Red oxide and binoxide of lead also, when heated, either with or without sulphuric acid, yield oxygen gm, but the quantity obtained is small, and the processes are on several accounts ineligible. When nitrate of potash also is heated to redness, it gives out a considerable quantity of oxygen gas, from the decomposition of the nitric acid ; but as it comes over mixed at different periods of the operation with variable quantities of the nitrogen gas of the nitric acid, this is not a method usually resorted to.
The oxygen gas obtained from binoxide of manganese is sufficiently pure for all the usual purposes of experiments intended merely to illustrate the properties of the gas; but when the gas is required for chemical analysis or accurate investigations, it is then obtained from the salt formerly called oxymuriate of pot eh, but now chlorate of potash.
Chlorate of potash is composed of one equivalent of chloric acid 75'45, and one of potash and when heated in a retort, is resolved into an equivalent of chloride of potassium 74.45, which remains in the retort, and 6 equivalents of oxygen, 5 from the acid and 1 from the potash= 48, which measure 140 cubic inches ; so that there is scarcely any other substance which yields eo much oxygen gas or of so great purity.
The evolution of the oxygen is greatly facilitated by mixing with the chlorate of potash about one-sixth of Its weight either of binoxide of manganese or oxide of copper, and the process thus modified is now aimed exclusively used for the preparation of oxygen gas.
Oxygen possesses great power of combination with other elementary bodies, there being only one (fluorine) with which it is not known to combine, either by direct union or indirect chemical action. The com pounds to which it gives rise by combining with certain metals, and also indeed with some other bodies, may be classed under the three heads of oxides, acids, and alkalies. There are many bodies which, by a moderate degree of oxidation, become first oxides, and by an increased degree, acids; such substances are charcoal, phosphorus, chromium, &c. : but there is no instance of its forming with different proportions of the same element an acid and an alkali.
The properties of oxygen gas are, that it is devoid of colour, taste, or smell, and being transparent, it is also Invisible. It possesses the mechanical properties of common air ; it is capable of being respired, and a given volume of it will support life much longer than an equal bulk of common air; on this account the name of vital air was bestowed upon it. It is heavier than atmospheric air, 100 cubic inches at a
medium temperature and pressure weighing 34.203 grains, whereas an equal volume of atmospheric air weighs 3P074 grains. It is but slightly soluble in water, requiring about 20 times its bulk for solution. Light has no effect upon this gas; by heat, likeall gaseous bodies, it is merely expanded; and electricity is incapable of effecting any change in its properties. Oxygen gas has not been rendered either liquid or solid by the united agency of cold and pressure, and not having been divided into two or more kinds of matter, it is considered as elementary in its nature. Its equivalent, atomic) or combining weight, is 8.
It will be proper here to notice the production of oxygen gas by the action of electricity, especially voltaic electricity, which, as is well known, possesses the power of evolving both oxygen and hydrogen from water. Oxygen gas is also evolved by the action of the sun's rays on the moistened leaves of trees, which by this agency decompose the carbonic acid diffused in the atmosphere from various sources, and by combining it with their carbon, flourish and increase in size. When compounds containing oxygen are decomposed by voltaic electricity, the oxygen is evolved in the gaseous state at the anode or positive polo.
The most remarkable property of oxygen gas, and that on account of which it was once called Fire-air, is the facility and splendour with which bodies, when previously ignited, burn in it. Substances which do not undergo combustion in the air, will readily do so, and with great brilliancy, in oxygen gm ; iron for example burns very readily in it when previously made red-hot. The intensity of the light emitted by burning phosphorus in oxygen gas is such as to be scarcely tolerable.
In eouchision, we may remark, that until after the discovery of this gas nothing was or could be known respecting the nature of the air, of water, or of earth, all of which, formerly reckoned as elementary bodies, aro now known to be compound, and to contain oxygen as one of their constituents. It has also thrown great and unexpected light on the nature of combustion and respiration.
The compounds which oxygen forms with metals and other elementary bodies, are treated of in the respective articles on those elements, whether they are oxides, acids, or alkalies. The importance of the oxygen of the AIR as affecting RESPIRATION, VEGETATION, COMBUSTION, and EnraeCAUSIS, is fully enlarged upon under those words.