Oxygen is perfectly quiescent and passive as a gas in the atmosphere, and as a constituent of water and solid bodies, yet that inactivity conceals the most intense energy, which only requires to be called into action. Thus combustion of extreme intensity takes place when ignited sulphur is put into a vessel containing oxygen gas; the metal potassium is instantly inflamed by it on touching water; some of its combinations with chlorine are highly explosive, and phosphorus burns in it with dazzling splendour. Thus a stupendous amount of energy is latent in oxygen under the most tranquil appearance.
M. Schönbein of Basle discovered that oxygen exists in another state, which has neither the extreme quiescence on the one hand, nor the intense violence on the other, of its ordinary form; and to express that intermediate condition, in which its activity is less in amount and different in quality, it has been called by another name, viz. ozone, from the following peculiarity.
It had long been observed that there is a peculiar smell when an electric machine is in activity, and when objects are struck by lightning; that smell Professor Schönbein ascertained to arise from the change of oxygen into ozone, and actually produced ozone by passing electric sparks through that gas. Ozone differs from oxygen in having a strong smell and powerful bleaching property; it purifies tainted air, changes vegetable colours, and stains starch prepared by iodide of potassium blue, which thus becomes a test of its presence; yet it certainly is oxygen in an allotropic or changed state, for it readily oxidizes or rusts silver and other metals, and when ozonized gas is sent through a red-hot tube, it comes out pure oxygen. According to the experiments of Messrs. Tait and Andrews, oxygen gas loses six eighths of its volume, and becomes four times more dense by the change; it contracts more readily with obscure electricity than with the spark. The experiments of Professor Tyndall on the absorption of radiant heat by gases give reason to believe that ozone is produced by the packing of the atoms of elementary oxygen into oscillating groups, and that heating dissolves the bond of union and restores the ozone to the form of oxygen. Ozone chiefly exists in air that has passed over a great expanse of sea, and the quantity is increased during the aurora, which alone might lead to a surmise of that phenomenon being electric.
The change of oxygen into ozone is not the only instance of Allotropism,—that is to say, the existence of the same substance in two states differing from each other in every respect,—for ozone itself is allotropic. Professor Schönbein has discovered that there are two kinds of ozone standing to one another in the relation of positively and negatively active oxygen; namely ozone and antozone, which neutralize each other into common oxygen when brought into contact. In this respect they are analogous to electricity, and, like electricity too, one kind cannot be produced without a simultaneous development of the other.
When a metal, such as silver for example, is oxidized or rusts, it gives polarity to the atoms of oxygen in the atmosphere and divides them into the opposite states of ozone and antozone; the ozone combines with the silver and rusts or oxidizes it, at the same time that the antozone is dissolved in the moisture or aqueous vapour in the air and forms peroxide of hydrogen. The oxidized or rusted silver, as well as every other oxidized substance, is an ozonide, while the peroxide of hydrogen is an antozonide.
Since both kinds of ozone are produced during the decomposition of water by electricity, and as sea air is always found to contain more or less free ozone, the ocean is probably an antozonide, for all the antozone formed by electricity during thunderstorms must be either dissolved in the sea-water, or carried into it in the form of peroxide of hydrogen by the rain. Ozone must be exceedingly abundant in the zone of calms and light breezes near the equator known as the variables, which is subject to heavy rains and violent thunderstorms, and also in the regions of the monsoons. On land one of the benefits arising from these formidable phenomena is the production of ozone, which oxidizes decomposing organic matter and hastens its decay, while the antozone, which is dissolved in the atmospheric vapour, forms the peroxide of hydrogen and frees the air from the antagonist principle.
The peroxide of hydrogen thus produced is a transparent colourless inodorous liquid with a metallic taste, and contains one equivalent of hydrogen and two of oxygen. It retains its liquid state under a great degree of cold, and mixes with water in any proportion. It has a strong bleaching property, instantly destroying vegetable colour. If exposed suddenly to a temperature of boiling water it is decomposed with violent explosion, and readily gives off oxygen at 59° Fahr. The mere touch of an oxidized metal, as the oxide of silver, completely and instantaneously decomposes it, and oxygen gas is evolved by the union of the ozone and antozone so rapidly as to produce a kind of explosion attended by an intense evolution of heat.
During the combustion of phosphorus in the atmosphere both kinds of ozone appear, and Professor Schönbein considers the slow combustion of that substance, which unites with the ozone and sets the antozone free, as the type of all the slow oxidations which organic and inorganic bodies undergo in moist atmospheric air; that true oxidation is always preceded by the appearance of the peroxide of hydrogen, and that this compound acts an important part in slow oxidations, and is deeply concerned in animal respiration, and in many other chemical actions going on in nature.
In confirmation of these views, it is certain that ozone is a powerful minister in the work of decay. If wood be made explosive like gun-cotton by a similar process, it becomes pulverulent after a time, and burns without exploding, though it still retains its shape. In the natural state of the wood the oxygen is passive and quiescent, for oxygen is a constituent of wood; in its second state it is explosive, and after a time that is succeeded by the semi-active state of ozone, which by a slow imperceptible combustion causes the wood to decay. Mr. Faraday observes that the force which would have been explosive had it been concentrated into one effort, expends itself in a long continued progressive change.
‘The majestic phenomena of combustion bespeak our admiration and rivet our attention because of their imposing grandeur; yet these are but spasmodic efforts in the grand economy of the material world, occurrences of now and then. The slower but continuous progress of the elements to their appointed resting-place, the silent, tranquil, ever progressing metamorphic changes involved in the phenomena of decomposition and decay, these we count for nothing and pass unheeded by. Yet with all their majesty, with all their brilliancy, all their development of tremendous energy, what are the phenomena of combustion in the grand scheme of the universe compared with these? When the loud crash of the thunder or the lightning’s flash awakens us from our thoughtless abstractions or our reveries, our feelings become impressed with the grandeur of Omnipotence and the might of the elements he wields, yet the whole fury of the thunderstorm—what is that in comparison with the electric energies which silently and continually exert themselves in every chemical change? Why, the electric force in a single drop of water, and disturbed when that water is decomposed, is of itself greater than in the electricity of a whole thunderstorm. Those of us who limit our appreciation of the powers of oxygen to the energies displayed by this element in its feebly active state, form but a very inadequate idea of the aggregate results accomplished by it in the economy of the world.’ Oxygen is the only known gas that is allotropic, and is the only known substance that is doubly allotropic, that is existing in three different states similar to oxygen, ozone, and antozone.