Oxyd, a term, in the antiphlogistic system of chemistry, denoting those compound bodies which are formed by the decomposition of oxygen-gas, either by means of metals, or certain other substances.

All oxydes are the basis of some metallic bodies, the most remarkable of which, were formerly known under the name of calces (or magi-steries, if dissolved in acids), and have received their present appellation from the acidifying principle which they are believed to contain. Metals are converted into oxydes by combustion, and by solution in acids ; but, many of them acquire this form, by the action of the atmosphere alone; though they assume it with greater facility, when the latter is aided by moisture.- During the process of conversion into oxydes, metals are divested of their lustre ; and, after increasing considerably in weight, they exhibit an earthy appearance.—Speculative chemists have, therefore, lately conjectured, that all earths are mctallic oxydes, and that they sire all susceptible of reduction to a metallic state, provided there be any matter for which oxygen has more powerful elective attraction than that, by which it is kept in combination with the bases of such supposed oxydes. As this opinion, however, is unsupported by actual proof, it cannot be. admitted in the present system of chemistry.