6 THE WORLD OF THE SEA. 
and sometimes onctuosité. The numerous salts which are present 
in the sea can never be deposited upon its bed, nor yet drawn 
up with the vapours to be returned to the earth with the rain; 
for certain agents are in action which retain, transform, and pre- 
vent them from accumulating. In this manner the waters always 
possess the same degree of saltness and bitterness, and to-day 
the ocean presents the same chemical and physical characters as 
it did in ages long past. 
If all the salts which are in solution in the sea were taken from 
the water and dried, the quantity is so enormous that, if spread on 
the surface of North America, it would cover that vast continent 
with a layer half a mile thick! The common salt (sodium chloride) 
alone would form a mass only one-third less than the Himalayas, 
and five times the size of the Alps! 
The saltness of the Mediterranean is greater than that of 
the ocean, probably because this sea loses more by evaporation 
than it receives from its rivers. On the contrary, the Black and 
Caspian Seas are less saline. The quantity of salt contained in 
the Dead Sea is so great that its density will enable a man to 
rest upon its surface like a piece of cork on fresh water. 
The sea appears generally less saline towards the poles than at 
the equator; yet there are exceptions in certain localities. In the 
Irish Sea, off the coast of Cumberland, the water contains ;5 of 
its weight of salt; off the French coasts, #;; the water of the 
Baltic, 45; at Teneriffe, 3;; and in the neighbourhood of Spain, +. 
In many places the sea is less salt at the surface than at the 
bottom. In the Straits of Constantinople the proportion is as 72 
is to 62; in the Mediterranean, as 32 is to 29. It is said that ata 
certain depth where the saltness increases, the bitterness decreases. 
At the mouths of great rivers, it is scarcely necessary to say, the 
water is always less saline than upon coasts which do not receive 
currents of fresh water. 
The ocean is in ceaseless motion; its extended surface rises 
and falls, as if softly breathing ; its movements, gentle or violent, 
slow or rapid, are determined by the variations of temperature. 
Heat affects the volume, and, consequently, the weight of the 
water as it expands or contracts. As the temperature falls, the 
