Mr. Lowe seems to have proved that ozone is always present in larger or smaller amounts, and Mr. Smyth maintained that the variation in quantity is trifling. In this latter conclusion, however, Mr. Smyth appears to have been mistaken, since it has been found to be present in very small relative amounts when northerly winds prevail. Mr. Smyth's mistake probably arose from the great velocity of the current of air which he passed over his iodized starch test-papers-a circumstance which, as we shall see in the sequel, materially affects the result. Its amount is likewise small when the air is much contaminated with the products of decomposing organic matter, but in such circumstances errors may have crept into the calculations owing to the ozone having been absorbed as soon as it was formed. From the results of observations in seventy different localities we find that, in a majority of cases, the maximum occurs in February and May, and the minimum in July and October. Prestel maintains that there are two seasons of maximum intensity, a greater and a less, and two corresponding minima. The greater maximum and lower minimum occur respectively about the vernal equinox and summer solstice; the lower maximum and higher minimum about the autumnal equinox and winter solstice. Reslhulber thinks the conditions producing a maximum are low temperature and barometric pressure, damp atmosphere, dull overcast sky, and much snow, while a minimum is produced by a warm temperature with mean barometric pressure, clear sky, dry atmosphere, and thunderstorms. The Rouen observations give the mean of the first three months of the year as 22, that of the next three as 56, the next three as 37, and the last three as 19. But this summer maximum seems to be explained by the presence of large forests in the immediate neighbourhood, the ozone-producing effects of which are necessarily greater in summer owing to the increased activity of the vegetative processes during that period-an explanation which probably also applies to Versailles and such other places as exhibit a summer instead of a spring or winter maximum. For the majority of observers have found ozone more abundant in winter than

in summer; perhaps they failed to make sufficient allowance for the fact that a larger quantity is consumed during the hot season, owing to the increased amount of organic impurities in the air. Scoutetten, moreover, asserts that the heat in summer raises the ozone to the upper regions of the air, but it is difficult to see why it should be elevated in larger proportion than the other gases. There seems little doubt that ozone is more abundant by night than by day, the greater and lesser maxima occurring at sunrise and sunset respectively (owing, perhaps, to the precipitation of dew at these periods), and the minimum about noon. Regarding the interval between 9 a.m. aud 9 p.m. as day, and the remaining twelve hours as night, the amount of ozone in the former is to that in the latter in the proportion of 183 to 286.

It might à priori be expected that in foggy weather much electricity and ozone would be produced from the mutual friction of the particles of water suspended in the atmosphere, but in the present state of our knowledge we cannot positively assert this to be the case. The conditions determining the production of electricity and ozone appear to be identical; since both reach their maximum with a low temperature, moderate degree of humidity, in elevated localities, and at sunrise and sunset. Another striking point of coincidence is that cloth will not dye when no ozone is present, and the effect of thunderstorms in favouring the action of dyers' mordants is well known. Thunderstorms, halos, aurora, hail, and snow cause an increase of ozone when the barometer is falling, but according to Moffat the amount is diminished when the glass is rising. This latter statement, however, requires confirmation.

Much difference of opinion prevails as to the effect of the direction and force of winds in the production of ozone. Dr. Moffat endeavours to establish the simultaneity of the periods of maximum and minimum ozone production with those of the greatest and least intensity of phosphoric luminosity; i. e., with the times when the southern and northern winds respectively prevail. A majority of observa

tions assigns the maximum to west and south-west winds, yet at Rouen we have it with north-east, perhaps owing to the presence of forests in that direction. The truth seems

to be that the observations at any particular place are so materially influenced by the nature of the surrounding country, the direction in which the sea lies, the proximity of forests, or, on the other hand, of marshes or putrescent organic matter, in any special quarter, that it is only by comparing with the minutest care a series of observations made in a very large number of different localities, paying strict regard the while to the circumstances just mentioned and many others, that we can hope to come to any satisfactory conclusion. Bérigny goes so far as to assert that the force and direction of the wind are, in themselves, quite unimportant; whereas Prestel, probably with greater justice, thinks both these circumstances of consequence, but especially the latter.

We learn from the observations of Bérigny and Decharmes that the quantity of ozone present in the atmosphere varies directly as the cloudiness of the sky; a maximum is reached with cirro-strati and a minimum with cirro-cumuli; and the higher the clouds the greater the amount of ozone. The quantity of ozone, moreover, varies inversely as the barometric pressure, as might have been anticipated from the fact that a low barometric pressure coincides with the occurrence of rain or storms, both of which are favorable to the production of ozone. A similar explanation may perhaps account for the alleged abundance of ozone at the periods of new and full moon. It seems also to be present in large amount during eclipses and at the time of the passage of asteroids. The influence of earthquakes is disputed.

The general conclusion at which Dr. Fox arrives is that ozone exists in large amount during spring and winter, because these are the seasons of rain, snow, hail, storms, and cold. Summer and autumn presenting diametrically opposite meteorological conditions, and being, furthermore, seasons during which the atmosphere is contaminated with the maximum of decomposing organic matter, owing to

their high temperature stimulating every form of chemical activity, are characterised by the presence of a minimum of

ozone.

Speaking generally, ozone is more plentiful on mountain tops than in valleys, at the coast than inland, in the country than in town, and in well-drained cities than in those in which sanitary precautions are neglected. In manufacturing towns a slight elevation of situation is not attended with an increased amount of ozone, because the noxious vapours have a tendency to ascend. The annual mean is higher at Silloth on the Solway than at any other place where observations have been recorded, while Lyons, on the other hand, has earned the unenviable soubriquet of "the town without ozone."

The sources of atmospheric ozone appear to be (1) electrical disturbances, as thunderstorms, &c.; (2) the condensation in their northerly course of the heated equatorial aërial currents; (3) the phosphorous oxidation of the noctilucene or excretory product of certain medusæ, infusoria, and other marine animal organisms, which has been shown by Panari and Phipson to be the cause of the phosphorescence of the sea; (4) the electrisation of oxygen escaping from aqueous solution, and of that secreted or given off by plants, and disengaged in chemical action; and (5) more especially that given off from salt water, which is electrified not merely by its escape from solution, but also by the disengaging of the various salts left behind by the water evaporating from the surface.

It is of great importance, and, at the same time, exceedingly difficult, to determine the amount of ozone, as distinguished from that of other oxidizing and purifying principles, present in the air at any given time and place. The following are among the chief tests which have been adopted with the view of solving this interesting problem: (1) Iodide of potassium, which is only useful as a means of ascertaining the sum of purifying principles contained in the air, but fails to determine the amount of ozone as distinct from the rest; a reddish-brown is produced by liberation of iodine; (2) red litmus paper in a solution of

iodide of potassium; this is the only trustworthy ozone test, and has hitherto been solely used by French chemists; here, in proportion as iodine is liberated, potash is formed, which, of course, restores the original blue colour of the litmus; (3) pure silver, which is oxidized by ozone when the latter is present in considerable amount, and (4) sulphate of protoxide of manganese, which is further oxidized under similar circumstances; but neither of these tests is sufficiently delicate for the estimation of atmospheric ozone, the quantity of which is, even when at its maximum, very small; (5) oxide of thallium, useless for meteorological purposes, since its indications are destroyed by the carbonic acid present in the atmosphere; (6) the formation of acetate of copper from copper and acetic acid,―too laborious a method to be practically useful; (7) black sulphide of lead, which is oxidized by ozone (but also, unfortunately, by some of the oxides of nitrogen) into the colourless sulphate; (8) guaiac resin, which ozone renders blue, but this reaction also occurs under the sole influence. of light; (9) indigo, which is bleached; and (10) certain fungi, as boletus luridus, which are coloured blue by ozone; and (11) the celebrated starch and iodide of potassium test, which has been chiefly used hitherto. This last is, however, open to all the following objections: 1. The chemicals are often impure, the iodide of potassium containing some carbonate, or an excess of free iodine. 2. The paper often gives rise to error, being impregnated with lime, silica, oxide of iron, &c.; and yet, common writing paper, with all these and other contaminations, is used by Mr. Moffat, and recommended by the British Meteorological Society! 3. A certain amount of the liberated iodine is converted into iodic acid. 4. The force of the wind is another fruitful cause of error, as manifestly a much larger amount of air must come into momentary contact with the test-paper during a gale than during a calm, and the papers will therefore register the action of a greater quantity of ozone, when the actual percentage present in any given bulk of air may even have been actually smaller. This may be in a great measure obviated VOL. XXXII, NO. CXXIX.—JULY, 1874.

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