Lead, for instance, combined with oxygen becomes the well─known red and heavy substance used by painters, under the name of minium or red lead (239). This, if deprived of its oxygen, loses its red colour, and returns to its former metallic state. Some of the metals are oxidized by merely being exposed to moisture. Thus the rust which is so readily contracted by iron is an oxide of that metal, produced by its attracting oxygen from the air or from water. ACIDS. 22. It is one of the most remarkable properties of oxygen to impart to most of those bodies called _acids_ their peculiar character of acidity. Oxygen does not itself possess the properties of an acid, nor is it an essential ingredient in all acids, though it is the acidifying principle in the greater number of them. 23. _Acid_ is a word originally synonymous with sour. It has, however, been gradually extended in its signification, and now comprehends all substances possessed of the properties of exciting upon the tongue the sensation called sour; of changing the blue colours of vegetables to red; of uniting with water in almost any proportion; of combining with alkalies (42), metallic oxides, and earths, and of forming with them certain compounds called _salts_. 24. _Sulphuric Acid_, or _Spirit of Vitriol_, as it is commonly called, is a liquid of a somewhat oily consistence, transparent and colourless as water, formed by a combination of oxygen (21) with sulphur (46). Like other acids, it never occurs in nature in a pure state, for it can no sooner be formed than it unites with earths (31), alkalies (42), or metals, and forms, with them, several well─known salts, which have the name of _sulphats_. Thus alabaster (192) and Epsom salts (199) are respectively formed by an union of sulphuric acid with lime and magnesia, and are denominated by chemists sulphat of lime and sulphat of magnesia. In like manner, blue vitriol (209) is sulphat of copper; green vitriol (208), sulphat of iron; and white vitriol (210), sulphat of zinc. 25. _Phosphoric Acid_ is produced by a combination of oxygen (21) with phosphorus (47); and, when obtained in a state of purity, is not a fluid, but a white and flaky substance. This acid, when combined with mineral productions, forms those salts which have the name of _phosphats_. It is very soluble in water; and, in dissolving, makes a hissing noise, similar to that produced by plunging hot iron into water.