CHAPTER XV.
OZONE AND ITS INDICATORS.
=139. Nature of Ozone.=--During the action of a powerful electric machine, and in the decomposition of water by the voltaic battery, a peculiar odour is perceptible, which is considered to arise from the generation of a substance to which the term ozone has been given, on account of its having been first detected by smell, which, for a long time after its discovery, was its only known characteristic. A similar odour is evolved by the influence of phosphorus on moist air, and in other cases of slow combustion. It is also traceable, by the smell, in air,--where a flash of lightning has passed immediately before. Afterwards it was established that the same element possessed an oxidising property. It was found to be liberated at the oxygen electrode when water was decomposed by an electric current; and has been regarded by some chemists as what is termed an _allotropic_ form of oxygen, while others speak of it as oxygen in the _nascent_ state, and some even regard it as intimately related to chlorine. So various are the existing notions of the nature of this obscure agent.
Its oxidising property affords a ready means for its detection, even when the sense of smell completely fails. The methods of noting the presence and measuring the amount of ozone present in the air, are very simple; being the free exposure to the air, defended from rain and the direct rays of the sun, of prepared test-papers. There are two kinds of test-papers. One kind was invented by Dr. Schonbein, the original discoverer of ozone; and the other, which is more generally approved, by Dr. Moffat.
=140. Schonbein's Ozonometer= consists of strips of paper, previously saturated with a solution of starch and iodide of potassium, and dried. The papers are suspended in a box, or otherwise properly exposed to the air, for a given interval, as twenty-four hours. The presence of ozone is shown by the test-paper acquiring a purple tint when momentarily immersed in water. The amount is estimated by the depth of the tint, according to a scale of ten tints furnished for the purpose, which are distinguished by numbers from 1 to 10. The ozone decomposes the compound which iodine forms with hydrogen, and, it is presumed, combines as oxygen with hydrogen, while the iodine unites with the starch, giving the blue colour when moist.
=141. Dr. Moffat's Ozonometer= consists of papers prepared in a somewhat similar manner to Schonbein's; but they do not require immersion in water. The presence of ozone is shown by a brown tint, and the amount by the depth of tint according to a scale of ten tints, which is furnished with each box of the papers.
Moffat's have the advantage of preserving their tint for years, if kept in the dark, or between the leaves of a book; and are simpler to use.
=142. Sir James Clark's Ozone Cage= (fig. 92), consists of two cylinders of very fine wire gauze, one fitting into the other; the wire gauze being of such a fineness as to permit the free ingress of air, at the same time that it shuts out all light that would act injuriously on the test-paper, which is suspended by a clip or hook attached to the upper part of the inner cylinder.
=143. Distribution and Effects of Ozone.=--Mr. Glaisher has found that "the amount of ozone at stations of low elevation is small; at stations of high elevation, it is almost always present; and at other and intermediate stations, it is generally so. The presence and amount of ozone would seem to be graduated by the elevation, and to increase from the lowest to the highest ground. The amount of ozone is less in towns than in the open country at the same elevation; and less at inland than at sea-side stations." It seems to abound most with winds from the sea, and to be most prevalent where the air is considered the purest and most salubrious. This may seem, says Admiral FitzRoy, in _The Weather Book_, to point to a connection between ozone and chlorine gas, which is in and over sea-water, and which _must_ be brought by any wind that blows from the sea. It prevails more over the ocean and near it than over land, especially land remote from the sea; and, says the Admiral, it affects the gastric juice, improves digestion, and has a tanning effect. Dr. Daubeny, in his _Lectures on Climate_, writes: "Its presence must have a sensible influence upon the purity of the air, by removing from it foetid and injurious organic effluvia. It is also quite possible that ozone may play an important part in regulating the functions of the vegetable kingdom likewise; and although it would be premature at present to speculate upon its specific office, yet, for this reason alone, it may be well to note the fact of its frequency, in conjunction with the different phases which vegetation assumes, persuaded that no principle can be generally diffused throughout nature, as appears to be the case, with this, without having some important and appropriate use assigned for it to fulfil."
=144. Registering Ozonometer.=--Dr. E. Lancaster has contrived an ozonometer, the object of which is to secure the constant registration of ozone, so that the varying quantities present in the atmosphere may be detected and registered. For this purpose, an inch of ozone paper passes in each hour, by clock-work, beneath an opening in the cover of the instrument.