Oxygen is certainly the most abundant element in nature. It exists all around us, and the animal and vegetable worlds are dependent upon it. It constitutes in combination about one-half of the crust of the earth, and composes eight-ninths of its weight of water. It is a gas without taste or colour. Oxygen was discovered by Priestley and Scheele, in 1774, independently of each other.
Fig. 333.—Oxygen from oxide of mercury.
Oxygen can be procured from the oxides of the metals, particularly from gold, silver, and platinum. The noble metals are reducible from their oxides by heat, and this fact assists us at once. If we heat chlorate of potash, mixed with binoxide of manganese, in a retort in a furnace, the gas will be given off. There are many other ways of obtaining oxygen, and we illustrate two (figs. 333, 335).
The red oxide of mercury will very readily evolve oxygen, and if we heat a small quantity of the compound in a retort as per illustration (fig. 333) we shall get the gas. In a basin of water we place a tube test-glass, and the gas from the retort will pass over and collect in the test tube, driving out the water.
Fig. 334.—Showing retort placed in furnace.
The other method mentioned above,—viz., by heating chlorate of potash, etc., in a furnace, is shown in the following illustration. Oxygen, as we have said, is a colourless and inodorous gas, and for a long time it could not be obtained in any other form; but lately both oxygen and hydrogen have been liquified under tremendous pressure at a very low temperature. Oxygen causes any red-hot substance plunged into it to burn brightly; a match will readily inflame if a spark be remaining, while phosphorus is exceedingly brilliant, and these appearances, with many others equally striking, are caused by the affinity for those substances possessed by the gas. Combustion is merely oxidation, just as the process of rusting is, only in the latter case the action is so slow that no sensible heat is produced. But when an aggregate of slowly oxidising masses are heaped together, heat is generated, and at length bursts into flame. This phenomenon is called “spontaneous combustion.” Cases have been known in which the gases developed in the human body by the abuse of alcoholic drinks have ended fatally; in like manner the body being completely charred. (Combustion must not be confounded with ignition, as in the electric light.) Oxygen then, we see, is a great supporter of combustion, though not a combustible itself as coal is. When the chemical union of oxygen with another substance is very rapid an explosion takes place.
Fig. 335.—The generation of oxygen from oxide of manganese and potash.