Physical geography. By Mary Somerville ...

200 PIIYSICAL GEOGRAPHY. CHAP. XVI
nished gold. The waters of the ocean also derive their colour from
animalcules of the infusorial kind, vegetable substances, and minute
particles of matter. It is white in the Gulf of Guinea, black round
the Maldives; off California the Vermilion Sea is so called on account of the red colour of the infusoria it contains; the same red
colour was observed by Magellan near the mouth of the river la
Plata. The Persian Gulf is called the Green Sea by eastern geo.
graphers, and there is a strip of green water off the Arabian coast
so distinct that a ship has been seen in green and blue water at the
same time. Rapid transitions take place in the Arctic Sea, from
ultramarine to olive-green, from purity to opacity. These appearances are not delusive, but constant as to place and colour; the
green is produced by myriads of minute insects, which devour one
another and are a prey of larger animals. The colour of clear
shallow water depends upon that of its bed; over chalk or white
sand it is apple-green, over yellow sand dark-green, brown or black
over dark ground, and grey over mud.
The sea is supposed to have acquired its saline principle when the
globe was in the act of subsiding from a gaseous state. The
density of sea-water depends upon the quantity of saline matter it
contains: the proportion is generally a little above 3 per cent.,
though it varies in different places; the ocean contains more salt in
the southern than in the northern hemisphere, the Atlantic more
than the Pacific. The greatest proportion of salt in the Pacific is
in the parallels of 22~ N. lat. and 170 S. lat.; near the equator it
is less, and in the Polar Seas it is least, from the melting of the ice.
The saltness varies with the seasons in these regions, and the fresh
water, being lightest, is upppermost. Rain makes the surface of
the sea fresher than the interior parts, and the influx of rivers
renders the ocean less salt at their estuaries; the Atlantic is brackish
300 miles from the mouth of the Amazon. Deep seas are more
saline than those that are shallow, and inland seas communicating
with the ocean are less salt, from the rivers that flow into them; to
this, however, the Mediterranean is an exception, occasioned by the
great evaporation, and the influx of salt currents from the Atlantic.
The water in the Straits of Gibraltar at the depth of 670 fathoms
is four times as salt as that at the surface.'
Fresh water freezes at the temperature of 32~ of Fahrenheit;
the point of congelation of salt water is much lower [28~ F.]. As
the specific gravity of the water of the Greenland Sea is about
102664, it does not freeze till its temperature is reduced to 28~ of
Fahrenheit, so that the saline principle preserves the sea in a liquid
This anomalous result, given on the authority of Dr. Wollaston, has not
been confirmed by the recent analysis of water taken near the Straits by
M. Coupvent de Bois, and examined by the eminent chemist, M. Barral.