Popular Science Monthly/Volume 17/July 1880/Atmospheric Dust
EVERY one is aware that the atmosphere holds quantities of dust in suspension. The dust betrays its presence by settling upon our clothes, furniture, and other objects, but, on account of the minuteness of its particles, it can not be seen as it floats in the air, except under the illumination of a strong light, as in the case of a sunbeam shining into a dark room. Besides the grains of dust which may be seen in this manner, there are others that can be perceived only through the microscope, and others smaller still, little nothings like nebulosities in the sky, which seem to become more numerous as they are sought for with more powerful instruments. These bits of dust, lifted up and carried hither and thither by the atmospheric currents, must not be overlooked, for they play a part of considerable importance in terrestrial economy, and give rise to real geological formations. Clouds of impalpable dust, falling from the air in showers of considerable abundance, are not uncommon in some countries, and have been noticed in all periods of history. Showers of blood have also been mentioned quite often from the times of Homer down; they are showers of rain-water made muddy with the atmospheric dust, and bearing a yellow or reddish deposit. Showers of dust, both dry and wet, are quite frequent in the Cape de Verd Islands, and are called red fogs by the sailors. They are also common in Sicily and Italy, and occur so often in some parts of China as hardly to attract remark. The Chinese account for them by saying that the dust is lifted up by whirlwinds in the Desert of Gobi, is carried by the aërial currents into the higher regions of the atmosphere, falls at a distance, and is then swept up by rain-waters and carried by the rivers to be deposited at the bottom of the Yellow Sea. A shower of very fine dust which fell in southern France in October, 1846, was found, by the analyses of M. Dumas and the microscopic tests applied by M. Ehrenberg, to be composed of the fine sands of Guiana and to contain the characteristic diatoms and microscopic shells of South America.
Some of these showers originate in volcanic eruptions, from which fine ashes are projected up into the atmosphere, transported to a distance, and deposited over regions of considerable extent. A volcano of the Island of Sumbawa, in 1815, covered with ashes a space of three or four times the extent of France; the event left such an impression that the people at Bruni, in Borneo, made it an epoch from which they reckoned their dates. In some places the atmospheric currents of dust exert a perceptible mechanical action. Sir Joseph Hooker remarked, when he was in South America, that the aërial sand-currents on the tops of high mountains were competent to wear down and polish the trunks of trees and produce striæ upon them like that made upon rocks by glacial action. M. Videt d'Aoust has found evidence in Mexico of the formation of beds by the settlement of the dust, covering the largest mountains, as Popocatepetl and Orizaba, rising to the height of about 12,000 feet on the slopes and reaching a thickness in the valleys of from 250 to 325 feet. These formations are occasioned by whirlwinds of dust which are frequent on the Mexican plain, and are stopped by the elevated chain of the mountains, as the mud in a river is stopped by a sand-bar. The action of the prevailing winds in other parts of the globe promotes the formation of similar deposits, which may be called aërial lands.
These are the dusts of the lower regions of the atmosphere. But the air contains other particles in the most minute degree of division. The waters of the sea, impinging on the coast in waves, are broken into thousands of particles which are taken up and evaporated by the winds. The saline residue of these particles adds a new element to the atmospheric dust; the vapors furnished by the sea go to the formation of clouds and fogs. Rising beyond the regions of ordinary clouds, these vapors ascend to the colder strata, where they are converted into a dust consisting of minute crystals of ice which form the cirrhus clouds and the ice-fields of the upper regions. These masses of frost, which can only be distinguished when they are approached by a balloon, but the existence of which is well established, are agents in the formation of halos and parhelia, and descend in cold winds to the surface.
If we pass these heights, into the extreme limits of the atmosphere, we shall find ourselves in the presence of dust from a new source—of that which is furnished by the combustion of incandescent aërolites. The fruits of the study of meteoric astronomy prove that the surface of the earth is continually receiving cosmic materials either in the form of meteorites or shooting-stars, or of an impalpable dust. A ship passing to the south of Java in January, 1859, was assailed by a very fine ferruginous dust. Ehrenberg examined some of it with the microscope, and found that it was formed of melted globules of oxide of iron, and did not hesitate to regard it as consisting of particles of a mass of meteoric iron, which had been melted off by the operations of atmospheric friction. In other cases the particles may originate in the disintegration of the substance of the meteors, when the soluble salts with which they are cemented are dissolved by atmospheric moisture, as Daubrée observed in the case of a meteorite at Orgueil. Showers of fire have been mentioned, or of sparks which seem to be formed of the incandescent dust of aërolites. Baron Reichenbach collected on the summit of Lahisberg a black ferruginous dust containing traces of nickel and cobalt, which incontestably indicated its cosmic origin. Mr. Nordenskjöld collected a similar dust from off the snow of the polar regions.
For several years I have paid attention to the study of atmospheric dust, and believe that I have proved that more or less considerable quantities of dust derived from cosmic bodies are constantly present in the air. The greater part of my experiments have been performed in the meteorological observatory of Sainte-Marie-du-Mont, Manche, where M. Hervé-Mangon has placed the resources of his establishment and laboratory at my disposition. I have endeavored to collect large quantities of dust, so that I might carry on microscopic and chemical analyses with precision. For this purpose I used a surface of paper of two square metres, which I exposed horizontally to the air; I collected from it the dust which fell from the air, sweeping it up with a small brush. I called this apparatus a dust-table. The weight of the dust collected upon this surface at Sainte-Marie-du-Mont varied from two to nine milligrammes (.03 to .14 grains troy) in twenty-four hours. The particles shown in Fig. 1 represent minute grains of magnetic oxide of iron, which were drawn out from the dust by the magnet. They are greatly magnified, their real diameter being only 10 of a millimetre 1000 of an inch).
If we evaporate considerable volumes of rain-water, we shall obtain a sediment which represents the air-dust; if, then, we draw a magnet through this sediment, we shall nearly always find little globules of magnetic oxide of iron in it. Fig. 2 represents some of these globules, which I extracted from one hundred quarts of rain-water at Sainte-Marie-du-Mont.
We can distinguish several of these grains which have the form of spherules, or granules that have undergone fusion. The sediment from rain-water collected in Paris at the School of Bridges and Highways at the Trocadéro and the dust collected in one of the towers of Notre Dame gave identical results (Fig. 3).
These facts prove conclusively that the air holds in suspension minute microscopic particles of oxide of iron, some of which assume the form of well-defined spherules.
Similar spherules may be found everywhere in the dust of the air and in rain-water and snow-water. I have found them in the sediment of snow-water collected at the height of 8,800 feet on Mont Blanc (Fig. 4). Since I have called attention to the existence of these ferruginous bodies in the air, several men of science have confirmed my observations, particularly M. Yung, of the University of Geneva, and MM. Schoenaur and Pierre Miquel, who have pursued the study of the
air-dust at the observatory of Montsouris under the direction of M. Marie-Davy.
Whence are these ferruginous particles derived? The spherules have been melted: we can prove this by burning particles of iron and observing them under the microscope (Fig. 5); those which fly off on the striking of a flint are like them (Fig. 6). The spherules we are considering are probably derived from the showers of fire that escape
from incandescent meteorites. I have been confirmed in this view by observing with the microscope the crust of the aërolites in the collection of the Museum of Natural History, in which were perceived rounded grains having considerable resemblance to those we have just noticed. Moreover, the magnetic particles withdrawn from atmospheric sediments have given on analysis reactions indicating the presence
of nickel; of a character, consequently, to cause them to be regarded as partaking of the nature of meteorites. It may be objected that metallurgical operations and the production of oxides in iron-works give rise to similar ferruginous corpuscles. This is true; but how can we explain the presence of the spherules in geological formations that have not been worked over, where they must have been left previous to the existence of man on the earth? M. Stanislaus Meunier and I have found in the grits below the Lias, in the micaceous slates of the Trias, magnetic globules like the spherules which now fall from the atmosphere upon the earth; we may, according to our theory, consider them fossil meteorites.
Beautiful and peculiar crystalline forms (Fig. 7) are obtained when a drop of rain-water or snow-water is evaporated to dryness, the substance of which consists of the nitrate of ammonia contained in meteoric waters.
The dusts that are produced by man in works of industry expose him to terrible dangers. Stone-dressers breathe very minute particles of grit
which perceptibly injures the lungs; the dust of white-lead, and that of the arsenite of copper, which is used to color cloths and papers, have often produced genuine poisonings. The dust of coal, with which the galleries of coal-mines are filled, is breathed by the miners, and produces an affection which Dr. Riembault, of St.-Etienne, has designated as the carbonaceous obstruction of the lungs of miners. He has dissected the lungs of workmen who had labored in the mines for a greater or less length of time, and has found that this pulmonary obstruction goes on continually increasing till it becomes very dangerous. Sections of the lungs of miners show a gradual progress of coloration, from the fresh, rosy color of the lungs of a person who has always lived in the open air, to gray after a few years, and a blackness approaching, that of the coal itself after forty years of labor in the mines. The dust of coal in the mines, when raised up and ignited, either by a blast or by the burning of a little carburetted hydrogen, has sometimes spread fires to great distances in the galleries, burning the workmen and producing terrible catastrophes. M. Galloway has made some important researches on this subject, and has shown that, even if the dust of coal is not directly inflammable, it becomes very combustible when the atmosphere contains traces of carburetted hydrogen. Other dusts are directly combustible, and sometimes produce genuine catastrophes by the fact of their suspension in the air. In 1869 a sack of starch was accidentally thrown down from the top of a staircase in the Rue de la Verrerie, Paris; it burst and scattered through the air a cloud of dust which took fire from the contact with a gaslight at the bottom of the stairs, and caused an explosion. M. Berthelot has observed that special conditions of mixture are required for the actual production of such explosions, and that a hundred cubic metres of air, containing about thirty kilogrammes of oxygen, will completely burn twenty-seven kilogrammes of starch-powder, or eleven kilogrammes of coal-dust. The terrible explosion in the flour-mills at Minneapolis, Minnesota, in May, 1878, was of a similar nature with the explosion of the starch in the Rue de la Verrerie.
- Translated and abridged from the "Revue Scientifique."