In this branch of inquiry great progress has been made, and sea water
is now known to contain at least 32 elementary bodies. Its chief
constituents are found to consist of the chlorides and sulphates of
sodium, magnesium, potassium and calcium. It also contains air and
carbonic acid.
The salinity and density of sea water have been investigated very
thoroughly, particularly in the Atlantic. As the salinity of the sea
water is an index of its density, changes in the former naturally
affect the latter. The salinity has been found generally to decrease in
the neighborhood of coasts, where rivers discharge their water into the
sea, and it is a maximum in the trade zones, and a minimum in the
equatorial rain belt. The salinity is affected by the degree of
evaporation and by the frequency of rainfall, and is now recognized as
an important factor in the biologic conditions of the sea.
Of the three great oceans, the Atlantic, with a salinity of 3.69 per
cent., shows a slight preponderance over that of the Pacific and Indian
Ocean, whose average salinity is 3.68 and 3.67, respectively.
In the trade belts the great evaporation augments the salinity, and
hence, also, the density, and in the polar zones the formation of ice
brings about the same result, though in a lesser degree. In the
equatorial calm region the frequent rainfall diminishes salinity and
density through the dilution of the salt water. Density and salinity
are thus in a certain degree subject to seasonal changes.
In the _Atlantic_ the density increases in general from the higher
latitudes towards the equator, but the maxima are separated by a zone
of lesser density. The maximum in the North Atlantic ocean is found
between the Azores, the Canaries and the Cape Verde Islands, and the
minimum between the equator and 15° N.
In the South Atlantic two maxima occur, one to the north of Trinidad,
and the other near St. Helena and between that island and Ascension.
Taking pure water at 4° C. for unity, the maximum density in the
Atlantic is 1.0275 and in the Pacific, 1.0270.
In the _North Pacific_ the maximum density occurs between 30° and 31°
N., and the minimum in about 7½° N., in the equatorial counter current,
where it was found as low as 1.02485.
In the _South Pacific_, which has a slightly greater density than the
North Pacific, the maximum has been found in the vicinity of the
Society Islands.
The density of the waters of the _Indian Ocean_ is not yet as well
known as that of the Atlantic and Pacific, but the results ascertained
indicate a lesser density in its northern part, with a maximum in the
region between 20° and 36° S. and long. 60° to 80° E.
In the vicinity of Java and Sumatra, probably on account of the extreme
humidity of the atmosphere and of frequent rainfall, the density has
been found as low as 1.0250.
In regard to the density of the water at various depths, it has been
ascertained that as a general rule it decreases from the surface down
to about 1,000 fathoms, after which it increases again slowly to the
bottom. In the equatorial calm regions, however, where the heavy rains
dilute the surface water, the density decreases from the surface down
to between 50 and 100 fathoms, after which it follows the law found for
other parts of the ocean. The bottom densities of the South Atlantic
and Pacific have been found about alike, varying only from 1.02570 to
1.02590; those of the North Atlantic, however, show a greater value,
varying from 1.02616 to 1.02632.