Popular Science Monthly/Volume 45/September 1894/The Work of Dust
|THE WORK OF DUST.|
WHEN a beam of sunlight enters a darkened room through a hole in the window shutter, it can be seen along its whole course. The light is reflected to every side, and made to reach the eye by the dust in the air of the room. We do not see the sunbeam itself, but the dust which is illuminated by it; and individual bodies can be perceived on a closer inspection floating in the beam. The dust may be much more plainly observed in still air, as it settles on objects. It is extremely slow in falling to the ground, although it consists of matter which in larger masses falls very speedily. This we can test by collecting dust and compressing it into a ball. In this process of compression a very large part of the exposed surface which the particles presented to the air is caused to disappear; and it was by means of this great extent of surface that the air bore enough upon the particles to support them against falling. The finer the dust the more extended is its surface in proportion to its mass, and the more it is delayed in falling through the air. It may seem useless to speak of the part played by this dust in Nature; for what noticeable effect can this insignificant stuff bring about? We have, however, as can be shown, no right to regard it as a little thing.
Dust has a very large share in nearly all the phenomena of the earth's atmosphere. It is what makes the clear sky appear blue; and when we look up into the sky we see the dust in the atmosphere illuminated by the sun. There is nothing else before us that can permit the light to reach the eye. Light goes invisible, straight through all gases, whatever their chemical composition. The dust catches it, reflects it in every direction, and so causes the whole atmosphere to appear clear, in the same way that it makes the sunbeam visible in the darkened room. Without dust there would be no blue firmament. The sky would be as dark as or darker than we see it in the finest moonless nights. The glowing disk of the sun would stand immediately upon this dark background, and the same sharp contrast would prevail upon the illuminated surface of the earth—blinding light, where the sun's rays fall, and deep black shadows where they do not. Only the light of the moon and the stars, which would remain visible in the daytime, would be able to temper this contrast in a slight degree. The illumination of the earth's surface would be like that we see with the telescope on the lunar landscapes; for the moon has no atmospheric envelope that can hold floating dust. We then owe to dust the even moderately tempered light, adapted now to our eyes; and it is that which contributes much to the beauty of our landscape scenery.
But if dust makes the sky appear clear, why is the color of the sky blue? Why does dust, of the different constituents of white sunlight, reflect the blue rather than the green, yellow, and red? This fact is connected with the size of the dust particles. Only the finest dust settles so slowly that it can be spread everywhere by means of the air currents, and can be found constantly in all strata of the atmosphere; and special importance can be ascribed only to these finest particles. The coarse parts soon fall to the ground. Let us consider the fine mechanism of light, the extremely short ether waves which determine its existence. These waves, although they are of even less than microscopic size, are not all equally long. The shortest are those that give blue light, while all the other colors are produced by longer waves. The fine atmospheric dust contains many particles which are large enough to reflect the short blue ether waves, fewer that can reflect green and yellow, and still fewer large enough to reflect the long' red waves. The red light, therefore, goes on almost without hindrance, while the blue is more liable to be diverted, and thus to reach the eye. A similar phenomenon may be observed on a larger scale on water which is roughened with waves of different lengths, and on which pieces of wood are floating. The pieces of wood stand in the same relation to the water waves as the dust particles to the ether waves. The great long waves pass the blocks undisturbed, only rocking up and down; while the finer ripples of the water are turned back, as if the blocks were firm walls. The finest dust thus appears blue. Look at the smoke that rises from the glowing end of a cigar. It appears on a clear day, especially in the presence of much blue light, of the most beautiful sky blue. But that part of the smoke that is drawn through the cigar, and is seen at the other end, is composed of coarser particles, which are large enough to reflect the longest ether waves, including all the constituents of white light. It therefore appears whitish. The same difference is found between the dust in the country and that in the town. There is much coarse dust in large towns, when the sky over them is often gray, while only the finest blue dust is carried up in the country. The dust is also variously assorted at different heights above the surface of the earth. The coarser dust will be found at the lower levels, where it is produced. On mountains we have most of the dust beneath us, while the rarefied air can sustain only the finest floating particles. Hence the sky on high mountains is clear and deep blue, even almost black, as if it were without dust. Only when we look at the lower strata, toward the horizon, does the color pass into gray.
Why is the sky in Italy and the tropics of a so much deeper blue than that of western Europe? Is the dust there finer? It is really so; not that a finer quality of dust is produced there, but because in the moist climate of the North Sea countries the dust can not float long in the air without being charged with water and made coarser, while in warmer countries water exists in the air as vapor, and does not become condensed as a liquid on the dust. Only when it is carried by the air currents into the higher strata and is cooled there, does it thicken into clouds? With this we come to the most important function of dust in our atmosphere—the part which it has in the function of rain by reason of vapors condensing upon it. It can be affirmed with certainty that all the water which the sun causes to evaporate on the surface of the sea and on the land is condensed again on dust, and that no raindrop falls unless it had a particle of dust as its primary nucleus.
When we speak of "vapor" we always mean water in the gaseous condition, transparent and invisible, like all other gases but cloudy steam, such as is seen escaping from the boiler of a locomotive. The latter, like the clouds and fogs, is liquid water split up into innumerable fine drops. If the walls of a steam-boiler were of glass, we should be able to see clearly through the part of it occupied by steam. Then we have water in the gaseous form. But when the steam escapes from it into much colder air, it is condensed into liquid drops. The process is precisely the same when the vapor which the sun has drawn up in the lower warm strata of the atmosphere is cooled on rising, and forms clouds. It is usually said that the upper atmospheric strata are colder than the lower, because they permit a perfect passage of the solar rays through them, and are therefore not warmed, while the rays, on the other hand, warm the surface of the earth, and that warms the air. This is true, but it does not explain why the upper strata of the air do not become warmed in the course of time. The supposition of a cooling of these strata by space does not afford a sufficient cause, for a body which, like the air, stores up little heat, likewise by a fixed law sends little out. Were the atmosphere perfectly still it would, in fact, be warmed all through from the earth's surface. But it is in constant motion, and the heat is consequently very unevenly distributed through it. When a column of air rises into the heights from the earth's surface, it expands greatly, for the pressure to which it is subjected is much less in the higher regions than below; and whenever a gas expands it becomes colder. A quantity of heat is withdrawn from it corresponding with the force which it spends in expanding in pushing itself into the surrounding region. Ascending air, therefore, becomes cooler, descending air warmer. Thus the fact is explained that by reason of the continuous motions in the atmosphere the equality of temperature, which would exist if all the strata were equally warmed, never can come to pass.
When the rising columns of air contain a sufficient quantity of vapor, it will at a certain height be condensed into drops and form clouds. We say that the cooling is the cause of the condensation. But it is now maintained and proved by experiment that cooling alone is not adequate to do this, and that condensation takes place only on the surface of some solid or liquid body; not in the free, pure air, but on the surface of the dust particles scattered through it. Every drop of a steam-jet or a cloud is a particle of dust covered with water. The experimental proof of this is easily made. We fill a large flask with dustless air by pressing ordinary air through wadding and conducting it into the flask till all the air originally therein has been replaced with filtered air. The wadding holds back all the dust particles. We then let a jet of steam from a boiler into the dustless air of the flask. It remains invisible. Not a sign of the usual cloudy appearance is perceptible. All that we observe is that the inner walls of the flask begin to trickle; the steam is condensed only on them, for there is no other fixed surface. If, now, some ordinary dusty air is blown into the flask, it at once appears to be filled with a thick, rolling cloud. The cloud is composed of as many drops as dust particles have been admitted. If only a little dust is admitted, all the vapor is precipitated upon it, and so loads it with water in a short time that it sinks in heavy drops to the ground. It is raining in our flask. It will soon become clear, and the vapor will be invisible as before.
Without dust there would be no condensation of water in the air—no fog, no clouds, no rain, no snow, no showers. The only condensing surface would be the surface of the earth itself. Thus the trees and plants, and the walls of houses, would begin to trickle whenever cooling began in the air. In winter all would be covered with a thick, icy crust. All the water which we are accustomed to see falling in rain-pours or in snow would become visible in this way. We should at once feel on going out of doors that our clothes were becoming wet through. Umbrellas would be useless. The air, saturated with vapor, would penetrate the interior of houses and deposit its water on everything in them. In short, it is hard to conceive how different everything would be, if dust did not oft'er its immeasurable extent of surface everywhere to the air. To this we owe it that the condensation of water is diverted from the surface of the earth to the higher, cooler atmospheric strata.
Since the importance of dust in meteorological phenomena has been recognized, experiments have been made in counting the particles in the air. Pasteur had already begun an investigation in that direction. He filtered a measured quantity of air through gun cotton, which retained all the particles of dust. This was then dissolved in a mixture of ether and alcohol, and the solution was dried to a sheet of clear and transparent collodion, in which the particles could be observed under the microscope and counted. The chief purpose of this experiment was to secure the yeast germs in the air. A better process for counting dust is based on our experiment with the dustless flask, and, like that, was devised by Mr. John Aitken, in Edinburgh. A measured quantity of the air to be tested—say, about a hundredth part of the contents of the flask—is let into it. The counting is facilitated by this dilution. The air in the flask has been already saturated with moisture, while it has been compressed by forcing in some dustless air. A faucet is suddenly opened, when the air expands, is cooled by the expansion, and the vapor settles on all the dust particles, weights them, and causes them soon to sink to the bottom. The bottom of the flask is made of a bright silver or a glass plate, on which a network of square millimetres is scratched. On this network as many drops of water fall as there were dust particles, and they can be counted with a lens. The number of dust particles in a cubic centimetre of air is—in London, for example, even at the borders of the city, and when the wind is blowing toward it from without—nearly a quarter of a million. About the same number are found in the air of Paris, and half as many at the top of the Eiffel Tower. The air of the Alps is very much purer. On the top of the Rhigi there were about two hundred particles to a cubic centimetre, and a few less after a fall of rain. In the relatively pure air of mountain tops the breath is not condensed into a visible cloud, even in cold weather. As we descend and approach villages whose chimney tops are smoking, the accustomed breath clouds appear again. But a steam jet is visible everywhere, for perfectly dustless air is not found anywhere.
Dust is usually spoken of as something peculiar to the earth. It is, however, present in space. Our solar system has its dust atmosphere, although it is extremely thinly sown. Besides the large blocks of matter, the meteoric stones, meteoric dust is incessantly falling from space upon the earth. Attention was first directed to this fact in 1869, when a meteorite fell at Upsala, and a shower of black dust at the same time. The dust was collected, and exhibited the same composition as the meteorite—carbon and iron. Since then several falls of cosmic dust of identical composition have been observed where no meteorites were seen. The recent advance of celestial photography has furnished images of externally faint clouds floating in space. These clouds do not participate, like earthly things, in the revolution around the axis, but remain fixed among the stars through the night. When near enough to the earth they can be seen with the naked eye as luminous clouds, long after sunset, till they are covered by the earth's shadow.
The presence of dust in planetary space is not strange. In the midst of it is the sun, the surface of which is like an immense volcano. We can only ask how the dust clouds of the solar eruptions can be diffused in space, against the attraction of the sun. An answer to this question is afforded by the electro-magnetic theory of light, and we can rely upon it because the theory has been confirmed by experimental demonstration. It teaches that the lighter undulations of the ether are of an electrical nature, and that consequently light exerts a pressure on all bodies upon which it falls. The illuminated body is repelled from the source of the light. We have also learned the amount of this pressure. It is so small that the scale of the most sensitive balance is not moved by it when the clear sunshine falls upon it from above; but it increases with the extent of the surface exposed to the light. Let us now suppose a body isolated anywhere in planetary space. It is subject to general attraction and is drawn toward the sun. The force with which the light of the sun repels it is slight as compared with the attraction. Let us imagine this body divided into smaller and smaller fragments. It then offers the sun a larger and larger surface, and in the same measure the force increases with which all the parts collectively are repelled from the sun. The amount of attractive force is, on the other hand, not changed, for it depends upon the mass of the body, and that has not been altered. It will be seen that the division of the body has only to be carried far enough for the repulsive force ultimately to exceed the attraction. Calculation shows that this is already the case when the body is changed into a dust cloud of not excessive fineness. Such a dust cloud will be no longer attracted toward the sun, but will be driven away by its light. It will be like the comets' tails, which consist chiefly of dust, radiate from the nuclei, and are always turned away from the sun.
Thus, even insignificant, common dust has its considerable part in the processes of Nature; and there is as much of the wonderful and mysterious concealed in it as in anything else.—Translated for The Popular Science Monthly from Die Gartenlaube.