Ozone (Gr. ozō, 'I smell'). It was remarked long ago that a peculiar odour was produced by the working of an electrical machine. Van Marum found that, when electric sparks were passed through a tube containing oxygen, the gas became powerfully impregnated with this odour—which he therefore called the 'smell of electricity.' Subsequent writers attributed the phenomenon to the formation of nitric acid, due to a trace of nitrogen mixed with the oxygen; especially as the gas was found to act energetically upon mercury. Thus supposed to be explained, these curious results were soon forgotten. But in 1840 Schönbein (q.v.) with remarkable acuteness made a closer investigation of the question, and arrived at many most curious results, all of which have not even yet been satisfactorily accounted for. The problem remains, in fact, one of the most perplexing, as well as interesting, questions imperfectly resolved in chemistry. The earlier results of Schönbein were these: (1) When water is decomposed by the voltaic current, the electrodes being of gold or platinum, the oxygen (which appears at the positive pole) possesses in a high degree the smell and the oxidising power developed by Van Marum by means of friction-electricity. (2) When the positive electrode is formed of an oxidisable metal these results are not observed, but the electrode is rapidly oxidised. (3) The oxygen collected at a platinum electrode retains these properties for an indefinite period if kept in a closed vessel; but loses them by heating, by the contact of an oxidisable substance, and even by contact with such bodies as charcoal and oxide of manganese. To the substance, whatever it may be, which possesses such powerful chemical affinities, Schönbein gave the name ozone, from its smell. In 1845 he showed that the same substance can be produced by the action of phosphorus on moist air, and hinted that it might be a higher oxide of hydrogen.
De la Rive and Marignac shortly afterwards, repeating the experiments of Van Marum, showed that electric sparks produce ozone even in pure and dry oxygen, and came to the conclusion that ozone is oxygen in an allotropic state, as diamond is a form of coke or charcoal. Baumert, in 1853, endeavoured to show that there are two kinds of ozone—one formed from pure oxygen by electric sparks, which he allowed to be allotropic oxygen; the other formed in the voltaic decomposition of water, which he endeavoured to prove to be a teroxide of hydrogen. Andrews, in 1856, refuted this view, by showing that no such oxide of hydrogen (at least in a gaseous form) is produced in the electrolysis of water; and that ozone, from whatever source obtained, is the same body, and is not a compound, but an allotropic form of oxygen.
In 1860 Andrews and Tait published the results of a series of Volumetric experiments on this subject, which led to some remarkable conclusions—among which are the following: When the electric discharge is passed through pure oxygen it contracts, hence ozone must be denser than oxygen. A much greater amount of contraction, and a correspondingly greater quantity of ozone, are produced by a silent discharge of electricity between fine points than by a brilliant series of sparks. The contraction due to the formation of the ozone is entirely removed by the destruction of the ozone by heat; and this process can be repeated indefinitely on the same portion of oxygen.
Soret subsequently determined the density of ozone as compared with that of oxygen, first by absorbing the ozone from the oxygen with which it was mixed by means of oil of turpentine or oil of cinnamon, and observing the contraction produced; and later by determining the relative rates of diffusion of chlorine and ozone. He ascertained that its density is one and a half times that of oxygen. Andrews showed later that ozone is rapidly destroyed when shaken up with dry fragments of glass, &c. He also proved that the effect which is (almost invariably, and sometimes in fine weather powerfully) produced by the air on what are called ozone-test papers—papers steeped in iodide of potassium which are rendered brown by the liberation of iodine—is really due to ozone. He did so by showing that it acts upon mercury as ozone does, and that it is destroyed by heat at the same temperature.
The quantity of ozone in the atmosphere is never great, and it varies within wide limits. Little or nothing is known as to its function in the air, but it is believed to be active in destroying unwholesome substances, owing to its intensely oxidising properties. Ozone has been liquefied by the application of pressure, at a temperature of about . It is stated to be blue in the liquid state, and to be liable to decomposition into oxygen, with explosive violence, on sudden diminution of pressure.
P
is the sixteenth letter in our alphabet. The symbol was derived from the hieroglyphic picture of a shutter (see ALPHABET). When taken over by the Semites, the sign was called pe, 'the mouth,' a name explained by the fact that in the Egyptian hieratic, from which the Phoenician sign was obtained, there are strokes resembling teeth, which, however, disappeared before the date of the earliest extant Phoenician inscriptions. The oldest Greek form was ϣ, which differs little from the Phoenician letter. In the Latin alphabet the hook gradually became a loop, giving the form P, and this being the old form of R, the latter acquired a tail to distinguish it. In the later Greek alphabet P continued to be the sign for r, and the sign for p was differentiated by lengthening the hook, giving ultimately the form Π for the letter pi.
The sound of p is the sharp labial mute. Hence it interchanges with other labials, especially with b, the flat labial mute. Most languages give a preference to one of these two sounds. Thus, the Etruscans preferred p, and have no b in their alphabet, whereas the Teutonic languages dislike p, especially as an initial. Only six primitive Teutonic words, all probably loan words, begin with p, and in Beowulf and Cædmon, taken together, only three such words are found. In Mæso-Gothic the Greek p was used by Ulphilas, but only for foreign words, such as Paul, Pontius Pilate, prophet, and presbyter. Most of our English words beginning with p, such as plough, parish, people, or prince, are loan words from Greek, Latin, or Celtic. A primitive Aryan p corresponds to a Teutonic f, and it is only a primitive b, a very rare letter, which can correspond to a Teutonic p. A Welsh p corresponds to a Gaelic e and an English f. Thus, the Gaelic mae, 'son,' is the Welsh map or ap. The Gaelic eathair is the Welsh pedwar, and the English four; and the Gaelic coic is the Welsh pump, and the English five. Owing to French influence the English prejudice against p begins to disappear in the 13th century, and we get gossip instead of the older godsib, apricot for abricot, and purse for bourse, though even here the b is retained in the derived verb to disburse. A p also intrudes between m and t, as in empty for the Old English æmtig, and in tempt from the Old French tenter. In Latin p intrudes also between m and l, as in the words exemplum and templum. In like case, as in humble from humilis, b is usually the intrusive letter in English words.