Popular Science Monthly/Volume 23/May 1883/On the Colors of Water
|ON THE COLORS OF WATER.|
OF THE UNIVERSITY OF LIÉGE.
VIEWED in relatively shallow masses, clear water appears wholly colorless. In our daily dealings with the liquid we seldom have occasion to observe it in great depths; hence it has been generally believed that water is quite destitute of color. The ancients were accustomed to explain the transparency of some bodies by assuming that they partook of the nature of water; and we now speak of a diamond as of the first water, to emphasize its perfect transparency and colorlessness. If, however, we regard the larger masses of water in nature—the seas, lakes, and rivers—we shall receive a different impression. In these, the water not only appears colored, but of various colors, and of a rich diversity of shades. The Mediterranean is of a beautiful indigo, the ocean is sky-blue, the Lake of Geneva is celebrated for its lovely and transparent azure waters; the Lake of Constance and the Rhine, the Lake of Zurich and the Lake of Lucerne, have waters quite as transparent, but rather green than blue; and the green waters of the little Lake of Kloenthal, near Glaris, can hardly be distinguished from the surrounding meadows. Other waters are of a darker color, like those of the Lake of Staffel, at the foot of the Bavarian Alps, which was quite black the day I saw it, though clear in shallow places.
These facts start the questions whether water, after all, has not a color; if it has, what the color is, and what causes the varied tints under which it is seen. The solution of these questions has long occupied the minds of scientific inquirers, and it can not yet be said that they have been answered. Disagreement still prevails respecting them.
M. Durocher, in his "Studies of the Glaciers of Northern and Central Europe," has expressed the opinion that the blue color of some waters is of glacial origin, and that it is so peculiar to water from snow-fields and glaciers as to constitute a mark by which to distinguish whence it has proceeded. "If the color of water is really blue," he adds, "the substitution for it of gray or greenish tints proceeds in the majority of cases from organic substances, chiefly vegetable rather than animal."
M. Durocher's view is disputed by M. Th. Martins, who points to the snow-fed Lakes of Sioron and the Bachalpsee, as one azure blue, the other yellowish green, and the Lake of Brienz, whose yellowish green waters, after crossing the Isthmus of Interlachen, become blue in the Lake of Thun.
Bunsen was the first one to deny, with any real knowledge, the absence of color in water. Struck with the green-blue color of the hot water of the Icelandic geysers, he examined pure water in a tube, found it blue, and concluded that that was the true color of the liquid, while other colors observed were derived from foreign matters or by reflection from a colored bottom.
Tyndall, Soret, and Hagenbach took up the question about twenty years after Bunsen. Tyndall found by experiments on polarization that the blue of the atmosphere was caused by reflection of the shorter blue light-waves at the expense of the longer waves, from particles of aqueous vapor in an extreme state of division, which he called nascent cloud. If the particles were larger, longer waves would be reflected, and the color would approach white. Soret, seeking to learn if the blue color of the Lake of Geneva had not an analogous origin, applied the polarization experiments to it, and concluded that it contained minute particles similar to those found by Tyndall in the atmosphere. Hagenbach repeated the experiments, with like results, in the Lake of Lucerne; and Tyndall, a year later, with water of the Mediterranean and the Lake of Geneva, sent to him in London. Mr. Hayes examined the water of the Lake of Geneva, to see if it did not contain a coloring substance, and found none.
The later of these experiments indicate that, contrary to Bunsen's belief, water by itself may be colorless, but nothing is less certain. M. Soret says that the lake was still blue in cloudy weather, when he could not get a trace of polarization effects. Is this not enough to prove that reflection is not the only cause of color in water? Moreover, if the blue in water were wholly of the same origin as that of the sky, the light transmitted by water should be crimson, as that which is shown on the tops of high mountains, or which is transmitted through the clouds at the rising and setting of the sun; but nothing of the kind is the case. Professor Tyndall states this, and Father Secchi has established the absence of the red and yellow from the absorption spectrum of water. It is also well known to those who have had occasion to make submarine excursions, or who have visited the glacial grottoes in Switzerland, that the transmitted light has a blue tone, and the red is so weak that the figures have a livid aspect.
These facts show that the question is still waiting a definite solution. We now turn to the explanations which have been offered of the diversities in the colors of natural waters.
According to Arago, water has two colors, "a color of transmission and a color of reflection, wholly different from the other. It appears blue by reflection, and its transmitted color is green." This supposition can not be reconciled with optical laws, but Arago used it to explain the variations of tint in the water of a shallow, white-bottomed sea. "When the sea is deep, light is reflected from the water and appears blue; but, if it is not very deep, the sand at the bottom receives the light through a stratum of water. The light then reaches the bottom, already green, and, in returning from the sand to the air, the green color is deepened, frequently so much as to predominate, on coming out, over the blue. This, probably, is the whole secret of those shades which are in calm weather the sure and valuable index to the experienced sailor of the depth of the bottom. This explanation fails when it is applied to other quarters than those for which Arago conceived it. The Swiss lakes are green, or blue, independently of their depth. Arago suggests, after Davy, that the change from blue to green may be caused by the presence of vegetable, M. Durocher of colored, matters. These suppositions are gratuitous, and supported by no evidence. H. Sainte-Claire Deville, in 1848, analyzed a number of natural waters, and found that the blue ones gave hardly perceptible colored residues, while the green ones yielded such considerable quantities of organic matter that the soluble salts became yellow after evaporation. According to this, green waters, and, a fortiori, yellow or brown waters, owe their color to the presence of a small quantity of yellow mud. If pure water is really blue, the presence of a small quantity of yellow matter would be enough to turn its color to green or yellow. The same idea was advanced some time ago by M. Wiltsthein, who believed he had proved, by analyses of the waters of a number of Bavarian streams and lakes, that brown and yellow waters contained more organic matter than green ones, and that they were less hard than the latter. He thought that mineral substances of themselves had no effect on the color of the water, but that organic substances, naturally brown, existing in it as humic acids, were held in solution through the presence of alkaline matters with them, and that they made the water, according to their abundance, green, yellow, brown, or black. His views are not supported by the results of my analyses, which indicated that the colors of different waters on which they were made bore no relation to the quantity either of organic matter or of alkalies held in them. I have also not been able to find any relation between the color of water and its hardness or softness. It is, however, probable that very dark water may owe its color to dark organic matter dissolved in it.
M. Schleinitz attributes the diversities in the color of sea-water to variations in the quantity of salt dissolved in it. During a voyage in the Gazelle, from Ascension to the Congo, he observed that the blue water had a higher specific gravity than that which was of a greenish tinge. This observation leads to an erroneous conclusion, but affords a confirmation of some results which I have reached.
M. J. Brun has noticed in the water and the ice of the Lake of Neufchâtel an alga which is green, orange, red, or brown, according to the stage of growth it has reached, and black after it is dead. Its presence would not be without influence on the color of the lake.
This review shows that the problem of the color of water still calls for more investigation. It may be useful to speak of a few researches that I have made. My object was to determine the color of pure water, and to observe the variations in color produced by the presence of different substances. I used glass tubes, five metres long and four centimetres in interior diameter, closed at the end with glass plates, and passing through a black sheathing that intercepted the side-light. They abutted against a ground-glass pane in the window of my laboratory, so as to receive diffused light in the direction of their axes.
M. V. Meyer, who used a similar arrangement, found the color of distilled water to be a blue-green. I found it pure blue. In my first experiment, I also found distilled water of a blue-green color, like that of a diluted solution of ferric sulphate. A second experiment, with freshly distilled water, gave a pure sky-blue, which in the course of seventy hours became blue-green like the former water, without losing any of its transparency. This indicated that the distilled water of laboratories is not perfectly pure, but that it contains substances that will change in time. These substances might be mineral or organic, or even living organisms. Wishing to ascertain whether the last was the case, I added to the water in one of the tubes 10000 of bichloride of mercury, while I left that in the other tube unchanged. The small quantity of bichloride did not at all affect the color of the water to which it was added. In the course of six days the water which had been left alone became blue-green, while that to which bichloride of mercury had been added preserved a fixed blue, and exhibited no sign of change for three weeks; but, when the salt was put into the water that had turned blue-green, that began slowly to turn blue again, and this process continued for nine days, when it stopped, without the blue color having been quite restored. Inasmuch as the bichloride of mercury is extremely deadly to minute organisms, we have a right to conclude that life exists in the distilled water of laboratories, and that such water contains also the aliments required for its development. How can organic germs exist in water that has just passed through the process of distillation? Tyndall has shown the possibility of vapor taking up germs as it passes through the air. M. Stas has proved that distilled water may contain volatile organic matters which after a little while become spontaneously fixed. We may, then, conclude that our distilled water continued blue as long as the organic matters contained in it continued volatile, but that it turned green as they became fixed.
It was necessary to obtain distilled water certainly free from organic matter. I did this by an adaptation of M. Stas's process of distilling spring-water over a mixture of manganate and permanganate of potash into a cooling-vessel of platinum. The resultant water, which met every test of its purity, when placed in the tubes, displayed a color to which only the clearest blue of the sky, as seen from a mountain-top on a perfect day, can be compared, the hue of which was not changed after it had been left in the tubes for two weeks. The color was evidently not due to reflection from minute particles, for it was a color of transmission and had not a tinge of red in it; moreover, if it was due to the presence of foreign particles, all liquids under the same conditions ought to have a bluish tinge. But amylic alcohol, distilled under circumstances favorable to the absorption of fine particles, was colorless, while acetic acid and ethylic alcohol were yellow, when seen through a thickness of five metres; and, though the color was effaced as the thickness of the masses was reduced, no trace of green or blue appeared in the liquids. It seemed proved to me that water, as pure as we can get it, has a blue color, which proceeds, not from reflection, but from an absorption of the yellow.
To perfectly clear lime-water I added enough of a solution of carbonic anhydride to cause the formation of a barely visible precipitate, and then poured the liquid into one of my tubes of observation. The liquid was entirely opaque, as much so as if it had been ink I had put into the tube. I then took the mixture from the tube, diluted it to a suitable degree with pure water, and introduced a current of carbonic anhydride sufficient to precipitate the lime as a carbonate, and finally to dissolve the carbonate as an acid carbonate of lime. The current of carbonic anhydride was interrupted from time to time, and the liquid was clarified and examined in the tube, and, as I did so, I could see the opacity slowly disappear, letting in first a brown light, then clear brown, then yellow, then green, and at last, after eighteen hours of circulation of the carbonic anhydride, the liquid had again become blue, but with a tendency to green. Thus, by the combined action of carbonic anhydride and carbonate of lime, it is possible to produce all the colors of natural waters, from opacity to greenish-blue. I reversed the process, and from a green saturated solution of bicarbonate of lime and carbonic acid, by gradually expelling the carbonic acid, obtained a succession of colors, in inverse order, to complete opacity. Similar processes with solutions of other salts gave results agreeing with these.
My experiments enabled me to verify several facts. First, we find that not all of a luminous ray can pass through a considerable mass of a liquid holding foreign bodies in suspension, even when the latter are transparent or colorless. Further, it is not necessary that the body in suspension be in the solid state. The important point is, that it be competent to reflect light. Then the light-rays of feebler intensity 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. It is not essential to the production of this phenomenon that the medium be liquid. It may be observed in our atmosphere, where the shadow of a cloud of smoke will appear yellow or brown according to the thickness of the smoke. It may be that the reflecting particles can be dispensed with, and we may say generally that, when light passes through an optically resistant medium, the yellow rays are the last to be extinguished.
Other experiments have satisfied me that the yellow tint exists not only when the liquids contain matter in suspension, but also when they contain it in solution to the point of saturation, or when precipitation is about to begin, at which point there still remains enough of this color to form with the natural blue of the water a green. This condition may be called, in analogy with the nascent cloud of Tyndall, that of nascent precipitation. We now come to another view, which is supported by a small number of experiments I have made with reference to it, that the obstruction of light, inducing the yellowish tint, which is produced by any salt, depends less on the quantity of the salt present than on its being near the stage of precipitation. Small quantities of a feebly soluble salt produce the same effect as large quantities of a more soluble salt. The variety in the colors of natural waters may, then, be thus explained: Absolutely pure water, viewed in masses of sufficient thickness, is of a beautiful blue color. If it holds in complete solution colorless salts in small mass, its color is not changed; but, in proportion as it may contain matter on the verge of precipitation, the light traversing 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 green-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.
In nature, generally, the feebly soluble substances contained in natural waters, and appearing, perhaps, in the state of nascent precipitation, are carbonate of lime or magnesia, silica, silicate of aluminum, and alumina. A blue water should contain carbonate of lime more completely dissolved in proportion as it is more distinctly blue, and should consequently have in it enough carbonic anhydride to produce the acid carbonate of lime. A green water, on the other hand, should contain carbonate of lime in less complete solution, as would be the case if there were a less relative proportion of carbonic anhydride in it. The blue waters of the Rhône and the green waters of the Rhine, as analyzed by Sainte-Claire Deville, illustrate and confirm this rule. It may also be presumed that a blue water, containing limestone in full solution, should become green when lime is added to it. This is illustrated on the north shore of the Lake of Achen, where the blue waters of the deep lake become chrome-green when they break over the limestone pebbles of the strand, and, generally, in the greener color of the bottom and shore waters of seas and lakes. Other substances than lime, particularly silica and alumina, may produce the same effects, but their action is more complicated. These substances, without being really soluble in water, are pseudo-soluble, or form an emulsion with it; and water which has taken them up from the ground over which it flows does not become perfectly clear on standing. If, however, it meets a solution of chloride of sodium, alumina, or silicate of alumina, it is precipitated rapidly; and this is what takes place at the mouths of rivers, and is the immediate cause of the deposits out of which deltas are built up. The changes in the color of the sea-water observed by M. Schleinitz, on board the Gazelle, may be accounted for by reference to this fact.—Translated for the Popular Science Monthly from the Revue Scientifique.