is the term used in the new chemical nomenclature to express a very numerous class of bodies formed by the union of certain bases with a smaller proportion of oxygen than what is necessary for their conversion into acids. (See OXYGENE). The most remarkable of these bodies are what were formerly called metallic calces, and have for their base some metallic substance. It is in this state that metals are contained in their ores, from which they are extracted, and converted into the reguline or metallic form, by the process called reduction. Metals are converted into oxyds by combustion, and by solution in acids; and many of them assume this form from the action of the atmosphere alone, but more readily when this is assisted by moisture. During their conversion into oxyds, metals lose their splendor, and, acquiring a considerable increase of absolute weight, put on an earthy, pulverulent appearance. It has of late been supposed that all earths are metallic oxyds, and that all of them would be capable of reduction, were we possessed of any body for which oxygen had a stronger elective attraction than that by which it is kept in conjunction with the bases of these supposed oxyds. But this opinion, being perfectly unsupported by experiment, cannot be admitted in a science which, like the chemistry of the present day, aspires to demonstration.

The term oxyd, however, is not confined to the combinations of metals with oxygen, but expresses that first degree of oxygenation in all bodies which, without converting them into acids, causes them to approach the nature of salts; and of these there is a prodigious variety; as the oxyd of phosphorus, which is the white concrete substance into which that body is converted by combustion; the oxyd of azote, or nitrous air of Dr Priestley; and a great many others. Most of the oxyds from the vegetable and animal kingdoms have bases compounded of different simple combustible bodies. Thus sugar, all the gums, mucous, and starch, are vegetable oxyds; the bases of which are hydrogen and carbon, combined in various proportions. We find accordingly, that all these bodies are, by farther additions of oxygen, convertible into acids; and it is probable that these acids differ from each other only in the proportion of the hydrogen oxydation, and carbon in their bases. The bases of the animal oxyds are still more complicated; all, or most of them, consisting of various combinations of azote, phosphorus, hydrogen, carbon, and sulphur. See CALX, CHEMISTRY, and TABLE OF CHEMICAL NOMENCLATURE.

OXIDATION, is a term employed by the later chemists to express the process by which bodies are converted into oxyds; and it is allowed on all hands to be exactly similar to combustion. The nature of this process has been much disputed; and the question on this subject involves in itself great part of the controversy between the followers of the immortal Stahl and the justly celebrated Lavoisier, the founders of the phlogistic and antiphlogistic theories, which have for some years divided the chemical world. A view of this question, sufficiently distinct, may be taken from the case of metals and their oxyds. Metallic calces (oxyds say the phlogistians) are simple bodies, which, when united with phlogiston, form metals. The process of reduction consists in exposing the ores of metals to an intense heat in contact with some inflammable body, most commonly charcoal. During this operation, say they, the charcoal being inflamed, parts with its phlogiston, which is immediately absorbed by the calx, and a metal is formed. Lavoisier and his followers, on the contrary, contend that metals are simple bodies; but that in the state of oxyds, that is, as they commonly exist in their ores, they are combined with oxygen. But as oxygen at a high temperature is more strongly attracted by charcoal than by most metals, during the process of reduction the oxyd is decomposed, and the oxygen unites with the charcoal to form carbonic acid, leaving the regulus or metal free. On this point hinges the great question, the decision of which must materially affect almost every part of chemical theory. Without presuming to decide between these two opinions, the former of which is still supported by one or two chemists of the first rank, we agree with Dr Black in thinking that, though there still remain a few facts which have not been thoroughly explained on antiphlogistic principles, this theory is much more simple, and better supported by facts, than any that preceded it. It has this great advantage over the doctrine of Stahl, that it requires not the supposition of an arbitrary body, which does not affect our senses, and of the existence of which we have not even a shadow of proof. Perhaps we may farther venture to assert, that though it may be extremely difficult, or even impossible, to refute the phlogistic theory, influenced as we have all been by a strong prejudice in its favour; yet had it been brought forward for the first time, when our knowledge had arrived at the point which it now holds it never would have been generally received. See CALCINATION, CHEMISTRY, COMBUSTION, INFLAMMATION.