20 THE OCEAN WORLD. 



deposited,- accumulating in the large depressions of the soil. The 

 seas of "the primitive globe were thus formed of rain water, holding 

 in solution all that the earth had given up, collected in large basins. 

 Chloride of sodium, sulphates of soda, magnesia, potassium, lime, 

 arid silex, this latter in the form of a soluble silicate ; in a word, 

 every soluble matter that the primitive globe contained formed part 

 of the mineral contingent of this water. If we reflect that through 

 all time up to the present day none of the general laws of Nature 

 have changed if we consider that the soluble substances contained 

 in the water of the primitive seas have remained there, and that the 

 fresh water of the rivers constantly replaces the water which disap- 

 pears by evaporation we have the true explanation of the saltness 

 of sea water. " It is a very simple theory, it is true," adds M. Figuier, 

 "but one that we have found nowhere, and the responsibility of 

 which we therefore claim. The chloride of sodium is by no means 

 the only substance dissolved in sea water. It contains, besides, 

 many other mineral substances ; in short, every soluble salt on the 

 face of the globe, and, along with them, portions of different metals 

 in infinitely small quantities." 



The mean temperature of the surface of the sea is nearly the 

 same as the atmosphere, so long as no currents of heat or cold in- 

 terpose their perturbing influence. In the neighbourhood of the 

 Tropics, it appears that the surface of the water is slightly warmer 

 than the ambient air, but experiments on the temperature of the sea 

 from the surface to the bottom reveal, according to our author,* 

 "some evidence which establishes a curious law. In very deep 

 water a perfectly uniform temperature of 4 below zero prevails, 

 which corresponds, as physics have established, to the maximum 

 density of water. Under the Equator this temperature exists "at 

 the depth of 7,000 feet. In the Polar regions, where water is 

 colder at the surface, this temperature is maintained at 4,600 feet. 

 The isothermal lines of 4 form a line of demarcation between the 

 zones, where the surface of the sea is colder, and those where it is 

 warmer than the bed of four degrees below zero." This is more 

 clearly shown in Fig. 4, which represents a section of the ocean, the 

 curved line which touches two points at the surface indicating the 

 depths where the temperature is constantly fixed at 4. 



Dr. Maury's account of this phenomenon is asserted with less 

 confidence. The existence of an isothermal floor of the ocean, as he 

 calls it, was first suggested by the observations of Kotzebue, Admiral 



* " La Terre et les Mers," p. 517, troisieme edition. 



