FRESH-WATER ALG. 265 
But this is not all! Wonderful as it may seem, 
the very realms of the air are peopled with dia- 
toms. The atmosphere we breathe contains hun- 
dreds of species, which float about on every breeze, 
and are wafted hither and thither. Many of them 
remain for years in the highest strata of the at- 
mosphere, until carried down in the full capacity 
of life to the nourishing waters of the stream and 
the lake, by descending currents of air. They 
have been found in immense numbers in the 
impalpably fine dust, which at certain seasons 
broods like a thick haze over the island of St. 
Domingo, and occasionally falls in great quantities 
on the decks of vessels far out on the Atlantic. 
The sirocco and trade winds convey immense 
quantities of them for hundreds of miles. Clouds 
of diatomaceous dust, giving the atmosphere an 
orange or ochre hue, have repeatedly been ob- 
served coming in various directions from the coast 
of Africa, falling on vessels, and diffusing around 
a darkness so dense as often to cause them to run 
ashore. Similar showers are not unfrequent in 
China, and spread over several provinces at once 
and far out to sea. They are raised from the 
Mongolian steppes—regions of sand more than 
2000 miles long and 400 broad—and falling into 
the waters of the Yellow Sea, give it that peculiar 
tinge from which it derives its name. During the 
