36 The Sea 
siderable amounts, particularly nitrogen, carbon dioxide, and 
oxygen. In the depths of the Black Sea there is hydrogen sul- 
phide, a poisonous gas which suppresses life except in the up-_ 
permost layers of that body of water. No animals or plants 
can exist in the lower reaches of the Black Sea. 
The fluids of ‘our bodies are very similar in composition to 
sea water. In fact, salt water is sometimes used for trans- 
fusions when blood of the correct type is not available. The 
similarity is part of the evidence that all living beings, hu- 
mans included, are descended from marine animals of a re- 
mote epoch. Sea water is not a good beverage, however, be- 
cause prolonged drinking of it tends to concentrate too much 
salt in the body. Too much salt (by a process called osmosis ) 
tends to shrink the cells of the body tissues, resulting in 
severe illness or death. 
WHERE DID THE SALT COME FROM? 
All the salt in the sea is supposed to have originated in 
rocks and minerals from which it has been leached by count- 
less rains for millions of years. In addition, quantities of 
chlorides and fluorides may have been supplied by volcanic 
vents. The story of the little salt mill that could not be 
topped is in scientific disrepute. Nevertheless, the total 
amount of salt in the sea is very great—so much that if it 
were dried out, it would cover all the present dry land to a 
uniform depth of 500 feet. It is difficult to imagine so much 
salt as that being washed out of the earth even after millions 
of years. 
DOES SALT IN WATER HELP THINGS TO FLOAT? 
It is common knowledge that objects float more easily in 
salt water than in fresh water. Swimmers frequently experi- 
