THE SALT NESS OF SEA. 19 



various marine animals form their solid covering, their shell or cara- 

 pace ; and the Infusoria make use of the lime, silex, and potassium, 

 for the same purpose. It is by knowing all the life-history and 

 habits of the C&lenterata that we explain the appearance of those 

 Coral Islands found in the ocean, the existence of which has been 

 a subject of much astonishment, and the history of which shall find 

 a place in another chapter. 



The Pacific and Indian Oceans are studded with islands some in 

 a state of formation which owe their origin to the coral polyps. 

 These extract from the sea water the lime and silex which are found 

 there in the state of soluble salts. In order to grow and develop, 

 they must be continually under water. They are constantly pro- 

 ducing calcareous deposits ; these deposits rise rapidly, and at last 

 reach the surface of the water. Then the seaweed and the rubbish 

 of all kinds that the sea carries along with it, arrested by these 

 emerging masses, cover them with a layer of fertile soil, which is 

 soon again covered with vegetation, as the birds and the waves bring 

 seeds thither. 



The coral islands of the Pacific, which are described in another 

 chapter, are formed in this manner. 



Besides the substances named, sea water also contains, but in 

 mfinitesimally small quantities, metals, such as iron, copper, lead, and 

 silver ; the old copper collecting round the keels of ships some- 

 times so much silver that it has been thought worth extracting ! A 

 curious calculation has been attempted, based on the age of ships 

 and the distance they have gone during all their voyages, to show 

 that the sea contains in solution 2,000,000 tons of silver.* 



The question has often been asked, Whence comes the salt and 

 other substances held in solution in sea water ? If our readers will 

 turn back to the first few pages of " The World Before the Deluge," 

 they will better understand the very simple geological explanation 

 that we are going to give of the origin of the different substances 

 dissolved in sea water. 



In the first stage of our planet, before the watery vapours con- 

 tained in the primitive atmosphere were condensed, and before they 

 had begun to fall on the earth in the form of a boiling rain, the shell 

 of the earth contained an infinite variety of heterogeneous mineral 

 substances, some soluble in water, others not. When rain fell on 

 the burning surface for the first time, the waters became charged 

 with all the soluble substances, which were reunited and afterwards 



* Sir J. Herschel's "Physical Geography," p. 22, gives the basis and details 

 of this calculation. 



