Popular Science Monthly/Volume 40/December 1891/Dust
By J. G. McPHERSON.
SOME of the most enchanting phenomena in nature are dependent for their very existence upon singularly unimportant things; and some phenomena that in one form or another daily attract our attention are produced by startlingly overlooked material. What is the agent that magically transforms the leaden heavens into the gorgeous afterglow of autumn, when the varied and evanescent colors chase each other in fantastic brilliancy? What is the source of the beautiful, brilliant, and varied coloring of the waters of the Mediterranean, or of the most extraordinary brilliant blue of the crystal waters of the tarns in the Cordilleras? What produces the awe-inspiring deep blue of the zenith in a clear summer evening, when the eye tries to reach the absolute? Whence come the gentle refreshing rain, the biting sleet, the stupefying fog, the chilling mist, the virgin snow, the glimmering haze, or the pelting hail? What raises water to the state of ebullition in the process of heat application for boiling? What is the source of much of the wound putrefaction, and the generation and spread of sickness and disease? What, in fact, is one of the most marvelous agents in producing beauty for the eye's gratification, refreshment to the arid soil, sickness and death to the frame of man and beast? That agent is dust.
And yet no significance is given to dust unless it appears in large and troublesome quantities. It requires the persistent annoyance of dust-clouds to excite any attention. Dust, however, demands to be noticed, even when not in that collected, irritating motion known in Scotland as stour. The dust-particles floating in the atmosphere or suspended in the water have a most important influence upon the imagination, as well as upon the comfort of man. Though so small that a microscope magnifying 1,600 diameters is required to discern them, they at times sorely tax the patience of the tidy housekeeper and the skill of the anxious surgeon. An æsthetic eye is charmed with their gorgeous transformation effects; yet some are more real emissaries of evil than poet or painter ever conceived.
Until the famous discovery made by Mr. John Aitken, of Falkirk, a few years ago, no one could reasonably account for the existence of rain. It was said by physicists that cloud-particles were attracted by the law of gravitation under certain conditions of temperature and pressure. But this famous experimentalist and observer found out that without dust there could be no rain; there would be nothing but continuous dew. Our bodies and roads would be always wet. There would be no need for umbrellas, and the housekeeper's temper would be sorely tried with the dripping walls.
A very easy experiment will show that where there is no dust there can be no fog. If common air be driven through a filter of cotton-wool into an exhausted glass receiver, the vessel contains pure air without dust, the dust having been seized by the cottonwool. If a vessel containing common air be placed beside it, the eye is unable to detect any difference in the contents of the vessels, so very fine and invisible is the dust. If both vessels be connected with a boiler by means of pipes, and steam be passed into both, the observer will be astonished at the contrast presented. In the vessel containing common air the steam will be seen, as soon as it enters, rising in a close white cloud; then a beautiful foggy mass will fill the vessel, so dense that it can not be seen through. On the other hand, in the vessel containing the filtered, dustless air, the steam is not seen at all; though the eye be strained, no particles of moisture are discernible; there is no cloudiness whatever. In the one case, where there was the ordinary air impregnated with invisible dust, fog at once appeared; whereas in the other case, the absence of the dust prevented the water-vapor from condensing into fog. Invisible dust, then, is required in the air for the production of fog, cloud, mist, snow, sleet, hail, haze, and rain, according to the temperature and pressure of the air.
The old theory of particles of water-vapor combining with each other to form a cloud-particle is now exploded. Dust is required as a free surface on which the vapor-particles will condense. The fine particles of dust in the air attract the vapor-particles and form fog-particles. When there is abundance of dust in the air and little water-vapor present, there is an over proportion of dust-particles; and the fog-particles are, in consequence, closely packed, but light in form and small in size, taking the more flimsy appearance of fog. But if the dust-particles are fewer in proportion to the number of molecules of water-vapor, each particle soon gets weighted, becomes visible, and falls in mist or rain.
This can be shown by experiment. Let a jet of steam be passed into a glass receiver containing common air, and it will be soon filled with dense fog. Shut off the steam and allow the fog to settle. The air again becomes clear. Admit more steam, and the water-particles will seize hold of the dust-particles that previously escaped. Fog will be formed, but it will not be so dense. Again, shut off the steam, and allow the fog to settle and the air to clear. Then admit some steam, and very likely the condensed vapor will fall as rain. If the experiment be often enough repeated, rain instead of fog will be formed, because there are comparatively few solid particles on which the moisture can condense. When, then, dust is present in large quantities, the condensed vapor produces a fog; there are so many particles of dust to which the vapor can adhere that each can only get a very small share—so small, in fact, that the weight of the dust is scarcely affected by the addition of the vapor—and the fog formed remains for a time suspended in the air, too light to fall to the ground. But when the number of dust-particles is fewer, each particle can take hold of a greater space of the water-vapor, and mist particles or even rain-particles will be formed.
This principle that every fog-particle has embosomed in it an invisible dust-particle led Mr. Aitken to one of the most startling discoveries of our day—the enumeration of the dust-particles of the air. Thirty years ago M. Pasteur succeeded in counting the organic particles in the air; these are comparatively few, whereas the number of inorganic particles is legion. Dr. Koch, Dr. Percy Frankland, and others have devoted considerable attention to the enumeration of the micro-organisms in the air, and Mr. A. Wynter Blyth, the public analyst in London, has done good service in counting the micro-organisms in the different kinds of water in the vicinity. Marvelous as are the results, still the process was comparatively easy. By generating the colonies in a prepared gelatin, the number of microbes can be easily ascertained.
But to attempt to count the inorganic dust seemed almost equal in audacity to the scaling of the heavens. The numbering of the dust of the air, like the numbering of the hairs of the head, was considered as one of the prerogatives of the Deity. Yet Mr. Aitken has counted the "gay motes that people the sunbeams." Though he could not enlarge the particles by a nutritive process, as in the case of the organic particles, he has been able to enlarge them by transferring them into fog-particles, so as to be within the possibility of accurate enumeration. His plan is to dilute a definite small quantity of common air with a fixed large quantity of filtered, dustless air, and allow the mixture to be supersaturated by water-vapor; the few particles of dust seize the moisture, become visible in drops, fall on a divided plate, and are there counted by means of a magnifying-glass.
The instrument employed by Mr. Aitken has taken various forms; in fact, he has so far improved it that it can be carried in the coat-pocket. But the original instrument, which we saw and used, is most easily described without the aid of diagrams. But, instead of his decimal system of measurements, we will use the ordinary system, that the dimensions may be more easily grasped by the general reader. Into a common glass flask of carafe-shape, and flat-bottomed, of thirty cubic inches capacity, are passed two small tubes, at the end of one of which is attached a square silver table, one inch long. A little water having been inserted, the flask is inverted, and the table is placed exactly one inch from the inverted bottom, so that the contents of the air above the table and below the bottom are one cubic inch. The observing table has been divided into a hundred equal squares, and is highly polished, with the burnishing all in one direction, so that during the observations it appears dark, when the fine mist-particles, falling on it, glisten opal-like with the reflected light, in order that they may be more easily counted. The tube to which the silver table is attached is connected with two stop-cocks, one of which can admit a small measured portion of the air to be examined. The other tube in the flask is connected with an exhausting syringe, of ten cubic inches capacity. Over the flask is placed a covering colored black in the inside. In the top of this cover is inserted a powerful magnifying-glass, through which the particles on the silver table can be easily seen and counted. A little to the side of this magnifier is an opening in the cover, through which light is concentrated on the silver table. This light, again, has had to pass through a spherical globe of water, in order to abstract the heat rays, which might vitiate the observations.
To perform the experiment, the air in the flask is exhausted by the syringe. The flask is then filled with pure filtered air. One tenth of a cubic inch of the air to be examined is then introduced into the flask, and mixed with the thirty cubic inches of dustless air. After one stroke of the syringe this mixed air is made to occupy an additional space of ten cubic inches; and this rarefying of the air so chills it that condensation of the water vapor takes place on the dust-particles. The observer, looking through the magnifying-glass upon the silver table, sees the mist particles fall like an opal shower on the table, and counts the number on a single square in two or three places, striking an average in his mind. Suppose the average number upon one of these squares were five, then on the whole table there would be 500; and these 500 mist-particles contain the 500 dust-particles which floated invisibly in the cubic inch of mixed air above the table. But, as there are forty cubic inches of mixed air in the flask and syringe, the number of dust-particles in the whole is 40 times 500 = 20,000; that is, there are 20,000 dust-particles in the small quantity of common air (one tenth of a cubic inch) which was introduced for examination; in other words, a cubic inch of that air contains 200,000 dust-particles—nearly a quarter of a million.
By this process Mr. Aitken has been able to count 7,500,000 of dust-particles in one cubic inch of the ordinary air of Glasgow. We counted with him 4,000,000 in a cubic inch of the air outside of the Royal Society Rooms, Princes Street, Edinburgh. Inside the room, after the Fellows had met for two hours, on a winter evening—the fire and gas having been burning for a considerable time—we found 6,500,000 in a cubic inch of the air four feet from the floor; but near the ceiling no fewer than 57,500,000 were counted in the cubic inch. He counted in one cubic inch of air immediately above a Bunsen flame the fabulous number of 489,000,000 of dust-particles. The lowest number he ever counted was at Lucerne, in Switzerland: 3,500 in the cubic inch. On the summit of Ben Nevis the observer, using Mr. Aitken's apparatus, counted from 214,400 down to 840 in the cubic inch. But on the morning of the 21st of July last there was a most marvelous observation made. Though at the sea-level the wind was steady, and the thermometer did not vary, at the summit the wind suddenly veered round to the opposite direction of that below, blowing out of a cyclone, and the temperature rose ten degrees. In consequence the extraordinarily low mean of only thirty-four dust-particles to the cubic inch was observed.
We now come to the most pleasant of the investigations in connection with dust. The very brilliant sunsets which began in the autumn of 1883, and continued during successive seasons with gradually decreasing grandeur, have arrested the attention of the physicist as well as of the general observer. What is the cause of the brilliant coloring in these remarkable sunsets? What is the source of the immense wealth of the various shades of red which have been so universally admired? Gazing on a gorgeous sunset, the whole western heavens glowing with roseate hues, the observer sees the colors melting away before his eyes and becoming transformed into different hues. The clouds are of different sizes and of all shapes. Some float virgin-like in silver folds, others voyage m golden groups; some are embroidered with burning crimson, others are like "islands all lovely in an emerald sea." And when the flood of rosy light, as it deepens into bright crimson, brings out into bold relief the circlet of flaming mountain peaks, it is like a gorgeous transformation scene. Stranger still, when the sun sinks below the horizon, and a dull ashen gray has possessed the western heavens, what occasions the hectic flush on the eastern horizon? Gradually the clouds are tinged with light red from the eastern horizon all over the zenith; whence comes the coloring?
It is a strange coincidence that these remarkably fine sunsets have been since the tremendous eruptions at Krakatoa, in the Straits of Sunda. Along with the lava eruption there was ejected an enormous quantity of fine dust. The decks of vessels, hundreds of miles away, were covered with it. Mr. Verbreek computed that no less than 70,000 cubic yards of dust actually fell round the volcano. This will give an idea of the enormous quantity of dust still floating in the atmosphere, and drifting all over the world. In the upper atmosphere, too, there must always be dust, for without the dust no clouds could be formed to shield us from the sun's scorching rays; and of cosmic dust there must be a considerable quantity in the air, produced by the waste from the millions of meteors that daily fall into it. Mr. Aitken has ably shown that the brilliancy and variety of the coloring are due to the suspended dust in the atmosphere.
Observers of the gorgeous sunsets and afterglows have been most particularly struck with the immense wealth of the various shades and tints of red. Now, if the glowing colors are due to the presence of dust in the air, there must be somewhere a display of the colors complementary to the reds, because the dust acts by a selective dispersion of the colors. The small dust-particles arrest the direct course of the rays of light and reflect them in all directions; but they principally reflect the rays of the violet end of the spectrum, while the red rays pass on almost unchecked. Overhead deep blue reigns in awe-inspiring glory. As the sun passes below the horizon, and the lower stratum of air, with its larger particles of dust which reflect light, ceases to be illuminated, the depth and fullness of the blue most intensely increase. This effect is produced by the very fine particles of dust in the sky overhead being unable to scatter any colors unless those of short wave-lengths at the violet end.of the spectrum. Thus we see, above, blue in its intensity without any of the red colors. When, however, the observer brings his eyes down in any direction except the west, he will see the blue mellowing into blue-green, green, and then rose color. And some of the most beautiful and delicate rose tints are formed by the air cooling and depositing its moisture on the particles of dust, increasing the size of the particles till they are sufficiently large to stop and spread the red rays, when the sky glows with a strange aurora-like light.
The dust theory of the splendor of sunset coloring is strengthened by the often glorious afterglows. The fiercely brilliant streaks of red have disappeared; over the mountain ridge a flush of orange hovers, and softens the approaching blue. The western hills, that once stood out bronzed against the glare of light, are somber-hued. But suddenly, as by a fairy's wand, the roseate flush of beauty rises in the east, and stretches its beautiful tints all over the sky. As the sun sinks, but before it ceases to shine on our atmosphere, the temperature of the air begins to fall, and its cooling is accompanied by an increase in the size of the particles floating in it by the condensation of the water-vapor. The particles to the east lose the sun first, and are thus first cooled. Accordingly, the rays in that direction are best sifted by the larger water-clad particles of dust, and the roseate coloring is there more distinct than in the north and south. As the sun sinks further, the particles overhead become cooler, and attract the water-vapor; thus they increase in size, and thereby reflect the red rays. Here the red hues, at first visible in the east, slowly rise, pass overhead, and descend in the west to form the charming afterglow. Sometimes a flood of glory will roll once more along the summits of the hills, entrancing the attention of the artistic spectator.
All examinations of the volcanic dust lately collected from the atmosphere show that a great quantity of it is composed of small glassy crystals. An abundance of these would quite account for the peculiarity in the visibility of the first glow; and the evidence seems to indicate that the quantity of such crystals is sufficient to produce the result. When these are fully illuminated, they become in turn a source of illumination, and reflect their reddish light all around. In winter sunsets, the water-clad dust-particles become frozen, and the peculiarly brilliant crimson is seen, coloring the dead beech leaves and red sandstone houses, and making them appear to be painted with vermilion.
If, then, there were no fine dust-particles in the upper strata of the atmosphere, the sunset effect would be paler; if there were no large particles in the lower strata., the beautiful sunset effects would cease. In fact, if our atmosphere were perfectly void of dust-particles, the sun's light would simply pass through without being seen, and soon after the sun dipped below the horizon total darkness would ensue. The length of our twilight, therefore, depends on the amount of dust in one form or another in our atmosphere. Not only, then, would a dustless atmosphere have no clouds, but there would be no charming sunsets, and no thoughtinspiring twilights.
There is a generally prevalent fallacy that the coloring at sunrise or sunset is much finer when seen from the summit of a mountain than from a valley. To this matter Mr. Aitken has been giving some attention, and his observations point the very opposite way, corroborative of his dust-theory. From the summit of the Rigi Kulm in Switzerland he saw several sunsets, but was disappointed with the flatness and weakness of the coloring; whereas in the valley, on the same evenings, careful observers were enchanted with the gorgeous display. The lower dusty humid air was the chief source of the color in the sunset effects. His opinon is strengthened by the fact that when from the summit he saw large cumulous clouds, the near ones were always snowy white, while it was only the distant ones that were tarnished yellow, showing that the light came to these clouds unchanged, and it was only the air between the far-distant clouds and his eye that tarnished them yellow. On the mountain-top it required a great distance to give even a slight coloring. The larger and more numerous dust-particles in the air of the valley are, therefore, productive of more brilliant coloring in sunrise or sunset than the smaller and fewer particles on the mountain-top.
It is now admitted that the inherent hue of water is blueness. Even distilled water has been proved to be almost exactly of the same tint as a solution of Prussian blue. This is corroborated by the fact that the purer the water is in nature, the bluer is the hue. But though the selective absorption of the water determines its blueness, it is the dust-particles suspended in it which determine its brilliancy. If the water of the Mediterranean be taken from different places and examined by means of a concentrated beam of light, it is seen to hold in suspension millions of dust-particles of different kinds. To this fine dust it owes its beautiful, brilliant, and varied coloring. Where there are few particles there is little light reflected, and the color of the water is deep blue; but where there are many particles more light is reflected, and the color is chalky blue-green. Along its shores the Mediterranean washes the rocks and rubs off the minute solid particles, which make the water beautifully brilliant.
That this is the case can be illustrated. If a dark metal vessel be filled with a weak solution of Prussian blue, the water will appear quite dark and void of color. But if some fine white powder be thrown into the vessel, the water at once becomes of a brilliant blue color; if more powder be added, the brilliancy increases. This accounts for the changes of depth and brilliancy of color in the several shores of the Mediterranean. In Lake Como, where there is an entire absence of white dust-particles, the water is of a deep blue color, but void of brilliancy; but, where the lake enters the river Adda, the increase of the current rubs down fine reflecting particles from the rocks; in consequence, there the water is of a finer blue. When the dust-particles carried down by the Rhône spread out into the center of the Lake of Geneva, the color assumes the deeper blue, rivaling in brilliancy any water in the world.
The phenomenon called a haze puzzled investigators until Mr. Aitken explained it on the principle of the condensing power of dust-particles Haze is only an arrested form of condensation of water-vapor. If one half of a dusty pane of glass be cleaned in cold weather, the clean part will remain undewed, while the dusty part is damp to the eye and greasy to the touch. Why is this?
Fit up an open box with two pipes, one for taking in water and the other for taking away the overflow. Inside fix a thermometer. Cover the top edge of the box with India rubber, and fix down with spring catches (so as to make the box water-tight) a glass mirror, on which dust has been allowed to collect for some time. Clean the dust carefully off one half of the mirror, so that one half of the glass covering the box is clean and the other half dusty. Pour cold water through the pipe into the box, so as to lower the temperature of the mirror, and carefully observe when condensation begins on each of the halves, taking a note of the temperature. It will be found that the condensation of the water vapor appears on the dust-particles before coming down to the natural dew-point temperature of the clean glass. The difference between the two temperatures indicates the temperature above the dew-point at which the dust condenses the water-vapor. Mr. Aitken found that the condensing power of the dust in the air of a smoking-room varied from 4° to 8° Fahr. above the dew-point, whenever that of the outer air varied from 3° to 512°.
Moisture is, therefore, deposited on the dust-particles of the air which is not saturated, and condensation takes place while the air is comparatively dry. before the temperature is lowered to the dew-point. The clearest air, then, has some haze; and, as the humidity increases, the thickness of the air increases. In all haze the temperature is above the dew-point. And in all circumstances the haze can be accounted for by the condensing power of the dust-particles in the atmosphere at a higher temperature than that required for the formation of fogs, or mists, or rain.
But whence comes the dust? Meteoric waste and volcanic débris have already been mentioned. On or near the sea the air is impregnated by the fine brine-dust lashed by the waves and broken upon the rocks and vessel-sides. But the most active of all substances as a fog-producer in towns is burned sulphur. No less than three hundred and fifty tons of the products of the combustion of sulphur from the coal are thrown into the atmosphere of London every winter day. But the powerful deodorizing and antiseptic properties of the sulphur assist in sanitation; and it is better to bear the inconvenience of fogs than be subjected to the evils of a pestilence. At the same time it should be known that smoke-particles can be deposited by the agency of electricity. If an electric discharge be passed through a jar containing smoke, the dust will be deposited so as to make the air clear. Lightning clears the air, restoring the devitalized oxygen and depositing the dust on the ground. Might it not, then, be possible for strong enough electrical discharges from several large voltaic batteries to attack the smoke in the air of large cities, and especially the fumes from chemical works, so as to bring down the dust In the form of rain instead of leaving it in the form of mystifying fog?
Organic germs also float in the air. Some are being vomited into the air from the pestilential hot-beds of the lowest slums. In a filthy town no less than thirty millions of bacteria in a year will be deposited by the rain upon every square yard of surface. A man breathes thirty-six germs every minute in a close town, and double that in a close bedroom. The wonder is how people escape sickness, though most of these germs are not deadly. In a healthy man, however, the warm lung surfaces repel the colder dust-particles of all kinds, and the moisture evaporating from the surface of the air-tubes helps the prevention of the dust clinging to the surface.
From this outline the reader will observe the increasing importance of careful attention to the influence of dust in the economy of nature. As a sickness-bearer and a death-bearer it must be attacked and rendered harmless; as a source of beauty unrivaled we must rejoice at its existence. The clouds that shelter us from the sun's scorching heat, the refreshing showers that clear the air and cheer the soil, the brilliancy of the deep blue sea and lake, the charms of twilight, and above all the glory of the colors of sunrise and sunset, are all dependent upon the existence of millions of dust-particles which are within the power of man's enumeration. No more brilliant achievement has been made in the field of meteorology than during the past few years by the careful observation and inventive genius of Mr. Aitken in connection with the importance of dust in air and water.—Longman's Magazine.