OXIDE, in chemistry. Metallic sub stances are not only of vast importance in the arts of civilized life, on account of the properties which belong to them in the metallic state ; but many of them are not less valuable in those changes which they undergo by new combinations, and the new properties they acquire, in con sequence of these changes. One of the first and most ordinary changes to which metallic substances are subject, is their combination with oxygen. This is called, in chemical language, oxydation. If a metal, as for instance, a piece of iron, is exposed to the air, when it is moist, it soon undergoes a remarkable change. It loses its metallic lustre, and the surface is covered with a brownish powder, well known by the name of rust. This change is owing to the combination of oxygen with the metal, and the rust of the metal in this state is known in che mistry by the name of oxide. The pro cess by which this compound of oxygen and a metallic substance is fbrmed, is called oxydation, and the product is de nominated an oxide. The process of oxy dation is effected more rapidly when metals are exposed to the action of heat ; and, indeed, many metals require a very high temperature to produce the combina tion, while it cannot be accomplished in others by the greatest degree of heat that can be produced. This process was for merly called calcination, or calcining the metal ; and the product now denominated an oxide, was distinguished by the name Of calx or calces, from its being reduced to the state of powder, in the same way as limestone, by burning. Metals differ very much from each other in the cir cumstances in which this oxydation takes place, as in the temperature which is necessary, the facility of the combination, the proportions of oxygen which combine, and the force of affinity between the con stituent parts of the oxide. Some metals are oxydated in the lowest temperature, as, for instance, iron and manganese ; while others require the greatest degree of heat that can be applied. Such are silver, gold, and platina.
The facility with which oxydation takes place in some metals is so great, such as in iron, tin, lead, copper, and manganese, that they must be completely defended from the action of oxygen; but in gold and platina, no perceptible change is ob served, for whatever length of time they are exposed to the atmosphere. This oxydation, and the quantity of oxygen absorbed, is proportionable to the tempe rature. 'there are, however, many metals which combine with a determinate pro portion of oxygen at certain tempera tures, and from this may be estimated the quantity or oxydation from the degree of heat which has been applied. The rapidi ty of the oxydation is almost always in creased by the elevation of temperature. In this way actual combustion or inflam mation is produced. Thus filings of metals thrown upon a body in a state of ignition, give out brilliant sparks ; and steel, struck upon a flint, burns with a vivid flame in the air, in consequence of the great heat which is communicated to it by percussion. Metallic substances
combine with very different proportions of oxygen ; and this quantity varies ac cording to the manner in which the pro cess has been conducted, or the tempe rature to which the metal has been ex posed.
In these different states and conditions of oxydation different phenomena are ex hibited. Sometimes the metal becomes red hot, and is inflamed ; sometimes the oxygen takes place without fusion, or does not combine with oxygen till after it has been melted; sometimes it is co vered with a brittle crust, or with a sub stance in the form of powder. At other times a pelicle, exhibiting different co lours, forms on the surface : but, in all cases, the metal is tarnished, loses its bril liancy and its colour, and assumes another; which announces the change that has taken place. Another difference which takes place among metals, is the different de grees of force with which the oxygen ad heres to the metal. The knowledge of this, and the different degrees of affinity between oxygen and metallic substances, is of great importance in many operations and.' chemical results. During the fixa tion of oxygen in metallic substances, it is absorbed by some in its solid state, and gives out a great deal of caloric. In others it is combined, without giving out the same quantity. This proportion of caloric given out corresponds to the facility with which oxides part with their oxygen, or are reduced to the metallic state. Those which have combined with oxygen, with the greater proportion of caloric, are most easily reduced ; but those, on the contrary, in which the oxy gen has been deprived of its caloric, are reduced to the metallic state by a great addition of caloric, and the greatest num ber of oxides require the addition of sub stances, whose affinity for oxygen is greater than that of the metal. Metallic oxides are extremely different in different metals, and even in the same metal, ac cording to the proportion of oxygen. They are, however, possessed of some common properties. They are all in the form of powder or earthy substance, or so brittle as to be easily reduced to this state. They exhibit every shade of co lour, from pure white to brown and deep red, and they are heavier than the metals from which they have been obtained. Some oxides are revived, as it is called, or are reduced to the metallic state, merely by being in contact with light or caloric. Some require the addition of a combustible substance and a high tem perature; while others have so strong an affinity for oxygen, that they cannot be deprived of' it by the strongest heat, but become fusible in the fire, and afford a glassy matter more or less coloured, and even serve as a flux to the earths. Some oxides are volatile, but the greatest num ber are fixed. Some have an acrid and caustic taste, are more or less soluble in in water, and even possess an acid quali ty ; others are insoluble and insipid.