24 GENERAL SCIENCE 



dusty roads, fine dust particles from volcanoes, the salt from the 

 ocean and from all things wearing away. More dust will be found 

 over cities where there is a great deal of smoke. This is often the 

 cause of hazy appearances in the vicinity of cities, and also in the 

 country after a period of drought. Hazy atmosphere is caused 

 by the dust from the roadside and fields which is blown into 

 the air. 



These dust particles give us our beautiful sunsets and cause 

 the air to look blue. Without them the atmosphere or sky would 

 look black. The tiny dust particles in the air may be shown easily 

 by darkening the room and allowing a little beam of light to enter 

 the window. 



Doctor Xavier, a Paris scientist, kept observation on individual specimens. 

 He found that an ordinary hairpin blew away in dust after 154 days. A steel 

 pen nib lasted just under fifteen months. A common pin disappeared in 

 eighteen months, while a polished needle lasted two and a half years. 



At sea the nuclei on which fog particles form are remarkably low in 

 number, ranging from 500 to 3000 as compared with 150,000 in port. On 

 certain days, however, the number rises to 50,000, the nuclei probably being 

 produced as sale particles by the evaporation of spray from waves. 



A block of dense fog 3 feet wide, 6 feet high and 100 feet long contains less 

 than one-seventh of a glass of water, which is distributed among 60,000,000,000 

 drops. During the densest fog of a voyage the diameter of fog particles was 

 found to be 10 microns; just about the limit of visibility with the naked eye. 



The dust in the atmosphere is cooled off by cold currents of 

 air. Drops of water condense on them. These tiny particles 

 of liquid are of such minute size that they float about in the atmos- 

 phere. It might be said that fog is dew on the tiny dust particles. 

 A splendid example of this is seen in the city of London. Foggi- 

 ness has increased with the growth of London and the increase in 

 the production of smoke. Undoubtedly the little tiny dust particles 

 from the smoke furnish something for the water to collect upon. 

 Sometimes this fog is so dense that all street traffic is stopped, and 

 ,some of the stores are closed. It is certain that the degree of den- 

 sity of the fog depends chiefly upon the dust particles. 



Another place which is famous for dense fogs is the Grand 

 Banks of Newfoundland. These fogs are produced by a cold 



