208 ALG. [CHAP. 
threads—Oscillatoriga, and Protococct, described in our 
first chapter, to which we refer our readers. 
Formerly these seaweeds yielded a rich harvest to 
the proprietors of coast-lands. They were collected 
in immense quantities, piled on the shore and burned, ~ 
the result being a solid cake of ash. This ash was 
known as “ Kelp,” and consisted largely of carbonate 
of soda and salts of potash, which are largely used 
in the manufacture of soap. This “kelp” formed a 
source of considerable revenue to those who possessed 
lands on the coast, by the sale of it to the soap 
manufacturers. But modern researches in chemistry 
have shown that carbonate of soda can be more 
cheaply obtained from common salt, and so the 
kelp industry has dwindled to very small proportions, 
and the chief use to which the “ harvest of the sea” 
is now put is in the manufacture of manure. But 
there is also another and a most important use to 
which seaweeds are put—that is, in the production of 
a valuable substance called /odine. The discovery 
was made in the early part of this century by a soap- 
maker of Marseilles, named Courtois, who noticed 
a blue vapour arising from his vats during the rapid 
evaporation of the ley. This circumstance led to 
the discovery of iodine, which is now such an im- 
portant substance in medicine and the arts. 
Certain species of seaweeds are also employed to 
feed sheep and cattle. But probably the greatest 
value of this class of plants consists in the fact that 
they form at once the home and the food of countless 
species of fish, molluscs, and crustaceans; and, 
further, without their presence and oxygenating 
