ATMOSPHERIC DUST. 345 



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 over- 

 looked, for they play a part of considerable importance in terrestrial 

 economy, and give rise to real geological formations. Clouds of im- 

 palpable dust, falling from the air in showers of considerable abun- 

 dance, 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 fre- 

 quent in the Cape de Verd Islands, and are called red fogs by the sail- 

 ors. 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 aerial 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 mi- 

 croscopic tests applied by M. Ehrenberg, to be composed of the fine 

 sands of Guiana and to contain the characteristic diatoms and micro- 

 scopic shells of South America. 



Fig. 1. Corpuscles extracted bt the Magnet from the Dust deposited on a Surface 

 of Twelve Square Metres at Sainte-Marie-du-Mont, Makche. (.600 Diameters.) 



Some of these showers originate in volcanic eruptions, from which 

 fine ashes are projected up into the atmosphere, transported to a dis- 

 tance, 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 Avhich 

 they reckoned their dates. In some places the atmospheric currents 

 of dust exert a perceptible mechanical action. Sir Joseph Hooker re- 

 marked, when he was in South America, that the aerial sand-currents 

 on the tops of high mountains were competent to wear down and polish 

 the trunks of trees and produce stria? upon them like that made upon 

 rocks by glacial action. M. Videt d'Aoust has found evidence in Mexico 



