126 THE ART OF CULTURE. 



exist, and without|^plants the alkalies would disap- 

 pear gradually from the surface of the earth. 



When it is considered, that sea-water contains 

 less than one-millit)nth of its own weight of iodine,* 

 and that all combinations of iodine with the metallic 

 bases of alkalies are highly soluble in water, some 

 provision must necessarily be supposed to exist in 

 the organization of sea-weed and the different kinds 

 of Fuci, by which they are enabled during their life 

 to extract iodine in the form of a soluble salt from 

 sea-water, and to assimilate it in such a manner, that 

 it is not again restored to the surrounding medium. 

 These plants are collectors of iodine, just as land- 

 plants are of alkalies ; and they yield us this ele- 

 ment, in quantities such as we could not otherwise 

 obtain from the water without the evaporation of 

 whole seas. 



We take it for granted, that the sea-plants require 

 metallic iodides f for their growth, and that their 

 existence is dependent on the presence of those 

 substances. With equal justice, then, we conclude, 

 that the alkalies and alkaline earths, always found 

 in the ashes of land-plants, are likewise necessary 

 for their development. 



CHAPTER VII. 



THE ART OF CULTURE. 



The conditions necessary for the life of all vege- 

 tables have been considered in the preceding part 



* This substance was discovered in 1812, and is obtained from marine 

 plants; it is found also in sea- water and several mineral springs in 

 combination with hydrogen, as hydriodic acid. With bases this acid 

 forms hydriodates. Iodine has not been decomposed. It is a solid, 

 and at about 350° F. passes into vapor of a beautiful violet color ; hence 

 its name. 



t Compounds of metals and iodine. 



