252 THE POPULAR SCIENCE MONTHLY, 



tant 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 aesthetic eye is charmed with their gorgeous trans- 

 formation 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 Fal- 

 kirk, 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 cotton- 

 wool. If a vessel containing common air be placed beside it, the 

 eye is unable to detect any difference in the contents of the ves- 

 sels, so very fine and invisible is the dust. If both vessels be con- 

 nected 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 ordi- 

 nary 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 press- 

 ure 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 con- 

 dense. The fine particles of dust in the air attract the vapor-par- 

 ticles and form fog-particles. When there is abundance of dust 

 in the air and little water-vapor present, there is an over propor- 

 tion of dust-particles ; and the fog-particles are, in consequence. 



