Page:Popular Science Monthly Volume 37.djvu/189

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HE 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