208 ALGJE. [CHAP. 



threads Oscillatorice, and Protococci, 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 Iodine. 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 



