however so strong an attraction for caloric in this state, that they will not part with it in the temperature of our atmosphere, or even at many degrees below our ordinary temperature. Caloric combines readily with many of the simple substances.-With oxygen it forms oxygen gas-With hydrogen, hydrogen gas-With nitrogen, nitrogen or azotic gas- With sulphur, sulphureous gas. The metals and many of the earths are reduced by it to a fluid state. It may then be laid down as a canon of chemistry, That all fluids, whether incompressible or gaseous, are combinations of caloric with some other substance simple or compound. II. OXYGEN is a matter which is never found in an uncombined state. It approaches nearest to purity in the state of oxygen gas (or combined only with caloric), which was formerly termed pure, vital, or empyreal air, from its property of sustaining animal life and combustion. Oxygen gas may be obtained by various easy processes. It may be procured by means of heat from all the oxides (or, as they were formerly called, calces) of metals; for it is the union of oxygen with the metals that reduces them to this calx, or cinder-like appearance. Thus red lead is an oxide or calx of lead produced by heating lead to a considerable degree in the open air, by which it attracts a quantity of oxygen from the atmosphere. The oxygen may again be extracted from the lead in the form of gas. To effect this, all that is necessary is to have a tub or vessel of water, with a shelf in it, on which a receiver may rest; a large tumbler will serve for a receiver *. The tumbler must then be filled with water, and be reversed with its brim under water, and resting on the shelf. Put then some of the red lead with a small quantity of sulphuric acid into a phial bottle in which is fixed airtight a bent tube†, which by an orifice in the wooden shelf may communicate with the receiver or tumblert. Apply heat to the bottom of the phial, (that of a wax-taper or common candle will be sufficient) and after the common air which the bottle contained is expelled, the oxygen gas will rise in bubbles and fill the receiver or tumbler, displacing the water as it enters. The oxygen gas thus obtained is not the purest, for it contains some carbonic acid gas, which, however, may be extracted by bringing the mouth of the receiver in contact with some lime-water in a bason, and slightly agitating it, when the carbonic acid gas will be extracted by the attraction which the lime-water has for that substance. Oxygen gas may be obtained in a still purer state by heating a small portion of the black oxide of manganese in a similar apparatus, only that as a stronger heat is required, the vessel containing the oxide should be of iron. * Pl. VI. fig. 19. ↑ The tube may be inserted into the cork. As it is possible I may not have explained myself with sufficient clearness in describing these experiments, and as some may be desirous of a more perfect apparatus, I shall beg leave to refer to Plate VI. fig. 19, where A represents the vessel or tub (sometimes filled with water, and sometimes with mercury); K K K the wooden shelf; B, G, F, are glass jars or receivers, inverted with their mouths downwards, and resting on the shelf. C is a glass bottle, which is supposed to contain the ingredients for furnishing the gas, and to the bottom of which heat is applied. D is a glass tube, generally fitted by grinding to the neck of the bottle, and curved so as to enter conveniently below the shelf, and communicate with the jar or receiver. E is a glass retort, which may be applied to the same purpose. Since oxygen is one of the component parts of water, any process that will decompose water will furnish it in the form of gas. Pump water, when exposed to the sun, will emit it in small quantities; and as all vegetables in a growing state decompose water, when there is a green slime (which is a vegetable matter) formed in water, in a bottle, &c. a quantity of oxygen gas will be separated when the water is exposed to the sun. Raw silk, and even very fine glass tubes, immersed in water, will also decompose it by means of the sun's heat, and produce oxygen gas. Oxygen gas forms a considerable part of the air of our atmosphere, (about 23 parts, in weight, in 100,) the rest is nitrogen, or azotic gas, which is not respirable, and will not support flame. Oxygen is necessary also to animal life. Being respired it unites with and removes the carbon of the venous blood; that which returns from the lungs to the heart, thereby having acquired a brighter colour. It has been supposed that during respiration, oxygen becoming condensed, parts with its caloric, and furnishes us with animal heat; and that this is proved by every person who uses laborious exercise breathing quick, and becoming proportionably heated. But the real origin of animal heat is not at present ascertained. It supports combustion. An iron wire, after being a little heated, and plunged in a vessel containing oxygen gas, will burn and flame like a candle. The oxygen, in this case, unites with the metal, and converts it into an oxide, or calx as it was formerly called, from its being produced by burning, and resembling a calx or cinder. Oxygen has indeed a strong attraction for all the simple acidifiable and combustible substances, hydrogen, nitrogen, sulphur, phosphorus, carbon, and boron, forming different compounds. With hydrogen it forms water; with nitrogen atmospheric air; with sulphur, sulphuric acid; with phosphorus, phosphoric acid; with carbon, carbonic acid gas, formerly called fixed air, and with boron, boracic acid, It is usually in the state of gas that oxygen enters into combination with these matters. Combustion is generally the result of this union; for as in most of these cases the oxygen gas is condensed, the caloric which retained it in the gaseous state is set free, and the evolution of heat and flame takes place. To effect this union, the combustible bodies must be reduced to their ultimate or minutest particles. This is done by the application of heat, which also expands or removes the particles further from the sphere of each other's attraction, whence their union with oxygen gas is facilitated. Hence flame has been called ignited vapour; that is, it is the combustible body reduced to the form of vapour, and in that state combining with oxygen gas. But one of the most extraordinary and important properties of oxygen is, that from which it derives its name. Oxus, in Greek, means sharp or acid; this substance, therefore, having been considered as that which imparts the acid character to those bodies which are denominated acids, was called oxygen. Thus sulphur in its primitive state is one of the mildest substances in nature, but in combination with oxygen it forms that corrosive and dangerous fluid called sulphuric acid, formerly oil or spirit of vitriol. It is not, however, to every substance that oxygen imparts the acid character; for instance, with hydrogen it forms only water, which has none of the properties of an acid. The only |