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.
Fig. 1.—Corpuscles extracted by the Magnet from the Dust deposited on a Surface of Twelve Square Metres at Sainte-Marie-du-Mont, Manche. (600 Diameters.)
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
