The saltness of sea water makes it more fitted to float ships, because its density is increased by the salts which are dissolved in it. Besides this, these salts contribute to prevent the water becoming contaminated with decomposed organic matter. By the table representing the composition of the water of the ocean and of that of the Mediterranean, we see that salts of lime and potassium, as well as iodine and silica, are only found in infinitely small quantities. Nevertheless, the lime and silica contained in the sea water are of very great importance; for these quantities, which appear to us so small in the table of a chemical analysis, become enormous in the entire extent of the ocean. The marine plants take in the lime, the silica, the potassa, and the iodides which are dissolved in the sea water; these mineral substances enter into their textures. It is from the carbonate of lime and silica that the marine animals form their solid covering, their shell or carapace. The infusoria make use of the lime, silica, and potassa for the same purpose. It is by the life and habits of the polypi that we explain those _Coral Islands_ found in the sea, the existence of which has been a subject of much astonishment, and ought, therefore, to find a place in this chapter. The Pacific and Indian Oceans are studded with islands in a state of formation, which owe their origin to the polypi and corallines. These zoophytes extract from the sea water the lime and silicium 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 producing calcareous deposits; these deposits rise rapidly, and at last reach the surface of the water. Then the seaweed and rubbish of all kinds that the sea carries along with it, arrested by these emerged masses, cover them with a layer of fertile soil; which is soon 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, in infinitesimally small quantities, metals, such as iron, copper, lead and silver. The old copper collecting round the keels of ships sometimes 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 two million tons of silver.[3] 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 different substances dissolved in sea water.