut read a paper before the Academie des Sciences describing some experiments bearing on this point. He had performed combustions of sugar-charcoal in oxygen dried by strong sulphuric acid, and found that the carbon did not undergo combustion as readily as it did if the oxygen was moist. He ascribed the incompleteness of the oxidation of the carbon to the presence of moisture which was inaccessible to our reagents, that is, moisture which did not cause a weighed sulphuric acid tube through which it was passed, to increase in weight. A few weeks later M. Dumas, who had determined the equivalent of carbon by burning a weighed quantity of pure graphite in pure oxygen, repeated the experiments, and, by using a large quantity of the partially dried oxygen, succeeded in burning the whole of a small quantity of graphite. In my first experiments on the combustion of carbon in oxygen, wood charcoal was employed. It was freed from hydrogen by heating in a current of chlorine for several hours. It was placed in glass tubes, into which phosphorus pentoxide had been previously introduced, the two substances being separated by a disc of platinum foil. The latter did not fit so closely as to prevent the free diffusion of the gaseous contents of the tubes. Oxygen dried by sulphuric acid was then passed through the tubes, which were then sealed at both ends. Experiments have shown that the long contact of phosphorus pentoxide with gases has an immeasurably greater drying effect than the mere passage of the gases through tubes containing that substance. After standing for some days each of these tubes was heated, side by side with a similar tube containing charcoal in moist oxygen, by the flame of a large Bunsen burner. The moist carbon always burnt with the scintillation characteristic of such a combustion, but the- dry carbon remained apparently unaltered. On analysis, however, it was found that a certain quantity of carbon had been burnt. The following Table gives the results of the analysis of the gaseous contents of the tubes, after the carbon had been heated to redness in them for about two minutes.