A Treatise on Meteorological Instruments Part 18

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 pa.s.sed 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 pota.s.sium, 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.

[Ill.u.s.tration: Fig. 92.]

=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 a.s.sumes, 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 a.s.signed 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 quant.i.ties present in the atmosphere may be detected and registered. For this purpose, an inch of ozone paper pa.s.ses in each hour, by clock-work, beneath an opening in the cover of the instrument.

CHAPTER XVI.

INSTRUMENTS NOT DESCRIBED IN THE PRECEDING CHAPTERS.

=145. Chemical Weather Gla.s.s.=--This curious instrument appears to have been invented more than a hundred years ago, but the original maker is not known. It is simply a gla.s.s vial about ten inches long and three quarters of an inch in diameter, which is nearly filled, and hermetically sealed, with the following mixture:--Two drachms of camphor, half a drachm of nitrate of pota.s.sium, half a drachm of chlorate of ammonium, dissolved in about two fluid ounces of absolute alcohol mixed with two ounces of distilled water. All the ingredients should be as pure as possible, and each vial filled separately. When the instruments are made in numbers and filled from a common mixture, some get more than the due proportion of the solid ingredients, and consequently such gla.s.ses do not exhibit that uniformity of appearance and changes, that undoubtedly should accompany similar influencing circ.u.mstances. It is in consequence of a want of precision and fixed principle of manufacture, that these interesting instruments are not properly appreciated, and more generally used.

The gla.s.s should be kept quite undisturbed, exposed to the north, and shaded from the sun. Camphor is soluble in alcohol, but not in water, while both water and alcohol have different solvent powers, according to the temperature; hence, the solid ingredients being in excess for certain conditions of solution, depending upon temperature chiefly, and perhaps electricity and the action of light also, appear as crystals and disappear with the various changes that occur in the weather.

The various appearances thus presented in the menstruum have been inferred to prognosticate atmospheric changes. The following rules have been deduced from careful study of the gla.s.s and weather:--

1. During cold weather, beautiful fern-like or feathery crystallization is developed at the top, and sometimes even throughout the liquid. This is the normal state of the gla.s.s during winter. The crystallization increases with the coldness; and if the structure grows downward, the cold will continue.

2. During warm and serene weather, the crystals dissolve, the upper and greater part of the liquid becoming perfectly clear. This is the normal state of the gla.s.s during summer. The less amount of crystallization, that is, the greater the clear portion of the liquid (for there is always some of the composition visible at the bottom), the greater the probability of continued fine dry weather.

3. When the upper portion is clear, and flakes of the composition rise to the top and aggregate, it is a sign of increasing wind and stormy weather.

4. In cold weather, if the top of the liquid becomes thick and cloudy, it denotes approaching rain.

5. In warm weather, if small crystals rise in the liquid, which still maintains its clearness, rain may be expected.

6. Sharpness in the points and features of the fern-like structure of the crystals, is a sign of fine weather; but when they begin to break up, and are badly defined, unsettled weather may be expected.

Admiral FitzRoy, in _The Weather Book_, writes of this instrument as follows:--"Since 1825, we have generally had some of these gla.s.ses, as curiosities rather than otherwise; for nothing certain could be made of their variations until lately, when it was fairly demonstrated that if fixed undisturbed in free air, not exposed to radiation, fire, or sun, but in the ordinary light of a well-ventilated room, or, _preferably_, in the outer air, the chemical mixture in a so-called storm-gla.s.s varies in character with the _direction_ of the wind--not its force, _specially_ (though it _may_ so vary in _appearance_, only from another cause, _electrical tension_).

"As the atmospheric current veers toward, comes from, or is only _approaching_ from the polar direction, this chemical mixture--if closely, even microscopically watched--is found to grow like _fir_, _yew_, fern leaves, or h.o.a.r-frost--or like crystallizations.

"As the wind, or great body of air, tends more from the _opposite_ quarter, the lines or spikes--all regular, hard, or crisp features--gradually diminish, till they vanish.

"Before, and in a continued southerly wind, the mixture sinks slowly downward in the vial, till it becomes shapeless, like melting white sugar.

"Before, or during the continuance of a northerly wind (polar current), the crystallizations are beautiful (if the mixture is correct, the gla.s.s a _fixture_, and duly _placed_); but the least motion of the liquid disturbs them.

"When the main currents meet, and turn _toward the west_, making _easterly_ winds, stars are more or less numerous, and the liquid dull, or less clear. When, and while they _combine by the west_, making westerly winds, the liquid is clear, and the crystallization well-defined, without loose stars.

"While _any hard_ or _crisp_ features are visible below, above, or at the top of the liquid (where they form for polar winds), there is _plus_ electricity in the air; a _mixture_ of polar current co-existing _in that locality_ with the opposite, or southerly.

"When nothing but soft, melting, sugary substance is seen, the atmospheric current (feeble or strong as it may be) is southerly with _minus_ electricity, unmixed with, and _uninfluenced_ by, the contrary wind.

"Repeated trials with a delicate galvanometer, applied to measure electric tension in the air, have proved these facts, which are now found useful for aiding, with the barometer and thermometer, in forecasting weather.

"Temperature affects the mixture much, but not solely; as many comparisons of winter with summer changes of temperature have fully proved.

"A confused appearance of the mixture, with flaky spots, or stars, in motion, and less clearness of the liquid, indicates south-easterly wind, probably strong to a gale.

"Clearness of the liquid, with more or less perfect crystallizations, accompanies a combination, or a contest, of the main currents, by the _west_, and very remarkable these differences are,--the results of these air currents acting on each other _from_ eastward, or from an entirely opposite direction, the _west_.

"The gla.s.s should be wiped clean now and then,--and once or twice a year the mixture should be disturbed, by inverting and gently shaking the gla.s.s vial."

[Ill.u.s.tration: Fig. 93.]

=146. Leslie's Differential Thermometer.=--A gla.s.s tube having a large bulb at each extremity, and bent twice at right angles, as represented in figure 93, containing strong sulphuric acid tinged with carmine, and supported at the centre by a wooden stand, const.i.tutes the differential thermometer as invented by Professor Leslie. The instrument is designed to exhibit and measure small differences of temperature. Each leg of the instrument is usually from three to six inches long, and the b.a.l.l.s are about four inches apart. The calibre of the legs is about 1/50 inch, not more; the other part of the tube may be wider. The tube is filled with the liquid, the bulbs contain air. When both bulbs are heated alike, each scale indicates zero. The scale is divided so that the s.p.a.ce between the freezing and the boiling-points of water is equal to 1,000 parts. When one bulb is heated more than the other, the difference of temperature is delicately shown by the descent of the coloured fluid from the heated ball. It is uninfluenced by changes in the temperature of the atmosphere; hence it is admirably adapted for experiments of radiant heat. The theory of the instrument is that gases expand equally for uniform increments of heat.

=147. Rumford's Differential Thermometer= differs from that just described in simply containing only a small bubble of liquid, which lies in the centre of the tube, when both bulbs are similarly influenced. The bulbs and other parts of the tube contain air. When one bulb is more heated than the other, the bubble moves towards the one less heated; and the scale attached to the horizontal part of the tube affords a measurement of the difference of temperature.

[Ill.u.s.tration: Fig. 94.]

=148. Glaisher's Thermometer Stand.=--The thermometer stand consists of a horizontal board as a base, of a vertical board projecting upwards from one edge of the horizontal one, and of two parallel inclined boards, separated from each other by blocks of three inches in thickness, connected at the top with the vertical, and at the bottom with the horizontal board, and the air pa.s.ses freely about and between them all. To the top of the inclined boards is connected a small projecting roof to prevent the rain falling on the bulbs of the instrument, which are carried on the face of the vertical board, with their bulbs projecting below it, so that the air plays freely on the bulbs from all sides. The whole frame revolves on an upright post firmly fixed to the ground, as shown in the engraving, fig. 94; and in use, the inclined side is always turned towards the sun.

=149. Thermometer Screen, for use at Sea.=--This screen, or shade, was designed by Admiral FitzRoy, and has been in use for several years on board H.M. vessels and many merchant-ships. It is about twenty-four inches long by twelve wide and eight deep; having lattice-work sides, door, and bottom; with perforation also at top, so contrived that the air has free access to the interior, while the direct rays of the sun, rain, and sea spray are effectually excluded from the thermometers mounted inside. There is ample s.p.a.ce for two thermometers placed side by side on brackets, at least three inches from each other or any part of the exterior of the screen. One thermometer should be fitted up as a "wet bulb" (see p. 105).

A small vessel of water can easily be fixed inside the screen so as to retain its position and contents under the usual motions of the ship; and by means of a piece of cotton-wick, or muslin rag tied round the bulb of the thermometer and trailing into the cup of water, keep the bulb constantly moist.