60 OF THE INORGANIC CONSTITUENTS OF PLANTS. 



source from which myriads of marine mollusca and corals are 

 supplied with materials for their habitations. 



Whilst the air contains only from 4 to 6 ten-thousandth parts 

 of its volume of carbonic acid, sea- water contains 100 times 

 more (10,000 volumes of sea-water contain 620 volumes of car- 

 bonic acid — Laurent, Bouillon-Lagrange). Ammonia* is also 

 found in this water ; so that the same conditions which sustain 

 living beings on the land are combined in this medium, in which 

 a whole world of other plants and animals exist. 



The roots of plants are constantly engaged in collecting from 

 the rain those alkalies which formed part of the sea-water, and 

 also those of the water of the springs penetrating the soil. With- 

 out alkalies and alkaline bases most plants could not exist, and 

 without plants the alkalies would disappear gradually from the 

 surface of the earth. 



When it is considered that sea- water contains less lhan one- 

 millionth 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 soa-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 element in quanti- 

 ties 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 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 deve- 

 lopment. 



* When the solid saline residue obtained by the evaporation of sea 

 water is heated in a retort to redness, a sublimate of sal-ammoniac if 

 obtai ned. — Marcet. 



