The Gentleman's Magazine/Volume 253/July 1882/The Colours of Water
ON the 6th of February last a paper was read at the Royal Society by Mr. John Aitken which "led to a considerable discussion among the Fellows." The subject was the varying colours of water, which have been explained rather learnedly by selective reflection, selective absorption, and polarisation of light. Mr. Aitken's explanation is much simpler, and closely approaches to my own, published five years ago in "Through Norway with Ladies." I there described particularly the waters of the Jolster Vand, which, seen from a short distance, appear of inky blackness. I have observed the same in other lakes. All these black lakes that I have seen in Norway, Scotland, Ireland, &c., are fed by rivers that flow through peat bogs, and dissolve from the peat a sufficient amount of bituminous matter to give the water when examined in small specimens the appearance of weak tea. The chief feeder of the Jolster Vand is as dark as very strong tea.
I have examined such water by looking through different depths, and find that its depth of colour goes on increasing proportionally; that these variations of tint are correctly represented by taking a solution of asphalte or common coal tar in turpentine, and painting it on white paper. A thin film stains the paper to about the same depth and character of tint as is shown by a tumbler of water dipped from a peat torrent, or from a bog pool; another coat over this represents the colour shown by looking through a greater thickness of the bog water, and so on until the blackness of the original pitch is obtained. The same is observable by looking through sections of the pitch itself. Thin films are semi-transparent and have the colour of strong tea, which grows darker with increased thickness and opacity. All this is due to invisibly minute particles of black carbon suspended in a resinous medium.
Thus the black colour of deep lakes fed by streams containing a weak solution of bitumen in water is well explained, and further observation has satisfied me that all the other varieties of the colour of water have a similar simple origin.
Water containing yellow particles in suspension is more or less yellow according to the quantity, and with white particles it is corresponding white. This is very grossly shown by the waves that break on the sandy and chalky shores of our south coast. I say grossly, because here the particles are big enough to be separately visible, and nobody can dispute the visible cause of the milkiness and pea-soupiness. When, however, the particles are so small as to be separately invisible, much keener observation is necessary. If any of my readers should visit the west coast of Ireland, they should carefully observe the waves that break upon the grand rocky barrier extending from Loop Head to Galway Bay, notably about Kilkee and the Cliffs of Mohir. There is no sand nor shingle here, and the water appears transparent; but the caverns, arches, and other torturings of the rock by the waves prove that they wear it away, and therefore must contain the minute particles they rub off. These waves, especially in rough weather, are not merely blue or green like ordinary sea-water, but are of deep indigo-purple, a colour magnificently displayed by contrast with the white foam of the breakers. A slice of the rock cemented to a piece of glass with Canada balsam and then ground down till translucent, displays the same colour as the water. Such sections are commonly made by microscopists.
I could name a multitude of other similar cases, as, in the course of my solitary pedestrian wanderings, I have noted again and again the deep ultramarine colour of a multitude of torrent-fed lakes and tarns filling the hollows of dark slaty rocks, or where gneiss or hornblende abounds. Allowance must always be made for the reflected colour of the sky shown in certain positions.
Other lakes are as nearly colourless as an imperfectly transparent liquid can be, and contrast remarkably with the intensely blue lakes above named.
All such colourless water that I have seen has either been supplied by the surface drainage of siliceous rocks, subject to little or no torrent grinding, or by springs passing through hard limestones. These contain limestone in solution, but no suspended coloured particles.
The most remarkable of the first that I now remember is the Aachensee, a Tyrolese lake, with water so transparent that taking a header into it from a steep bank demanded quite an effort of resolution; it seemed like a suicidal plunge over an aërial precipice.
The fountain of Cyane, the source of the Anapo near Syracuse, is a deep pool welling from the limestone; it is so clear, so colourless and air-like, that floating in a boat and looking down at the pebbles, seen with microscopic distinctness 40 or 50 feet below, is suggestive of sitting in the car of a balloon.
It is well known to observant mariners that the wide ocean itself, far beyond the sight of land, varies considerably in tint. Seawater has been justly described as "a weak broth," on account of the large quantity of organic matter, chiefly spores and microscopic zoophytic life, that it contains. If I am right, its colour is due to these, and must vary with them.