Worlds in the making: The evolution of the universe

by Svante Arrhenius

It should, perhaps, be mentioned that the most intense spectrum line of the polar lights has been found to be characteristic of the noble gas krypton. As this gas is found only in very small quantities in our atmosphere, it does not appear improbable that it has been carried to us together with the solar dust, and that its spectrum becomes perceptible during the discharge phenomena. The other spectrum lines of the polar lights belong to the spectra of nitrogen, argon, and of the other noble gases. The volumes of noble gases which are brought into our terrestrial atmosphere in this manner would in any case be exceedingly small.

The electrical phenomena of our terrestrial atmosphere indirectly possess considerable importance for organic life and, consequently, for human beings. By the electrical discharges part of the nitrogen is made to combine with the oxygen and hydrogen (liberated by the electric decomposition of water vapor) of our air, and it thus forms the ammonia compounds, as well as the nitrates and nitrites, which are so essential to vegetable growth. The ammonia compounds which play a most important part in the temperate zones appear especially to be formed by the so-called silent discharges which we connect with auroras. The oxygen compounds of nitrogen, on the other hand, seem to be chiefly the products of the violent thunder-storms of the tropics. The rains carry these compounds down into the soil, where they fertilize the plants.

The supply of nitrogen thus fixed amounts in the course of a year to about 1.25 gramme per square metre in Europe, and to almost fourfold that figure in the tropics. If we accept three grammes as the average number for the whole firm land of the earth, that would mean 3 tons per square kilometre, and about 400 million tons per year for the whole firm land of 136 million square kilometres. A very small portion of this fixed nitrogen, possibly one-twentieth, falls on cultivated soil; a larger portion will help to stimulate plant growth in the forests and on the steppes. We may mention, for comparison, that the nitrogen contained in the saltpetre which the mines of Chili yield to us has risen from 50,000 tons in 1880 to 120,000 tons in 1890, to 210,000 tons in 1900, and to 260,000 tons in 1905. The nitrogen produced in the shape of ammonium salts (sulphate) by the gas-works of Europe amounts to about one-quarter of the last-mentioned figure. To this figure we have, of course, to add the production of coal-gas ammonia in the United States and elsewhere. Yet even allowing for this item, we shall find that the artificial supply of combined nitrogen on the earth does not represent more than about one-thousandth of the natural supply.

As the nitrogen contents of the air may be estimated at 3980 billion tons, we recognize that only one part in three millions of the nitrogen of the atmosphere is every year fixed by electric discharges, presuming that the nitrogen supply to the sea is as great per square kilometre as to the land. The nitrogen thus fixed benefits the plants of the sea and of the land, and passes back into the atmosphere or into the water during the life of the plants or during their decay. Water absorbs some nitrogen, and equilibrium between the nitrogen contents of the atmosphere and of the sea is thus maintained. Hence we need not fear any noteworthy depletion of the nitrogen contents of the air. This conclusion is in accord with the fact that no notable accumulation of fixed nitrogen appears to have taken place in the solid and liquid constituents of the earth.

The reader may remember (compare page 57) that during the annual cycle of vegetation not less than one-fiftieth of the atmospheric contents in the carbon dioxide is absorbed. Since oxygen is formed from this carbon dioxide, and since the air contains about seven hundred times as many parts per volume of oxygen as of carbon dioxide, the exchange of atmospheric oxygen is about one part in thirty-five thousand. In other words, the oxygen of the air participates about one hundred times more energetically in the processes of vegetation than the nitrogen, and this is in accordance with the general high chemical activity of oxygen.

Before we close this chapter we will briefly refer to the peculiar phenomenon known as the Zodiacal Light, which can be seen in the tropics almost any clear night for a few hours after or before sunset or sunrise. In our latitudes the light is rarely visible, and is best seen about the periods of the vernal and autumnal equinoxes. The phenomenon is generally described as a luminous cone whose basis lies on the horizon, and whose middle line coincides with the zodiac. Hence the name. According to Wright and Liais, its spectrum is continuous. It is stated that the Zodiacal Light is as strong in the tropics as that of the Milky Way.

Fig. 44.—Zodiacal Light in the tropics

There can be no doubt that this glow is due to dust particles illuminated by the sun. It has therefore been assumed that this dust is floating about the sun in a ring, and that it represents the rest of that primeval nebula out of which the solar system has been condensed, according to the theory of Kant and Laplace (compare Chapter VII.). Sometimes a fairly luminous band seems to shoot out from the apex of the cone of the Zodiacal Light and to cross the sky in the plane of the ecliptic. In that part of the sky which is just opposite the sun this band expands to a larger, diffused, not well-defined spot of light covering about 12° of arc in latitude and 90° in longitude. This luminescence is called the counter-glow (Gegenschein), and was first described by Pezenas in 1780.

The most probable view concerning the nature of this counter-glow is that it is caused by small particles of meteorites or dust which fall towards the sun. Like the position of the corona of the aurora, the position of this counter-glow seems to be an effect of perspective; the orbits of the little particles are directed towards the sun, and they therefore appear to radiate from a point opposite to it.