Popular Science Monthly/Volume 37/June 1890/Atmospheric Dust
By Dr. WILLIAM MARCET, F. R. S.
THE infinitely small particles of matter we call dust, though possessed of a form and structure which escape the naked eye, play important parts in the phenomena of nature. A certain kind of dust has the power of decomposing organic bodies and bringing about in them definite changes known as putrefaction, while other kinds exert a baneful influence on health, and act as a source of infectious diseases. Again, from its lightness and extreme mobility, dust is a means of scattering solid matter over the earth. It may float in the atmosphere as mud does in water, and, blown by the wind, will perhaps travel thousands of miles before again alighting on the earth. Thus Ehrenberg, in 1828, detected in the air of Berlin the presence of organisms belonging to African regions; and he found in the air of Portugal fragments of infusoria from the prairies of America. The smoke of the burning of Chicago was, according to Mr. Clarence King, seen on the Pacific coast.
Dust is concerned in many interesting meteorological phenomena, such as fogs, as it is generally admitted that fogs are due to the deposit of moisture on atmospheric motes. Again, the scattering of light depends on the presence of dust, as is shown in one of Tyndall's interesting experiments. There is no atmosphere without dust, although it varies much in quantity, from the summit of the highest mountain, where the least is found, to the low plains, at the sea-side level, where it occurs most abundantly.
The origin of dust may be looked upon, without exaggeration, as universal. Trees shed their bark and leaves, which are powdered in dry weather and carried about by ever-varying currents of air; plants dry up and crumble into dust; the skin of man and animal is constantly shedding a fine material of a scaly form. The ground in dry weather, high roads under a midsummer's sun, emit clouds of dust consisting of very fine particles of earth. The fine river and desert sand, a species of dust, is silica ground down into a fine powder under the action of water. If the vegetable and mineral world crumbles into dust, on the other hand it is highly probable that dust was the original state of matter before the earth and heavenly bodies were formed; and here we enter the region of theory and probabilities. While it is best to avoid as much as possible stepping out of the track of known facts, there is a limit to physical observation, and in some cases we can do no more than glance into the possible or probable source of natural phenomena.
This has been done, as to the origin of the universe, by Prof. Norman Lockyer, in his article on the History of a Star. The author proposes there to clear in our imagination a limited part of space, and then set possible causes to work: that dark void will sooner or later be filled with some form of matter so fine that it is impossible to give it a chemical name; but the matter will eventually condense into a kind of dust mixed with hydrogen gas, and constitute what are called nebulae. These nebulae are found by spectrum analysis to be made up of known substances, which are magnesium, carbon, oxygen, iron, silicon, and sulphur. This dust comes down to us in a tangible form—dust shed from the sky on the earth, and large masses, magnificent specimens of meteorites, which have fallen from the heavens at different times, some of them weighing tons. There are swarms of dust traveling through space, and their motion may be gigantic. From photographs taken of the stars and nebulae, we are entitled to conclude that the swarms of dust meet and interlace each other, becoming raised by friction and collision to a very high temperature, and giving rise to what looks like a star. The light would last so long as the swarms collide, but would go out should the collision fail; or, again, such a source of supply of heat may be withdrawn by the complete passage of one stream of dust-swarms through another. We shall, therefore, have various bodies in the heavens, suddenly or gradually increasing or decreasing in brightness, quite irregularly, unlike those other bodies where we get a periodical variation in consequence of the revolution of one of them round the other. Hence, as Mr. Lockyer expresses it, "it can not be too strongly insisted upon that the chief among the new ideas introduced by the recent work is that a great many stars are not stars like the sun, but simply collections of meteorites, the particles of which may be probably thirty, forty, or fifty miles apart." These swarms of dust undergo condensation by attraction or gravitation; they will become hotter and brighter as their volume decreases, and we shall pass from the nebulæ to what we call true stars. Mr. Lockyer imagines such condensed masses of meteoric dust being pelted or bombarded by meteoric material, producing heat and light, the effect continuing as long as the pelting is kept up. To this circumstance is due the formation of stars like suns. Our earth originally belonged to that class of heavenly bodies, but from a subsequent process of cooling assumed its present character.
The dust scattered everywhere in the atmosphere, which is lighted up in a sunbeam or a ray from the electric lamp, is of an organic nature. It is seen to consist of countless motes, rising, falling, or gyrating, although it is impossible to follow any of them with the eye for longer than a fraction of a second. We conclude that their weight exceeds but very slightly that of the air; and, moreover, that the atmosphere is the seat of multitudes of minute currents, assuming all kinds of directions. One day last June, from the top of Eiffel's Tower in Paris, I amused myself by throwing an unfolded newspaper over the railing round the summit of the tower. At first it fell slowly, carried away by a light breeze; but presently it rose, and, describing a curve, began again to fall. As it was vanishing from sight, the paper seemed to me as if arrested now and then in its descent, perhaps undergoing again a slight upheaval. Here was, indeed, a gigantic mote floating in the atmosphere, and subject to the same physical laws, though on a larger scale, as those delicate filaments of dust we see dancing merrily in a sunbeam.
It is difficult to say how much of the dust present in the air may become a source of disease, and how much is innocuous. Many of the motes belong to the class of micro-organisms; and experiments show how easily these micro-organisms or sources of infectious diseases can reach the lungs, and do mischief if they should find a condition of the body on which they are able to thrive and be reproduced. Atmospheric motes, although it has been shown that they are really deposited in the respiratory organs, do not accumulate in the lungs and air-passages, but undergo decomposition and disappear in the circulation. Smoke, which is finely divided coal-dust, is clearly subjected to such a destructive process; otherwise the smoky atmosphere of many of our towns would soon prove fatal, and tobacco-smoke would leave a deposit interfering seriously after a very short time with the process of respiration. Dust, however in its physical aspect is very far from being always innocuous, and many trades are liable to suffer from it. The cutting of chaff, for horses' food, is one of the most pernicious occupations, as it generates clouds of dust of an essentially penetrating character. Persons engaged in needle manufacturing and steel-grinders suffer much from the dust of metallic particles. Stone-cutters, and workmen in plaster of Paris, coal-heavers, men engaged in the manufacture of cigars and rope, those employed in flour-mills and hat and carpet making, are liable to suffer from dust. A number of methods have been adopted, more or less successfully, to rid these trades of the danger due to this source. I observed many years ago that charcoal has the power of retaining dust in a remarkable degree, and having had respirators made of it, found them very effective in preventing dust reaching the lungs.
Micro-organisms—dust-like particles capable of cultivation or reproduction in certain media and at certain temperatures—are scattered everywhere in the atmosphere. Interesting inquiries into their distribution in air and water have been made by Dr. Miquel at the Montsouris Observatory, Paris, and by Dr. Percy Frankland in England. Dr. Frankland has found that the number present is much reduced in winter. Experiments made in inclosed places, where there is little or no aërial motion, show the number of suspended organisms to be very moderate; but as soon as any disturbance in the air occurs, from draughts or people moving about, the number rapidly increases and may become very great. Being slightly heavier than air, they have an invariable tendency to fall, and on that account collect on the surface of water. Hence rivers, lakes, and ponds are constantly being thus contaminated.
Important points connected with dust of organic origin are its inflammability and its liability to explode when mixed with air. The property of explosiveness was forcibly illustrated in the destruction of six flour-mills by this cause in Minneapolis, Minn., in May, 1878. Coal-dust in coal-mines is a cause of accident from explosions which has been closely investigated in England, Germany, and other mining countries. The subject was thoroughly treated by Sir Frederick Abeel, in a paper on Accidents in Mines, read before the Institution of Civil Engineers in 1888.
Extremely fine particles of mineral dust may exist in the atmosphere, and do exist there more frequently than is generally thought, while they escape detection by our senses. The author, while making experiments on the Peak of Teneriffe, in 1878, found the knife-edges of his balance so clogged with this invisible dust that the balance refused to act. When wiped off, the dust collected again in a few minutes, and it was only by continually wiping it away that he was able to go on with his investigation. Prof. Piazzi Smyth, while on the Peak of Teneriffe, witnessed strata of dust rising to a height of nearly a mile, reaching out to the horizon in every direction, and so dense as to hide frequently the neighboring hills. Prof. S. P. Langley, looking down from the height of fifteen thousand feet on Mount Whitney, California, into a region that had appeared clear from the valley below, saw "a kind of level dust ocean, invisible from below, but whose depth was six or seven thousand feet, as the upper portion only of the opposite mountain-range rose clearly out of it."
Dust storms are classified by Dr. Henry Cook, according to their intensity, as atmospheric dust, dust columns, and dust storms. Dr. Cook has observed in India that there are some days on which, however hard and violently the wind may blow, no dust accompanies it, while on others every little puff of air or current of wind forms or carries with it clouds of dust. If the wind which raises the dust is strong, nothing will be visible at the distance of a few yards, and the sun will be obscured. The dust penetrates everywhere, and can not be excluded from houses, boxes, and even watches, however carefully guarded. The individual particles of sand appear to be in such an electrical condition that they are ever ready to repel each other, and are consequently disturbed and carried up into the air. Dust columns are regarded by Dr. Cook as due to electrical causes. On calm, quiet days, when hardly a breath of air is stirring, and the sun pours down its heated rays with full force, little eddies arise in the atmosphere near the surface of the ground. These increase in force and diameter, catching up and whirling round bits of sticks, grass, dust, and lastly sand, until a column is formed of great height and considerable diameter, which usually, after remaining stationary for some time, sweeps away across country at great speed. Ultimately it loses gradually the velocity of its circular movement and disappears. In the valley of Mingochar, which is only a few miles in width, and surrounded by high hills, Dr. Cook, on a day when not a breath of air was stirring, counted upward of twenty of these columns. They seldom changed their places, and, when they did so, moved but slowly across the level tract. They never interfered with one another, and appeared to have independent existences. Mr. P. L. H. Baddeley, in his book on Whirlwinds and Dust Storms of India, tells of a gentleman at Lahore who fixed an electrometer apparatus, so adjusted as to report atmospheric electrical movements, and observed that it was strongly affected during dust storms.
Volcanic dust consists mainly of powdered vitrified substances reduced by the action of intense heat. It is interesting in many respects. The ashes or scoria shot out in volcanic eruptions are mostly pounded pumice, but they also originate from stones and fragments which are pulverized by striking against each other. Volcanic dust has a whitish-gray color, and is sometimes nearly white. Thus it is that, in summer, the terminal cone of the Peak of Teneriff e appears from a distance as if covered with snow; but there is no snow on the mountain at that season of the year, and the white cap of the peak is due to pumice ejected centuries ago.
The friction caused by volcanic stones and rocks as they are crushed in their collision develops a mass of electricity which shows itself in brilliant displays of branch lightning darting from the edges of the dense ascending column. During the great eruption of Vesuvius in 1822 they were constantly visible, and added much to the grandeur of the spectacle. It not unfrequently happens that the dust emitted from Vesuvius falls into the streets of Naples; but this is nothing in comparison with the mass of finely powdered material which covered and buried the towns of Pompeii, Herculaneum, and Stabiæ, in the year 79. The eruption of Krakatoa, in 1883, exceeded, in all probability, in its deadly effects, and as a wonderful phenomenon of nature, the outburst of Vesuvius in the year 79. It is shown, in the report of the Krakatoa Committee of the Royal Society, that the detonations caused by the explosive action were heard a hundred and fifty miles away. Captain Thompson, of the ship Medea, sailing at a point seventy-six miles northeast of Krakatoa, saw a black mass like smoke rising into the clouds to an altitude which was estimated as not less than seventeen miles. All the eye-witnesses agree as to the splendor of the electrical phenomena. The old crater of Krakatoa was eviscerated, and a cavity was formed more than a thousand feet deep. On the morning of the 27th of August three vessels at the eastern entrance of the Strait of Sunda encountered the fall of mingled dust and water, which soon darkened the air, and covered their decks and sails with a thick coating of mud. Some of the pieces of pumice falling on the Sir R. Sale were said to have been of the size of a pumpkin. All that day the three vessels were beating about in darkness, pumice-dust falling upon them in such quantities as to employ the crews for hours in shoveling it from the decks and in beating it from the sails and rigging. The speed and distance attained by the pumice ejected from the volcano may be conceived from the fact stated in Mr. Douglas Archibald's contribution to the report, that dust fell on September 8th more than thirty-seven hundred English miles from the seat of the eruption. The great mass of the pumice was of a dirty, grayish-white tint, and was very irregular in size.
The dust ejected from Krakatoa did not all fall back at the same time upon the sea and the earth. The lightest portion formed into a haze, which was as a rule propagated westward. Most observers agree in regarding this haze as the proximate cause of the twilight glows, colored suns, and large corona which were seen for a long time (more than two years) after the eruption. The haze was densest in the Indian Ocean and along the equatorial belt, and was often thick enough to hide the sun when within a few degrees of the horizon.
I hope I have succeeded in showing that infinitely small objects, no larger than particles of dust, act important parts in the physical phenomena of nature.
- Abstract of an address delivered before the Royal Meteorological Society, January 15, 1890.