21. Oxygen, like caloric, is a fluid never found in an uncombined state. It forms one of the component parts of the air that we breathe, and of the water we drink; but it approaches nearest to a state of purity in combination with caloric ([18]), when it has the name of oxygen gas. It was formerly called vital air, because no breathing animal can live for a moment in any air or gas which has not in it a mixture of oxygen; every kind of combustible burns with great splendour in it, and without it ceases to burn. It unites with a great number of substances, and changes both their appearance and properties in a very remarkable manner. Of the metals it entirely destroys the metallic lustre, and gives them an earthy form and texture. Substances in this state have the name of oxides.
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.
26. Carbonic Acid is a compound of oxygen ([21]) and carbon, or pure charcoal ([48]): and in a state of gas ([20]) it forms a constituent part of the atmospheric air. It is also emitted in great abundance from wine, beer, and other liquors, in a state of fermentation, and is sometimes found in the lowest parts of mines, where it is known to the miners by the name of choke damp, from the circumstance of its immediately extinguishing flame, and suffocating all animals that are immersed in it. This gas, which was formerly called by chemists fixed air, is about twice the weight of common air. In combination with lime it forms chalk, marble, and limestone; and it constitutes part of several other mineral substances, which are thence denominated carbonats.
27. Fluoric Acid is a gas of very singular nature, which is held in combination with lime, in the Derbyshire or fluor spar ([194]); and may be separated from it by pouring sulphuric acid, or spirit of vitriol ([24]), upon powdered spar, in a leaden vessel called a retort, and applying to it a gentle heat. The salts formed by fluoric acid have the name of fluats.
28. The Boracic is a peculiar kind of acid, which, in combination with soda ([200]), forms the substance that we import from the East Indies under the name of borax ([204]). When extracted from borax this acid does not assume the form of a fluid, but appears in thin six-sided scales or flakes, of white colour, which adhere slightly together, and feel somewhat greasy in handling. To the taste it is at first sour, then bitterish; and at last it leaves an agreeable sweetness on the palate.