Oxides, a general term applied to the compounds of oxygen with other bodies, particularly the binary compounds with the other elements. Their number and variety are very great, for oxygen is the most widely diffused and abundant of all the elementary substances. Water is an oxide of hydrogen, and the geological formations are principally composed of various oxides of the metallic and non-metallic elements, as oxide of silicon or silica in quartz rock, oxide of iron in various iron ores, and oxides of aluminum and silicon in clay and feldspathic rocks. The oxides exist in all three of the physical forms of matter, the solid, liquid, and gaseous. The metallic oxides are solid at ordinary temperatures, and most of them retain this state at high temperatures. Oxide of hydrogen, water, is a liquid at the common pressure of the atmosphere between 212° and 32° F.; above 212° it has a gaseous, and below 32° a solid form. The oxides of carbon, carbon monoxide and carbon dioxide, commonly called carbonic oxide and carbonic-acid, are gases, the former never having been liquefied.

Carbonic acid has, however, been liquefied and frozen. (See Carbonic Acid, and Heat.) According to the proportion of the number of equivalents with which oxygen enters into union with other bodies, the oxides receive the names of monoxide or protoxide, dioxide (deutoxide or binoxide), teroxide or tritoxide, tetroxide, pentoxide, and also suboxide and peroxide, for lowest and highest oxides. The series of oxides of some of the elements are remarkable for regularity of composition, as the oxides of nitrogen and manganese. (See Nitrogen, and Manganese.) The oxides are conveniently divided into three principal groups, the first containing all those which resemble the oxides of potassium, sodium, silver, and the lower oxides of lead, and which are called basic oxides or bases. In the second 2Toup the oxides of sulphur and phosphorus may be taken as types. They are called acid oxides, and are capable of uniting with the basic oxides and forming compounds called salts. Thus when sulphuric oxide (anhydrous sulphuric acid, S03) is passed in a state of vapor over oxide of barium, BaO, combination takes place with evolution of heat and light, and sulphate of barium is formed containing all the elements of the original bodies (BaO + S03=BaS04 or BaO,S03). There is an intermediate group of oxides, called neutral oxides, because of their indifference to entering into combinations.

The black oxide, binoxide or dioxide of manganese, Mn02, is an example of this group, the monoxide of the same metal being basic and the higher oxides acid. The dioxide of lead (peroxide) may also be classed with these; for although it possesses feeble acid properties, and plumbates of the alkalies may be formed, they are decomposed by solution in water. Moreover, the dioxide of lead in the presence of acids is generally decomposed, with the formation of a salt of the monoxide. The three groups are not separated by very decided lines, although the well marked types are characteristic. They blend together upon their borders by imperceptible degrees, so that the same oxide will exhibit basic reactions toward one body, while it behaves like an acid toward another. In general it may be said that when oxygen combines in several proportions with a metallic element, the lower oxides are basic, while the higher ones have an acid character.