“APRIL 11, 1884.] 
THE COLORS OF NATURAL WATERS. 
Mr. W. Sprrivne, of the University of Liege, 
has greatly advanced our knowledge of this 
subject in a paper in the Revue scientifique for 
Feb. 10, 1883, a translation of which also ap- 
peared in the Popular science monthly for May. 
He begins with a careful and critical summary 
of the views of previous observers, and from 
these and his own experiments reaches the 
following important conclusions: 1. Water, in 
the purest state in which it can be obtained, 
has a distinct and beautiful blue color, which 
must be regarded as its essential or proper 
color, as the color of absolutely pure water ; 
2. The green, yellow, and brown colors ob- 
served in water are due to the reflection of 
light by matter held in suspension. This sus- 
pended foreign matter is very finely divided, 
and probably is usually in the state of nascent 
precipitation. It may be liquid or solid, trans- 
parent or opaque. 
‘* The important point is, that it be competent to 
reflect light. Then the light-rays of feebler intensity 
(violet, blue, green, etc.) suffer extinction, one after 
another, according to the thickness of the medium, 
till the yellow rays, the brightest to our eyes, are the 
last to survive the struggle. 
“The obstruction of the light, inducing the yel- 
lowish tint, which may be produced by any salt, de- 
pends less on the quantity of the salt present than on 
its being near the stage of precipitation. Small quan- 
tities of a feebly soluble salt produce the same effect 
as large quantities of a more soluble salt. The vari- 
ety in the colors of natural waters, then, may be thus 
explained: absolutely pure water, viewed in masses 
of sufficient thickness, has a beautiful blue color. If 
it holds in complete solution colorless salts, its color 
is not changed; but, in proportion as it may contain 
matter on the verge of precipitation, the light travers- 
ing it will be of a yellow or darker color, until a stage 
is reached when the liquid will let no light through, 
and becomes opaque or black. The yellow light will 
combine with the blue light of the water; and thus 
will be produced greenish-blue, bluish-green, and 
green tints, according to the strength of the yellow. 
If the latter is very strong, the dark blue will be 
wholly smothered, and the water will appear yellow, 
brown, or of a still darker color.’’ 
The less soluble bodies in natural waters — 
those which may be regarded as frequently 
in the state of nascent precipitation, and to 
which the colors are chiefly due — are the car- 
bonates of calcium, and silica, and also, proba- 
bly, the finest mechanical sediment or clay, 
which, although not properly soluble, forms an 
emulsion with water, and affects the light in 
the same way as an incipient precipitate. 
This theory appears to me to be the only one 
yet advanced affording a consistent explanation 
of all the phenomena; and my present pur- 
pose is to call attention to a general and im- 
SCIENCE. 
445 
portant fact concerning the color of natural 
waters (which appears to have been but little 
noticed or appreciated by scientific observers, 
and of which I have never seen any explana- 
tion), and to show that it harmonizes beauti- 
fully with Mr. Spring’stheory. Briefly stated, 
this general fact is as follows: tropical and 
warm seas are blue, and polar and cold seas 
are green; i.e., other things being equal, the 
color of the water is determined by the tem- 
perature. 
All voyagers in tropical seas must have no- 
ticed the magnificent blue color of the water ; 
the color seeming to be purest and most intense 
under the equator, or where the water is warm- 
est. On passing to higher latitudes and lower 
temperatures, the color changes to greenish 
blue, bluish green; and green, although the 
Gulf Stream and other warm currents carry 
the tropical color and temperature well up to- 
ward the frigid zones, and into the midst of seas 
whose prevailing tint is deep green. Probably 
nothing makes the Gulf Stream seem more real, 
especially to the unscientific observer, than the 
creat contrast in color that is presented within 
a very short distance, when we cross its north- 
ern wall. On one side is the cold, green water 
of the polar current. and on the other the 
warm, blue water of the Stream. The differ- 
ence in color between the Gulf Stream and the 
surrounding parts of the ocean is noticeable 
even in the North Atlantic, on the track of the 
transatlantic steamers; and I have found that 
this part of the sea is perceptibly greener in 
winter than in summer. 
As already stated, this general difference in 
color between warm and cold seas, although 
not explained by Mr. Spring, is a necessary 
corollary of his theory: for. warm water is, 
for most substances, a more powerful solvent 
than cold water; and if cold seas are green in 
consequence of some of the contained salts 
being imperfectly dissolved, i.e., in a state of 
nascent precipitation, then an increase of tem- 
perature in the water, by augmenting its solvent 
power, will tend to obliterate the green color, 
and restore the blue. Again: warm water 
possesses less adhesion than cold water, which 
would cause a more rapid deposition of fine 
clayey matter in warm water than in cold. 
Hence, if the green color of cold seas is due, 
not to imperfectly dissolved salts, but to the 
suspension of fine insoluble clays, forming an 
emulsion, an increase of the temperature of 
the water, by causing a more complete deposi- 
tion of the suspended clayey matter, will also 
tend to change the color of the water from 
green to blue. 
