IVTHE RADIATION PRESSURE

Next to simple measuring and simple calculations, astronomy appears to be the most ancient science. Yet, though man has worshipped the sun from the most remote ages, it was not fully comprehended before the middle of the past century that the sun is the source of all life and of all motion. Part of the veneration for the sun was transferred to the moon, with its mild light, and to the smaller celestial lights. It did not escape notice that their positions in the sky were always changing simultaneously with the annual variations in the weather, and all human undertakings depended upon the weather and the seasons. The moon and the stars were worshipped—we know now, without any justification whatever—as ruling over the weather, and consequently over man’s fate.[6]Before anything was undertaken people attempted first to assure themselves of the favorable aspect of the constellations, and since the most remote ages astrologers have exercised a vast influence over the ignorant and superstitious multitude.

In spite of the vehement enunciation of GiordanoBruno (1548-1600), this superstition was still deeply rooted when Newton succeeded in proving, in 1686, that the movements of the so-called wandering stars, or planets, and of their moons could be calculated with the aid of one very simple law: that all these celestial bodies are attracted by the sun or by their respective central bodies with a force which is proportional to their own mass and to the mass of the central body and inversely proportional to the square of their distance from that central body. Newton’s contemporary, Halley, applied the law of gravitation also to the mysterious comets, and calculating astronomy has since been based upon this, its firmest law, to which there has not been found any exception. The world was thus at once rid of the paralyzing superstition which exacted belief in a mysterious ruling of the stars. The contemporaries of Newton, as well as their descendants, have rightly valued this discovery more highly than any other scientific triumph of this hero’s. According to Newton’s law, all material bodies would tend to become more and more concentrated and united, and the development of the universe would result in the sucking up of the smaller celestial bodies—the meteorites, for instance—by the larger bodies.

It must, however, be remarked that Newton’s great precursor, Kepler, observed in 1618 that the matter of the comets is repelled by the sun. Like Newton, he believed in the corpuscular theory of light. The sun and all other luminous bodies radiated light, they thought, because they ejected minute corpuscles of light matter in all directions. If, now, these small corpuscles hit against the dust particles in the comets’ tails, the dust particles would be carried away with them, and their repulsion by the sun would become intelligible. It is characteristicthat Newton would not admit this explanation of Kepler’s, although he shared Kepler’s opinion on the nature of light. According to Newton, the deviation of the tails of comets from his law of general attraction was only apparent. The tails of comets, he argued, behaved like the columns of smoke rising from a chimney, which, although the gases of combustion are attracted by the earth, yet ascend because they are lighter than the surrounding air. This view, which has been characterized by Newcomb as no longer to be seriously taken into consideration, demonstrates the strong tendency of Newton to explain everything with the aid of his law.

The astronomers followed faithfully in the footsteps of their inimitable master, Newton, and they brushed aside every phenomenon which would not fit into his system. An exception was made by the famous Euler, who, in 1746, expressed the opinion that the waves of light exerted a pressure upon the body upon which they fell. This opinion, however, could not prevail against the criticisms with which others, and especially De Mairan, assailed it. That Euler was right, however, was proved by Maxwell’s great theoretical treatise on the nature of electricity (1873). He showed that rays of heat—and the same applies, as Bartoli established in 1876, to radiations of any kind—must exercise a pressure just as great as the amount of energy contained in a unit volume, by virtue of their radiation. Maxwell calculated the magnitude of this pressure, and he found it so small that it could hardly have been demonstrated with the experimental means then at our disposal. But this demonstration has since been furnished, with the aid of measurements obtained in a vacuum, by the Russian Lebedeff and by the Americans Nichols and Hull (1900, 1901). They have found that this pressure, the so-calledradiation pressure, is exactly as great as Maxwell predicted.

In spite of Maxwell’s great authority, astronomers quite overlooked this important law of his. Lebedeff, indeed, tried in 1892 to apply it to the tails of comets, which he regarded as gaseous; but the law is not applicable in this case. As late as the year 1900, shortly before Lebedeff was able to publish his experimental verification of this law, I attempted to prove its vast importance for the explanation of several celestial phenomena. The magnitude of the radiation pressure of the solar atmosphere must be equivalent to 2.75 milligrammes if the rays strike vertically against a black body one square centimetre in area. I also calculated the size of a spherule of the same specific gravity as water, such that the radiation pressure to which it would be exposed in the vicinity of the sun would balance the attraction by the sun. It resulted that equilibrium would be established if the diameter of the sphere were 0.0015 mm. A correction supplied by Schwarzschild showed that the calculation was only valid when the sphere completely reflects all the rays which fall upon it. If the diameter of the spherule be still smaller, the radiation pressure will prevail over the attraction, and such a sphere would be repelled by the sun. Owing to the refraction of light, this will, according to Schwarzschild, further necessitate that the circumference of the spherule should be greater than 0.3 times the wave-length of the incident rays. When the sphere becomes still smaller, gravitation will once more predominate. But spherules whose sizes are intermediate between these two limits will be repelled. It results, therefore, that molecules, which have far smaller dimensions than those mentioned, will not be repelled by the radiation pressure, and that therefore Maxwell’s law does not hold for gases. Whenthe circumference of the spherule becomes exactly equal to the wave-length of the radiation, the radiation pressure will act at its maximum, and it will then surpass gravity not less than nineteen times. These calculations apply to all spheres, totally reflecting the light, of a specific gravity like water, and to a radiation and attraction corresponding to that of the sun. Since the sunlight is not homogeneous, the maximum effect will somewhat be diminished, and it is nearly equal to ten times the gravity for spheres of a diameter of about 0.00016 mm.[7]

Before we had recourse to the radiation pressure for the explanation of the repulsion phenomena such as have been observed in the tails of comets, it was generally believed with Zöllner that the repulsion was due to electrical forces. Electricity undoubtedly plays an important part in these phenomena, as we shall see. The way in which it acts in these instances was explained by a discovery of C. T. R. Wilson in 1899. Gases can in various ways be transformed into conductors of electricity which as a rule they do not conduct. The conducting gases are said to be ionized—that is to say, they contain free ions, minute particles charged with positive or negative electricity. Gases can be ionized, among other ways, by being radiated upon with Röntgen rays, kathode rays, or ultra-violet light, as well as by strong heat. Since the light of the sun contains a great many ultra-violet radiations, it is indisputable that the masses of gases in the neighborhood of the sun (e.g., probably in comets when they come near the sun) will partly be ionized,and will contain both positive and negative ions. Ionized gases are endowed with the remarkable capability of condensing vapors upon themselves. Wilson showed that this property is possessed to a higher degree by the negative ions than by the positive ions (in the condensation of water vapor). If there are, therefore, water vapors in the neighborhood of the sun which can be condensed by cooling, drops of water will, in the first instance, be condensed upon the negative ions. When these drops are afterwards repelled by the radiation pressure, or when they sink, owing to gravity, as drops of rain sink in the terrestrial atmosphere, they will carry with them the charge of the negative ions, while the corresponding positive charge will remain behind in the gas or in the air. In this way the negative and positive charges will become separated from each other, and electric discharges may ensue if sufficiently large quantities of opposite electricity have been accumulated. By reason of these discharges the gases will become luminescent, although their temperature may be very low. Stark has even shown that low temperatures are favorable for the display of a strong luminosity in electric discharges.

We have stated that Kepler, as early as the beginning of the seventeenth century, came to the conclusion that the tails of comets were repelled by the sun. Newton indicated how we might, from the shape of the comets’ tails, calculate their velocity. The best way, however, is to determine this velocity by direct observation. The comets’ tails are not so uniform in appearance as they are generally represented in illustrations, but they often contain several luminous nuclei (Fig. 33), whose motions can be directly ascertained.

Fig. 33.—Photograph of Roerdam’s comet (1893 II.), suggesting several strong nuclei in the tail

From a study of the movements of comets’ tails, Olbers concluded, about the beginning of the last century, that therepulsion of the comets’ tails by the sun is inversely proportional to the square of their distance—that is to say, that the force of the repulsion is subject to the same law as the force of gravitation. We can, therefore, express the repulsion effect in units of solar gravitation, and this has generally been done. That the radiation pressure will in the same manner change with the distance is only natural. For the radiation against the same surface isalso inversely proportional to the square of the distance from the radiating body, the sun.

Fig. 34.—Photograph of Swift’s comet (1892 I.)

In the latter part of the past century the Russian astronomer Bredichin conducted a great many measurements on the magnitude of the forces with which comets’ tails are repelled by the sun. He considered himself, on the strength of these measurements, justified in dividing comets’ tails into three classes. In the first class the repulsion was 19 times stronger than gravitation; in the second class, from 3.2 to 1.5 times stronger; and in the third class, from 1.3 to 1 times stronger. Still higher values have, however, been deduced for several comets.Thus Hussey found for the comet of 1893 (Roerdam’s comet, 1893 II., Fig. 33) a repulsion 37 times as strong as gravitation; and Swift’s comet (1892 I.) yields the still higher value of 40.5 (Fig. 34). Some comets show several tails of different kinds, as the famous comet of Donati (Fig. 35). Its two almost straight tails would belong to the first class, and the more strongly developed and curved third tail to the second class.

Fig. 35.—Donati’s comet at its greatest brilliancy in 1858

Schwarzschild, as already stated, calculated that small spherules reflecting all the incident light and of the specific gravity of water would be repelled by the sun with a force that might balance ten times their weight. For a spherule absorbing all the light falling upon itthis figure would be reduced to five times the weight. The small particles of comets which, according to spectroscopic observations, probably consist of hydrocarbons are not perfectly absorbing, but they permit certain rays of the sun to pass. A closer calculation shows that in this case forces of about 3.3 times the gravity would result.

Larger spherules yield smaller values. Bredichin’s second and third classes would thus be well adapted to meet the requirements which the radiation pressure demands.

It is more difficult to explain how such great forces of repulsion as those of the first group of Bredichin or of the peculiar comets of Swift and of Roerdam can occur. When a particle or drop of some hydrocarbon is exposed to powerful radiation, it may finally become so intensely heated that it will be carbonized. It will yield a spongy coal, because gases (chiefly hydrogen) will escape during the carbonization, and the particles of coal will resemble the little grains of coal-dust which fall from the smokestacks of our steamboats, and which afterwards float on the surface of the water. It is quite conceivable that such spherules of coal (consisting probably of so-called marguerites, felted or pearly structures resembling chains of bacilli) may have a specific gravity of 0.1, if we make allowance for the gases they include (compare page 106.) A light-absorbing drop of this density of 0.1 might, in the most favorable case, experience a repulsion forty times as strong as the gravitation of the sun. In this manner we can picture to ourselves the possibility of the greatest observed forces of repulsion.

Fig. 36.—Imitation of comets’ tails. Experiment by Nichols and Hull. The light of an arc-lamp is concentrated by a lens upon the stream of finely powdered particles.

The spectra of comets confirm in every respect the conclusions to which the theory of the radiation pressure leads up. They display a faint, continuous spectrum which is probably due to sunlight reflected by the smallparticles. Besides this, we observe, as already mentioned, a spectrum of gaseous hydrocarbons and cyanogen. These band spectra are due to electric discharges; for they are observed in comets whose distance from the sun is so great that they cannot appear luminous owing to their own high temperatures. In the tail of Swift’s comet banded spectra have been observed in portions which were about five million kilometres from the nucleus. The electric discharges must chiefly be emitted from the outer parts of the tails, where, according to the laws of static electricity, the electric forces would be strongest. For this reason the larger tails of comets look as if they were enveloped in cloaks of light of a more intense luminosity.

When a comet comes nearer to the sun, other less volatile bodies also begin to evaporate. We then find the lines of sodium and, when the comet comes veryclose to the sun, also the lines of iron in its spectrum. These lines are evidently produced by substances which have been evaporated from the nucleus of the comet. Like the meteorites falling upon our earth, the nucleus will consist essentially of silicates, and particularly of the silicate of sodium, and, further, of iron.

We can easily imagine how the tails of comets change in appearance. When a comet draws near to the sun, we observe that matter is ejected from that part of the nucleus which is turned towards the sun. The case is analogous to the formation of clouds in the terrestrial atmosphere on a hot summer day. The clouds are provided with a kind of hood which envelops like a thin, semi-spherical veil that side of the nucleus which turns to the sun. Sometimes we observe two or more hoods corresponding to the different layers of clouds in the terrestrial atmosphere. From the farther side of the hood matter streams away from the sun. The tails of comets are usually more highly developed when they approach the sun than when they recede from it. That may be, as has been assumed for a long time, because a large part of the hydrocarbons will become exhausted while the comet passes the sun. We have also noticed that the so-called periodical comets, which return to the sun at regular intervals, showed at every reappearance a fainter development of the tail. Comets, further, shine at their greatest brilliancy in periods of strong solar-spot activity. We may, therefore, assume that in those periods the surroundings of the sun are charged to a relatively high degree with the fine dust which can serve as a condensation nucleus for the matter of the comets’ tails. It is also probable that in such periods the ionizing radiation of the sun is more pronounced than usual, owing to the simultaneous predominance of faculæ.

Nichols and Hull have attempted to imitate tails of comets. They heated the spores of the fungusLycoperdon bovista, which are almost spherical and of a diameter of about 0.002 mm., up to a red glow, and they thus produced little spongy balls of carbon of an average density of 0.1. These they mixed with emery-powder and introduced them into a glass vessel resembling an hour-glass (Fig. 36) from which the air had previously been exhausted as far as possible. They then caused the powdered mass to fall in a fine stream into the lower part of the vessel while exposing it at the same time to the concentrated light of an arc-lamp. The emery particles fell perpendicularly to the bottom, while the little balls of carbon were driven aside by the radiation pressure of the light.

We also meet with the effects of the radiation pressure in the immediate neighborhood of the sun. The rectilinear extension of the corona streamers to a distance which has been known to exceed six times the solar diameter (about eight million km.) indicates that repelling forces from the sun are acting upon the fine dust. Astronomers have also compared the corona of the sun with the tails of comets, and Donitsch would class it with Bredichin’s comets’ tails of the second class. It is possible to calculate the mass of the corona from its radiation of heat and light. The heat radiated has been measured by Abbot. At a distance of 30,000 km. from the photosphere, the corona radiated only as little heat as a body at -55° Cent. The reason is that the corona in these parts consists of an extremely attenuated mist whose actual temperature can be estimated by Stefan’s law at 4300° Cent. The corona must, therefore, be so attenuated that it would only cover a 190,000th part of the sky behind it. We arrive at the same result when we calculate the amount oflight radiated by the corona; this radiation is of the order of that of the full moon, being sometimes smaller, sometimes greater, up to twice as great. The considerations we have been offering apply to the most intense part of the corona, the so-called inner corona. According to Turner, its light intensity outward diminishes in the inverse ratio of the sixth power of its distance from the centre of the sun. At the distance of a solar radius (690,000 km.) the light intensity would therefore be only 1.6 per cent. of the intensity near the surface of the sun.

Let us assume that the matter of the corona consists of particles of just such a size that the radiation pressure would balance their weight (other particles would be expelled from the inner corona); then we find that the weight of the whole corona of the sun would not exceed twelve million metric tons. That is not more than the weight of four hundred of our large ocean steamships (e.g., theOceanic), and only about as much as the quantity of coal burned on the earth within one week.

That the mass of the corona must be extremely rarefied has already been concluded, from the fact that comets have wandered through the corona without being visibly arrested in their motion. In 1843 a comet passed the sun’s surface at a distance of only one-quarter the sun’s radius without being disturbed in its progress. Moulton calculated that the great comet of 1881, which approached the sun within one-half its radius, did not encounter a resistance of more than one-fifty-thousandth of its mass, and that the nucleus of the comet was at least five million times denser than the matter of the corona. Newcomb has possibly expressed the degree of attenuation of the corona in a somewhat exaggerated way when he said that it contains perhaps one grain of dust per cubic kilometre (a cube whose side has a length of three-fifths of a mile).

However small the quantity of matter in the corona may be, and however unimportant a fraction of this mass may pass into the coronal rays, it is yet certain that there is a constant loss of finely divided matter from the sun. The loss, however, is not greater than the supply of matter (compare below)—namely, about 300 thousand millions of tons in a year—so that during one billion years not even one-six-thousandth of the solar mass (2 × 1027tons) will be scattered into space. This number is very unreliable, however. We know that many meteorites fall upon the earth, partly as compact stones, partly as the finest dust of shooting-stars which flash up in the terrestrial atmosphere rapidly to be extinguished. These masses may be estimated at about 20,000 tons per year. According to this estimate, the rain of meteorites which falls upon the sun may amount to 300 thousand millions of tons in a year. All the suns have emitted matter into space for infinite ages, and it seems, therefore, a natural inference that many suns would no longer be in existence if there had not been a supply of matter to make up for this loss. The cold suns undergo relatively small losses, but receive just as large inflows of matter as the warm suns. As, now, our sun belongs to the colder type of stars, it is probable that the loss of matter from the sun has for this reason been overestimated by being presumed to be as great as the accession. The presence of dark celestial bodies may compensate for this overestimation.

Fig. 37.—Granular chondrum from the meteorite of Sexes. Enlargement 1: 70. After S. Tschermak

Whence do the meteorites come? If they were not constantly being created, their number should have diminished in the course of ages; for they are gradually being caught up by the larger celestial bodies. It is not at all improbable that they arise from the accrescenceof small particles which the radiation pressure has been driving out of the sun. The chondri, which are so characteristic of meteorites, display a structure as if they had grown together out of a multitude of extremely fine grains (Fig. 37). Nordenskiöld says: "Most meteoric iron consists of an extremely delicate texture of various alloys of metals. This mass of meteoric iron is often so porous that it oxidizes on exposure to the air like spongy iron. The Pallas iron, when cut through with a saw, shows this property, which is so distressing for the collector. Theiron of Cranbourne, of Toluca, and others—in fact, almost all the meteorites with a few exceptions—display the same texture. It all indicates that these cosmical masses of iron were built up in the universe by particle being piled upon particle, of iron, nickel, phosphorus, etc., analogous to the manner in which one atom of a metal coalesces with another atom when the metal is galvanically deposited from a solution. Most of the stony meteorites present a similar appearance. Apart from the crust of slag on the surface, the stone is often so porous and so loose that it might be used as a filtering material, and it may easily be crumbled between the fingers." When the electrically charged grains of dust coalesce, their small electrical potential (of about 0.02 volt) may increase considerably. Under the influence of ultra-violet light these masses of meteorites are discharged when they approach the sun, as Lenard has shown. Their negative charge then escapes in the shape of so-called electrons.

Since, now, the sun loses through the rays of the corona large multitudes of particles, and these particles probably carry, according to Wilson, negative electricity with them, the positive charge must remain behind in the stratum from which the coronal rays were emitted, and also on the sun itself. If this charge were sufficiently powerful, it would prevent the negatively charged particles in the corona from escaping from the sun, and all the phenomena which we have ascribed to the radiation pressure would cease. By the aid of the tenets of the modern theory of electrons, I have calculated the maximum charge that the sun could bear, if it is not to stop these phenomena. The charge would amount to two hundred thousand millions of coulombs—not by any means too large a quantity of electricity, as it would only be sufficient to decompose twenty-four tons of water.

By means of this positive charge the sun exerts a vast attractive power upon all negatively charged particles which come near it. We have already remarked that the grains of sun-dust which have united to form meteorites lose under the influence of ultra-violet light their charge in the shape of negative electrons, extremely minute particles, of which perhaps one thousand weigh as much as one atom of hydrogen (1 gramme of hydrogen contains about 1024atoms, corresponding to 1027electrons). These electrons wander about in space. When they approach a positively charged celestial body they are attracted by it with great force. If the electrons were moving with a velocity of 300 km. per second, as in Lenard’s experiment, and if the sun were charged to one-tenth the maximum amount just calculated, it would be able to draw up all the electrons whose rectilinear path (so far as not curved by the sun’s attraction) would lie at a distance from the sun 125 times as great as the distance between the sun and its most remote planet, Neptune, and 3800 times as great as the distance between the sun and the earth, which, after all, would only be one-sixtieth of the distance from our nearest fixed-star neighbor. The sun drains, so to speak, its surroundings of negative electricity, and this draining effect carries to the sun, as could easily be proved, a quantity of electricity which is directly dependent upon the positive charge of the sun. Thus, so far as electricity is concerned, ample provision has been made for maintaining equilibrium between the income and expenditure of the sun.

When an electrical particle enters into a magnetic field it describes a spiral about the so-called magnetic lines of force; when at a greater distance, the particles appear to move in the direction of the lines themselves. Therays of the corona emanating from the solar poles show a distinct curvature like that of the lines of force about a magnet, and for this reason the sun has been regarded as a big magnet whose magnetic poles nearly coincide with the geographical poles. The coronal rays nearer the equator likewise show this curvature (compare Fig. 30). The repelling force of the radiation pressure there is, however, at right angles to the lines of force and much stronger than the magnetic force, so that the rays of the corona are compelled to form two big streams flowing in the equatorial direction. This is especially noticeable at times of sun-spot minima. During the times of sun-spot maxima the strength of the radiation pressure of the initial velocity of the grains of dust seem to predominate so markedly that the magnetic force is relatively small.

The astronomers tell us that the sun is only a star of small light intensity compared to the prominent stars which excite our admiration. The sun further belongs to a group of relatively cold stars. We may easily imagine, therefore, that the radiation pressure in the vicinity of these larger stars will be able to move much larger masses of matter than in our solar system. If the different stars had at any time consisted of different chemical elements, this difference would have been equalized in the course of ages. The meteorites may be regarded as samples of matter collected and despatched from all possible divisions of space. Now, what bodies do we find in them?

In the comets (compare page 104) iron, sodium, carbon, hydrogen, and nitrogen (as cyanogen) play the most important part. We know, especially from the researches of Schiaparelli that meteorites often represent fragments of comets, and must therefore be related to them. ThusBiela’s comet, which had a period of 6.6 years, has disappeared since 1852—it had divided into two parts in 1844-1845. The comet was rediscovered in a belt of meteorites of the same period which approaches the orbit of the earth each year on November 27. Similar relations have been observed with regard to several other swarms of meteorites. We know also that the just-mentioned elements which spectrum analysis has proved to exist in comets are the main constituents of the meteorites, which, in addition, contain the metals calcium, magnesium, aluminium, nickel, cobalt, and chromium, as well as the metalloids oxygen, silicon, sulphur, phosphorus, chlorine, arsenic, argon, and helium. Their composition strongly recalls the volcanic products of so-called basic nature—that is to say, those which contain relatively large proportions of metallic oxides, and which have been thought for good reasons to hail from the deeper strata of the interior of the earth. Lockyer heated meteoric stones in the electric arc to incandescence and found their spectra to be very similar to the solar spectrum.

We therefore draw the conclusion that these messengers from other solar systems which bring us samples of their chemical elements are closely related to our sun and to the interior of our earth. That other stars and comets are essentially composed of the same elements as our sun and earth, spectrum analysis had already intimated to us. But various metalloids, like chlorine, bromine, sulphur, phosphorus, and arsenic, which are of importance for the composition of the earth, have so far not been traced in the spectra of the celestial bodies, nor in that of the sun. We find them in meteorites, however, and there is not the slightest doubt that we must likewise count them among the essential constituents of the sun and other celestial bodies. It is with difficulty, however, that the metalloids can be made to exhibit their spectra, and this is manifestly the reason why spectrum analysis has not yet succeeded in establishing their presence in the heavens. As regards the recently discovered so-called noble gases helium, argon, neon, krypton, and xenon, their presence in the chromosphere has been discovered on spectrograms taken during eclipses of the sun (Stassano). According to Mitchell, however, these statements would appear to be somewhat uncertain as to krypton and xenon.

The small particles of dust which the radiation pressure drives out into space to all possible distances from the sun and the stars may hit against one another and may accumulate to larger or smaller aggregates in the shape of cosmical dust or meteorites. These aggregates will partly fall upon other stars, planets, comets, or moons, and partly—and this in very great multitudes—they will float about in space. There they may, together with the larger dark celestial bodies, form a kind of haze, which partly hides from us the light of distant celestial bodies. Hence we do not see the whole sky covered with luminous stars, which would be the case if, as we may surmise, the stars were uniformly distributed all through the infinite space of the universe, and if there were no obstacle to their emission of light. If there were no other celestial bodies of very low temperature and very large dimensions which absorbed the heat of the bright suns, the dark celestial bodies, the meteorites, and the dark cosmical dust would soon be so strongly heated by solar radiation that they would themselves turn incandescent, and the whole dome of the sky would appear to us like one glowing vault whose hot radiation down to the earth would soon burn every living thing.

These other cold celestial bodies which absorb the solar rays without themselves becoming hot are known as nebulæ. More recent researches make us believe that these peculiar celestial bodies occur nearly everywhere in the sky. The wonderful mechanism which enables them to absorb heat without raising their own temperature will be explained later (in Chapter VII.). As these cold nebulæ occupy vast portions of space, most of the cosmical dust must finally, in its wanderings through infinite space, stray into them. This dust will there meet masses of gases which stop the penetration of the small corpuscles. As the dust is electrically charged (particularly with negative electricity), these charges will also be accumulated in the outer layers of the nebulæ. This will proceed until the electrical tension becomes so strong that discharges are started by the ejection of electrons. The surrounding gases will therefore be rendered luminescent, although their temperature may not much (perhaps by 50°) exceed absolute zero, -273° Cent., and in this way we are enabled to observe these nebulæ. Most of the particles will be stopped before they have had time to penetrate very deeply into a nebula, and it will therefore principally be the outer portions of the nebulæ which send their light to us. That would conform to Herschel’s description of planetary nebulæ, which display no greater luminosity in their centres, but which shine as if they formed hollow spherical shells of nebulous matter. It is very easy to demonstrate that only substances, such as helium and hydrogen, which are most difficult to condense, can at this low temperature exist in gaseous form to any noticeable degree. The nebulæ, therefore, shine almost exclusively in the light of these gases. There occurs in the nebulæ, in addition to these gases, a mysterious substance, nebulium, whose peculiar spectrum has not beenfound on the earth nor in the light of stars. Formerly the character of the nebular spectrum was explained by the assumption either that no other bodies occurred in nebulæ than the substances mentioned, or that all the other elements in them were decomposed into hydrogen—helium was not known then. The simple explanation is that only the gases of the outer layer of the nebulæ are luminous. How their interiors are constituted, we do not know.

It has been objected to the view just expressed that the whole sky should glow in a nebulous light, and that even the outer atmosphere of the earth should display such a glow. But hydrogen and helium occur only very sparely in the terrestrial atmosphere. We find, however, another light, the so-called auroral line, which may possibly be due to krypton in our atmosphere. Whichever way we turn the spectroscope on a very clear night, especially in the tropics, we observe this peculiar green line. It was formerly considered to be characteristic of the Zodiacal Light, but on a closer examination it has been traced all over the sky, even where the Zodiacal Light could not be observed. One of the objections to our view is therefore unjustified.

As regards the other objection, we have to remark that any light emission must exceed a certain minimum intensity to become visible. There may be nebulæ, and they probably constitute the majority, which we cannot observe because the number of electrically charged particles rushing into them is far too insignificant. A confirmation of this view was furnished by the flashing-up of the new star in Perseus on February 21 and 22, 1901. This star ejected two different kinds of particles, of which the one kind travelled with nearly double the velocity of the other. The accumulations of dust formed two spherical shells around the new star, corresponding in every respect to the two kinds of comets’ tails of Bredichin’s first and second classes, which we have sometimes observed together in the same comet (Fig. 35). When these dust particles, on their road, hit against nebular masses, the latter became luminescent, and we thereby obtained knowledge of the presence of large stellar nebulæ of whose existence we previously had not the faintest suspicion. Conditions, no doubt, are similar in other parts of the heavens where "we have not so far discovered any nebulæ—we believe, because of the small number of these charged particles straying about in those parts. On the same grounds we may explain the variability of certain nebulæ which formerly appeared quite enigmatical."

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