Page:Popular Science Monthly Volume 40.djvu/264

252 influence upon the imagination, as well as upon the comfort of man. Though so small that a microscope magnifying 1,600 diameters is required to discern them, they at times sorely tax the patience of the tidy housekeeper and the skill of the anxious surgeon. An æsthetic eye is charmed with their gorgeous transformation effects; yet some are more real emissaries of evil than poet or painter ever conceived.

Until the famous discovery made by Mr. John Aitken, of Falkirk, a few years ago, no one could reasonably account for the existence of rain. It was said by physicists that cloud-particles were attracted by the law of gravitation under certain conditions of temperature and pressure. But this famous experimentalist and observer found out that without dust there could be no rain; there would be nothing but continuous dew. Our bodies and roads would be always wet. There would be no need for umbrellas, and the housekeeper's temper would be sorely tried with the dripping walls.

A very easy experiment will show that where there is no dust there can be no fog. If common air be driven through a filter of cotton-wool into an exhausted glass receiver, the vessel contains pure air without dust, the dust having been seized by the cottonwool. If a vessel containing common air be placed beside it, the eye is unable to detect any difference in the contents of the vessels, so very fine and invisible is the dust. If both vessels be connected with a boiler by means of pipes, and steam be passed into both, the observer will be astonished at the contrast presented. In the vessel containing common air the steam will be seen, as soon as it enters, rising in a close white cloud; then a beautiful foggy mass will fill the vessel, so dense that it can not be seen through. On the other hand, in the vessel containing the filtered, dustless air, the steam is not seen at all; though the eye be strained, no particles of moisture are discernible; there is no cloudiness whatever. In the one case, where there was the ordinary air impregnated with invisible dust, fog at once appeared; whereas in the other case, the absence of the dust prevented the water-vapor from condensing into fog. Invisible dust, then, is required in the air for the production of fog, cloud, mist, snow, sleet, hail, haze, and rain, according to the temperature and pressure of the air.

The old theory of particles of water-vapor combining with each other to form a cloud-particle is now exploded. Dust is required as a free surface on which the vapor-particles will condense. The fine particles of dust in the air attract the vapor-particles and form fog-particles. When there is abundance of dust in the air and little water-vapor present, there is an over proportion of dust-particles; and the fog-particles are, in consequence,