Page:Handbook of Meteorology.djvu/144

 pulverized horse dung, and foliage dust. The dust carried by winds blowing over areas of orchard and shrubbery usually contains the spores of fungi, pollen in season, the spores of various moulds, the eggs of insects and the dust scales of moths. Winds blowing over swampy areas are apt to have a generous content of the micro-organisms common to swamps. Dry air contains the spores of micro-organisms; moist air is often rich in the organisms themselves.

Bacterium Content of Dust.—Dr. T. M. Prudden exposed Petri dishes, each varnished with a gelatine culture medium, for five minutes in different parts of New York City. The dishes were set aside for several days. Each micro-organism falling on the plates developed into a “colony.” The colonies were counted with the following result:

Examination of dust collected by the author in school rooms and from the book shelves of a public library yielded results similar to those obtained by Prudden.

The foregoing presents general principles worth noting: micro-organisms may fall to the ground and become a part of the dust of a public street. They also float a long time—some of them permanently—in the air. Exposures 1, 5, and 6 show that, when the air is in motion, the bacterium content is much greater than when the air is still. Measurements made at the direction of the Transvaal Chamber of Mines showed that dust particles 1 micromillimeter in dimension required about five and one-half hours to fall a distance of 7 feet. At the Mount Vernon laboratory, particles of the same dimension required from six to ten hours to fall 9 feet. Equally important is the fact that, when the air is stirred by sweeping, the tramp of footsteps, or the passage of vehicles, its bacterium content—and also its dust content—is much greater than when it is still.