sulphur with oxygen formed not directly but by the addition of a fresh quantity of oxygen to sulphurous anhydride, SO 2 , by passing it together with oxygen over incandescent spongy platinum. Carbonic anhydride (often termed ‘carbonic acid’), CO 2 , phosphoric anhydride, sulphurous anhydride, are all acid oxides, for they can combine with such oxides as lime or calcium oxide, magnesia or magnesium oxide, MgO, soda or sodium oxide, Na 2 O, &c.
If a given element form but one basic oxide, it is termed the oxide; for example, calcium oxide, magnesium oxide, potassium oxide. Some indifferent oxides are also called ‘oxides’ if they have not the properties of peroxides, and at the same time do not show the properties of acid anhydrides—for example, carbonic oxide, of which mention has already been made.
If an element forms two basic oxides (or two indifferent oxides not having the characteristics of a peroxide) then that of the lower degree of oxidation is called a suboxide—that is, suboxides contain less oxygen than oxides. Thus, when copper is heated to redness in a furnace it increases in weight and absorbs oxygen, until for 63 parts of copper there is absorbed not more than 8 parts of oxygen by weight, forming a red mass, which is suboxide of copper; but if the roasting be prolonged, and the draught of air increased, 63 parts of copper absorb 16 parts of oxygen, and form black oxide of copper.
Sometimes to distinguish between the degrees of oxidation a change of suffix is made in the oxidised element, -ic oxide denoting the higher degree of oxidation, and -ous oxide the lower degree. Thus ferrous oxide and ferric oxide are the same as suboxide of iron and oxide of iron.
If an element forms one anhydride only, then it is named by an adjective formed from the name of the element made to end in -ic and the word anhydride. When an element forms two anhydrides, then the suffixes -ous and -ic are used to distinguish them: -ous signifying less oxygen than -ic; for example, sulphurous and sulphuric anhydrides.52
When several oxides are formed from the same element, the prefixes mon, di, tri, tetra are used, thus: chlorine monoxide, chlorine dioxide, chlorine trioxide, and chlorine tetroxide or chloric anhydride.
The oxides themselves rarely undergo chemical transformations, and in the few cases where they are subject to such changes a particularly important part is played by their combinations with water. The majority of, if not all, basic and acid oxides combine with water, either by a direct or an indirect method forming hydrates—that is, compounds which split up into water and an oxide of the same kind only. It is well known that many substances are capable of combining with water. Oxides possess this property in the highest degree. We have already seen examples of this (Chapter I.) in the combination of lime, and of sulphuric and phosphoric anhydrides, with water. The resulting combinations are basic and acid hydrates. Acid hydrates are called acids because they have an acid taste when dissolved in water (or saliva), for then only can they act on the palate. Vinegar, for example, has an acid taste because it contains acetic acid dissolved in water. Sulphuric acid, to which we have frequently referred, because it is the acid of the greatest importance both in practical chemistry and for its technical applications, is really a hydrate formed by the combination of sulphuric anhydride with water. Besides