Popular Science Monthly/Volume 41/June 1892/The Colors of Water
|THE COLORS OF WATER.|
By CARL VOGT.
"GRANDPA," asked my two grandchildren, as if with one voice, "shall we pass over the blue lake when we go to Geneva?"
Our residence at Salvan, a charming village of the canton Wallis, about a thousand metres above the level of the sea, was nearing its end. The return journey was the subject of lively conversation, and we were almost entirely occupied with the fancies of the children, who asked no end of questions. "We shall go across the blue lake," I said. "First we shall go down to the station. Grandma and I will go in carriages, you others will walk. Then we shall take the railway train and go down to the lake. The steamboat will be there, and as soon as we are aboard—"
"Grandpa," interposed the others, "why is the lake so blue?"
I was somewhat confused by these questions. If a fool can ask more questions than ten men can answer, a child can perplex more than a hundred grandpas. An evasive reply, like "It is blue because it is not yellow like the Oder at home," was not available with my children. It is an old observation that the simpler a phenomenon of Nature appears at first sight, the more complicated it is in fact; but it is always well to recollect that there is no simple phenomenon in Nature, that all that ever happens or is perceived by our senses is only a result of very different or even opposing forces and causes, which we must not only learn by observation, but must also separate from one another by experiment, if we would come to a conclusion that has hands and feet. Every one can see that the Lake of Geneva is blue, and most persons regard the subject as quite simple and clear, without probing further into the causes of the blue color. But if a child in his naïve candor asks for the reason of this color which has struck him because the waters of his home are not like it, there floats before the mind of the expert an unanticipated multitude of problems in optics involving the most difficult laws and broad knowledge, and over which mathematicians and physicists and students of every kind, artists and poets, have racked their brains, without having ever reached a definite solution. How, then, shall he convey ideas to a child which shall give an answer to the question adapted to his powers of comprehension?
I was thinking of preparing a small water-color drawing, when the children asked their eager question. "Why should I not invoke the Muse to the help of art? A large cylindrical glass, holding a litre, stood on the table, filled with the superb water that gushes out of the slates fresh and cool, crystal clear, and chemically pure.
"Look at the water in the glass," said I; "of what color is it?"
"I don't see that it is of any color," said one. "It is red," said the other.
"But that comes from the flowers that are behind it," replied Annie. "Come round to where I am; it doesn't look red from here."
Lili ran round the table, and confessed, a little vexed, that the water was not red. She had a disposition, perhaps, like Lessing's, who was dissatisfied because the spring was always green, and not, by way of change, sometimes red.
"Is it not true, grandpa, that water has no color?"
"Yes, dear child, it is blue, but so little so that you can not see it."
"Can you see that it is blue? "
"No; but still it is blue. Look at this."
I took a little ultramarine on the end of the brush and mixed it with the water. "Does it look blue now? "
"No; I see nothing."
"Nor I. But you saw how I put a little blue color in it with the brush."
"Yes, but there was not enough of it. Put more in."
I silently took the glass and set it on a piece of white paper in the bright sunshine. "Now look from above down into it."
"It is blue!" said the little one, clapping her hands, "but only a very little."
"Look at it from the other side, where the sun is shining into it. Is it not a little bit red, like the bell-flowers which you picked yesterday? "
"That is wonderful," said the little one. "It is blue from above, a little bit red in the sun, and when we look at it from this side of the room we see nothing!"
"Think about it a little. The glass is as broad as my finger is long. But it is at least three times as high as my finger. When you look at it from the side, you see only a finger's length of water; but when you look down into it, you see through three fingers' length of water three times as much. You see it blue from the side, and three times as blue from above, don't you? "
"Is that really true?" said the little one, as she measured with her finger. She nodded that she was satisfied.
"Now imagine that the water is as deep as the height of the church-steeple, and deeper—that it reaches from here up into Salvan and down to Vernayaz. Then you would see the water from above it all blue."
"Is the lake, then, really so deep? "
"Yes, and deeper."
I will not continue the conversation any longer. It went on with various simple experiments, beginning with differently colored stones, which I let drop into the water, and then placed on the white, then with setting the glass with its weakly bluish contents on differently colored papers, and ended with my trying to make the children perceive how the colors changed when they were seen through the whole depth of the glass. I will not say that the little ones were brought to a full comprehension of the matter; but they stuck fast to the assertion that water is blue, of an infinitely weak blue, and that the blue color can not be seen till one looks into a certain depth of it.
Physicists first acquired this knowledge by means of an experiment of Bunsen's, who let a piece of white porcelain fall into a tube filled with distilled water, and satisfied himself that the descending piece looked bluer as it sank deeper. Bunsen had, of course, provided that only white light reflected from the ceiling of his room should fall into the tube, and not the blue light of the sky. The experiment has been modified in various ways, and made more convenient, but has always given the same result; and it is now established as a scientific truth that chemically pure water, free from all other constituents, either dissolved or floating, has a bright, clear, blue color.
But there is no such water in Nature, for rain-water, even distilled—water evaporated out of the sea and everywhere, and carried on in the form of clouds, and falling in drops—even that rainwater contains some dissolved substances, and still more of little microscopic bodies that are floating in the air which the drop carries with it in its fall.
Yet we can assure ourselves at least as to the dissolved salts, in which sea-water, for example, is rich, that they are all, particularly common salt, colorless in the crystalline condition, and therefore have not the least influence on the color of sea-water. Seamen and sailors, although uninstructed in this matter, and without knowledge, know very well that they, going away from the coast, in a short time reach the clear, the "blue water," and then sail over deeps till they can not reach the bottom with their anchors.
I have already said that every phenomenon in Nature is a complex affair, and depends on many causes and conditions. This is true of the coloring of large masses of water, as of lakes and seas, which are indeed, as is known, of very different shades. It may be permitted to consider here a few of the conditions that have an influence on the general effect.
A still-water surface forms a mirror which reflects those colors of the horizon that fall upon it at the same angle as that under which the eye stands to it. When I am at the shore of a quiet sea, or of a still lake, the water beams with the colors of the horizon. In a bay surrounded by woods, I see a deep gray; on the broad surface at sunset, the liveliest yellows and reds; looking straight down from my boat, the blue of the sky over my head.
These reflected colors concern the physicist the least, for he knows that every reflecting surface returns them; but they interest the painter almost exclusively. They constitute the tone of his landscape; they enliven the otherwise monotonous, dead surface, and he as well as the looker at his picture receives chiefly the impression of them. They are for the most part the colors of the lower horizon, for the point of view of the spectator is usually only a few metres above the level of the water.
Thus with the smooth mirror. But the scene changes immediately upon the slightest agitation. It is very seldom that the sea is quite still. The waves form hills and valleys, their surfaces are more or less inclined, and they reflect not the horizon with its down-toned colors, but the more saturated tints of the zenith. Whoever has seen the Mediterranean Sea or the Lake of Geneva under a cloudless sunset, and a slight rippling of the waves, will recollect that the surfaces glowing with burning yellows and reds, are broken up by sharp, deep-blue lines; they are the wave-valleys, which, by reason of their oblique inclination, turn the blue colors of the zenith into the eye. But this is not all. With the smooth mirror surface, and lower point of view, the eye not only receives the rays reflected from the surface, but it pierces through the inclined parts of the wave-valleys into the mass of the water, and thus perceives the proper color of the water, more intensely as the small surface of the wave-valley stands more perpendicularly to the eye. If the waves are very short, and follow one another rapidly, this impression of the color of the water will overcome that of the reflection. I can satisfy myself of this fact at any time.
The windows on the western front of my house look toward the Arve, which is here crossed by a dam that causes a fall of about a metre. Above the dam, the glacier-stream, colored a grayish yellow in summer and green in winter, is perfectly smooth; and from my windows, which are situated about six metres above the river, I can see hardly any but the mirror colors, yet a little mingled with the proper color of water, which appears considerably stronger when the sky is covered and its glaring light does not—as the painters are accustomed to say—"eat up" the softer tones. But below the dam the water is in lively motion with endless little racing wavelets, and here the green color comes out so vividly that the reflection almost wholly vanishes. Yet another fact is revealed by the movement. Only with a perfectly smooth mirror surface are the outlines of the reflected objects fully clear and sharp. The reflection is then so perfect that one often hardly knows whether he sees the objects themselves or only their mirrored images on the water; and the lines between water and shore are quite effaced. The slightest movement causes the outline of the mirror picture to appear notched; clearer lines from the horizon creep into the darker colors of the picture, but notches from these, too, spring out over the lines which the outline should have. This phenomenon is so common that we notch the borders of water reflections in colored pictures, as well as in those drawn only in black. I have no doubt that a relationship lies at the bottom of this phenomenon like the fact observed by Colladon, and now often remarked, that water in motion carries the light along with it. A stream of water, flowing through a dark tube out of an illuminated receiver, carries the light along, whether it be white or colored, and shines; why should not moving waves exhibit the same effect?
But enough of these painters' impressions, which, as we have said, are neglected by the physicist, but are still of the highest significance for the beholder as well as for the artist, and, as may result from our representation, are dependent on various factors, among which, besides mirroring, the real color of the water is to be considered.
Let us go a little closer into this matter.
Pure or colorless water containing salts in solution is beautifully blue and perfectly transparent, at least to a certain depth. It is, hence, clear that with the color of all objects visible at this depth, and constantly reflecting the rays of light, is associated under the water a blue tone, more intensive as the depth at which the object lies is greater. The gravels on the shore of a lake or the sea become, when seen through the blue water, as if they were observed through a pane of blue glass; and since all shore figures, with trifling exceptions, are of a yellowish color, they will shine of a more or less green color, and the water on the shore will likewise appear green.
I here lay aside all physical deductions concerning the nature of color. We know that it is not, as was once thought, a property of bodies, but that a transparent body like water, for example, shows a distinct color, because it lets certain colored rays through, but not others, and that a solid body reflects the rays which we perceive, but to a certain extent absorbs the others. The discussion of the nature of color is not of very great importance for our essay.
Blue water also takes on another tint when objects lying on the ground are seen through it, and this mixed color tone depends on the color of the ground. We can easily verify this by the simple experiments described above with blue colored water in a cylinder glass. White bodies, pieces of porcelain for instance, appear light-blue, yellow-green, red-violet, and, the deeper they sink, the more is this shading from blue washed out, till it is destroyed. The red shades vanish first of all.
The depth to which no trace of bottom-colors reaches us is certainly not little, and may, under favorable conditions, be estimated at several hundred metres. But the question is a large one, and we will consider a little more carefully to what extent the more or less favorable conditions I have mentioned have been determined.
I have already said that pure water does not exist in Nature. It always must contain dissolved or floating substances which will change its colors. Peat waters contain brown and blackish organic matters in solution. They may be perfectly clear and transparent, but the colors which the humus acids and similar substances lend them will always produce a certain effect upon them, which will be re-enforced by the dark-brown or black colors of the bottom of the peat lakes. It has also been observed that filtered water from a blue lake on evaporation leaves a white or light gray, and that from green lakes a yellow sediment; and that thus blue lakes contain white matters and green lakes yellow ones in solution, whose colors produce with those of the water mixed tints. The difference in the colors of the Lake of Geneva and of the Bodensee is explained on this principle, but the results of the experiments on which the conclusion rests have been disputed, and there is much room for doubt on the subject. Whatever may be thought of this, it is certain that no water in Nature is perfectly clear and transparent, but is more or less turbid by the presence of other substances floating in it. That this turbidity is of greater or less importance, that we can distinguish at greater or less depths objects swimming in the water, like fishes, or lying on the bottom, are taught by daily experience as well as by experiments which have been made by sinking solid bodies in sunlight and on cloudy days and at different seasons, or by letting down sources of light, such as burning lamps and incandescent electric lights, and ascertaining the depth at which a perceptible glimpse of them can be obtained. It is to be regretted that these as well as other experiments upon the penetrating power of light have been made only in waters not quite clear, as in a few Swiss lakes and the Mediterranean Sea. Whoever has traveled on the coasts of Norway must have been astonished at the transparency of the water in many of the fiords; it is also affirmed that in some of the North American lakes the eye can perceive objects on the bottom at the depth of several hundred metres. Visibility extends to no such depths in either the Lake of Geneva or the Mediterranean Sea. The water of the Lake of Geneva is more transparent in winter than in summer, but in this lake, as well as in the sea-waters that have been thus far examined, the extreme limits of visibility are at forty-five, and at most fifty metres' depth. Observations in diving apparatus have shown that one is there as in a blue cloud, and can only see some seven or eight metres in a horizontal direction, in exceptional cases twenty metres, and at most twenty-five metres. But the seeing man can dive with the apparatus only to a depth of thirty metres, and, although he can not see clearly, he is surrounded by diffuse light.
The light from above must therefore penetrate more deeply. A more closely approximate measurement has been made by such means as sinking sensitized photographic plates into the water, and exposing them to the light at fixed depths, or by sinking substances which are chemically acted upon, changed, or destroyed by light, so that the measure of the alteration may at the same time furnish the measure of the strength of the acting light. Photographic experiments have shown that a depth of four hundred metres in the Mediterranean Sea is the average limit to which a blackening of the plate can be verified.
Thus light penetrates to ten times as great a depth as our eye, and this is an important point—a whole zone, three hundred metres in thickness, receives light and thus also sends up rays which our eyes can not immediately distinguish, but in all probability perceives through the mixture of the color tones which they produce. It is known that there are other differences than those of blindness to certain colors in the eyes of men, and that our organs may be trained to an extraordinary degree of delicacy in the observation of the finer tints. I once visited the Gobelins tapestry factory in Paris in company with some painters; the workmen could distinguish with ease and indubitably tints which looked identical to our unskilled eyes. There must, to return to our subject, radiate up from that depth to the surface, light, of a bluish color, which makes far less impression on our eyes than the colors called warm, yellow and red, which—especially the latter—are absorbed by the water.
It was formerly believed that total darkness reigned in the greater depths of a thousand metres and more, and that the collected colors of deep water were seen on a black ground. But, in the light of the recent deep-sea investigations, this idea must be given up, along with the other one that once prevailed, that there is no animal life in great depths. Most animals living in dark caves have atrophied or no eyes; there are also living beings found on the surface of the earth, which hide themselves in dark places, under the ground, etc., and are blind. Similar conditions prevail in the great deeps. There are blind crustaceans there, which probably live in the mud and under stones, while others, moving animals, fishes, have large, well-formed eyes. It must be that they see, or in other words that there is light there. Whether this light is produced in the depth by means of the phosphorescent organs which many of these animals, even fishes, possess, or whether it penetrates from above, as might perhaps be concluded from the fact that some of the deep-sea animals whose organization compels them to creep on the ground have yellow and red colors on their backs, is of no importance so far as our inquiry is concerned. We can only reach the inevitable conclusion that we see the colors of water not on a dark or black ground but on one that is illuminated, if but faintly. This is of moment because, in the light of it, particles floating in water are illuminated not from above only, but from below too.
We can satisfy ourselves of the effects of the coarser floating matter of sand and mud, as well as of the fact that the color of masses of water depends to a large extent upon the color of such matter. The Arve, which flows in front of my windows, is grayish yellow in summer, and opaque, assuming a deeper color after rain-storms; in winter, on the contrary, it is green, semi-transparent, and greener and clearer the less water it carries; facts easily explainable upon principles which one of my pupils nearly established by observations continued through a whole year. In summer the Arve carries, with the surplus glacier-water, grayish-yellow fragments of the mountain rocks in great multitudes; after heavy rains, masses of yellow mud are added to these, having been washed away from the banks of the stream. In winter the amount of sediment derived from the glaciers is small, and the blue color of the water is transformed into the green mixed color. Every glacial stream has its individual color, derived from the disintegrated rocks; and it is not without reason that the two rivers which join at Zweilütschine, in the Bernese Oberland, are known as the Black and the White Lütschine. The one brings disintegrated white limestone, the other the emery of pulverized dark slates.
How extraordinarily strong the mixed colors produced by sedimentary matter may appear was shown me by an observation which I made at Nice at the end of December, 1889. The weather had been fine for a few days, and the sea, which I overlooked from my window to Cape Antibes, about fifteen kilometres away, had been unusually blue. Now came stormy weather, with sporadic showers in the mountains of the Var. The river, whose mouth is about six kilometres from my house, poured considerable masses of saturated ochre-colored water into the sea, and there was a sharp boundary of waves between the clay-yellow tongue which continually licked itself farther into the sea, and the deep-blue salt water. After a few hours the yellow tongue became bordered with a widening green band, so brightly, so poisonously green, that I was induced to apply my whole stock of green (vert Paul Veronese) to the completion of a study on which I tried to fix the phenomenon as truly as possible. Under the blowing of the west wind the tongue stretched itself out farther, to the rocky shore behind the harbor of Nice, around toward the bay of Villa-franca; and when I visited the latter place the next day the water appeared, not steel-blue as usual, but green, fully green; and the fishermen of the zoological station there complained that no marine animals could be found swimming around, because they had fled from the green water. The blue color returned after a few days. The green was produced by the finer yellow floating matter; the coarser particles had already sunk.
The finer matter keeps afloat for a very long time. G. Bischof put some of the flood waters of the Rhine in large casks, and deposited these in the cellar of the chemical laboratory at Bonn. The finer particles had not yet entirely settled, and the water had not become clear, after several months of absolute stillness. It is plain that in a lake, in which the continual inflow and outflow keep up a constant current, though it be slight and unremarked by ordinary observers, fishermen and rowers, these fine floating particles will never come to rest, and that, since they have a yellow color, this will appear more intense in the deeper parts, because a larger number of yellow particles are floating in the thicker layers of water there. But, farther away, the shades which the floating matters of single brooks and rivers exhibit vary endlessly between gray, yellow, and reddish, and there result the most diversified and delicately shaded mixed colors, with constant variations according to the quantity of floating matter that is carried into the water-basin. Also in the sea, which is never quiet, the fine floating matter keeps afloat for a long time, and is distributed over immensely large surfaces.
Organic matters, plants and animals, have effects similar to those of mineral substances. The shores are covered with numerous plants; they grow on the lakes in all stages of green and brown (many microscopic plants, which cover the rocks as with a slime, are yellow or brown); green plants grow on the sea-shores to a depth of thirty metres, yellow and red sea-weeds to a still greater depth, forming semblances of woods and meadows, and mingling their colors with those of the water. Even in northern seas there are numerous stationary animals, sponges, solens, mussels, masses of which develop a definite color; while visitors to southern seas are unable to say enough of the splendid colors conjured up by the coral reefs.
But even this is not all. All lakes and seas swarm with swimming or "pelagic" plants and animals. Green and yellow, one-celled, microscopic algæ are exceedingly common to a considerable depth; and green and yellow algæ sometimes come to the front in such masses that "the Red Sea" becomes no arbitrary designation, but the correct expression of an observed fact. I have seen the bay of Villafranca colored partly red by millions of swimming Anchinia rubra about as large as peas; I have seen mile-long strips, several metres broad, immediately along the shore on the Riviera, colored a deep royal blue by compressed masses of swimming salleemans (Velella spirans).
We can not absolve the transparent swimming water-organisms, from the larger medusa down to the infinitesimal microbes, from having a certain amount of influence on the color of water. We should not be able to see their crystal-clear bodies if they did not refract the rays of light in a different direction from the surrounding water. By this means they send out a multitude of refracted rays, which singly are of little importance, but in the aggregate must produce an effect through their accumulation when millions of these living beings are crowded into a cubic millimetre. To what purpose should we have in some parts of the retina of our eye a million of sensitive elements or rods to the square millimetre, if we could not seize single impressions and unite them into a view of the whole?
Finally, we will not forget the air that is mixed with the water. If we shake a viscous fluid in the air, it becomes whitish, and at last white, like milk. Yet the fluid and the air are both transparent. But the air-bubbles scattered through the water refract the light in another way. The wave looks whitish, quite white on its edges, from the inclosed air, and as the motion grows stronger the white becomes more prominent, with a greenish tone when the water is clear and the sky clouded, radiant yellow in sunshine, and clay-yellow when the water is not clear. All these tones mix with the colors of the deep, and with the mirror-colors of the surface. Thus the question of the causes of the colors of water rises to be one of the most complex problems of science as well as of art, the full solution of which has not yet been reached, in spite of the various efforts of men of science and of pictorial artists, because in order to meet the apprehension of the common eye they have to continue into a picture the endlessly changing colors and shapeless figures which the sea affords. But when I stand before a wave painted by Mazure in Paris (he is there usually called Mazure le Vague, the Wave-Mazure), and see how that artist, without help of shore, walls, buildings, or ships, which support the eye by their forms, shows me a wave from the sea with its reflected and refracted colors harmoniously mingled with the bottom tints issuing from the deep and with the proper color of the water itself, my arms, as they say, fall from my body. And it is then hard for me to realize that the colors of water in general are composed of a multitude of factors, among which the most important are the normal blue of pure water, the mirror-colors of the surface, the refracted colors of the moving parts, the proper colors of bodies swimming in the water, and the colors of the bottom or of only very softly illuminated parts shining up through the mass.
In this, as in everything, the principle is true that there are no simple phenomena in Nature, but that all are only the result of a number of single factors, the aggregate effect of which we observe and perceive with a very imperfect instrument—our eye.—Translated for The Popular Science Monthly from Die Gartenlaube.