ATMOSPHERIC DUST. 177 



ATMOSPHERIC DUST.* 



By Db. 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 be- 

 longing 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 phenom- 

 ena, such as fogs, as it is generally admitted that fogs are due to 

 the deposit of moisture on atmospheric motes. Again, the scat- 

 tering 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 abun- 

 dantly. 



The origin of dust may be looked upon, without exaggeration, 

 as universal. Trees shed their bark and leaves, which are pow- 

 dered 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 vege- 

 table 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 



* Abstract of an address delivered before the Royal Meteorological Society, January 

 15, 1890. 



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