Henry Craik, ed. English Prose. 1916. Vol. V. Nineteenth Century
Colours in Snow and Water
By Sir Humphry Davy (17781829)
Poietes. You, Halieus, must certainly have considered the causes which produce the colours of waters. The streams of our own island are of a very different colour from these mountain rivers, and why should the same element or substance assume such a variety of tints?
Halieus. I certainly have often thought upon the subject, and I have made some observations and one experiment in relation to it. I will give you my opinion with pleasure; and as far as I know, they have not been brought forward in any of the works on the properties of water, or on its consideration as a chemical element. The purest water with which we are acquainted, is undoubtedly that which falls from the atmosphere. Having touched air alone, it can contain nothing but what it gains from the atmosphere; and it is distilled without the chance of those impurities, which may exist in the vessels used in an artificial operation. We cannot well examine the water precipitated from the atmosphere, as rain, without collecting it in vessels, and all artificial contact, gives more or less of contamination; but in snow, melted by the sunbeams, that has fallen on glaciers, themselves formed from frozen snow, water may be regarded as in its state of greatest purity. Congelation expels both salts and air from water, whether existing below, or formed in, the atmosphere; and in the high and uninhabited regions of glaciers, there can scarcely be any substances to contaminate. Removed from animal and vegetable life, they are even above the mineral kingdom; and though there are instances in which the rudest kind of vegetation (of the fungus or mucor kind) is even found upon snows, yet this is a rare occurrence; and red snow, which is occasioned by it, is an extraordinary and not a common phenomenon towards the pole, and on the highest mountains of the globe. Having examined the water formed from melted snow on glaciers, in different parts of the Alps, and having always found it of the same quality, I shall consider it as pure water, and describe its character. Its colour, when it has any depth, or when a mass of it is seen through, is bright blue; and, according to its greater or less depth of substance, it has more or less of this colour; as its insipidity, and its other physical qualities, are not at this moment objects of your inquiry, I shall not dwell upon them. In general in examining lakes and masses of water in high mountains, their colour is of the same bright azure. And Captain Barry states, that the water on the Polar ice has the like beautiful tint. When vegetables grow in lakes, the colour becomes nearer sea-green, and as the quantity of impregnation from their decay increases, greener, yellowish green, and at length when the vegetable extract is large in quantity, as in countries where peat is found, yellow and even brown. To mention instances, the Lake of Geneva, fed from sources (particularly the higher Rhone) formed from melting snow, is blue; and the Rhone pours from it dyed of the deepest azure, and retains partially this colour till it is joined by the Saone, which gives it a greener hue. The Lake of Morat, on the contrary, which is fed from a lower country, and from less pure sources, is grass green. And there is an illustrative instance in some small lakes fed from the same source, in the road from Inspruck to Stutgard, which I observed in 1815, (as well as I recollect) between Nazareit and Reiti. The highest lake fed by melted snows in March, when I saw it, was bright blue. It discharged itself by a small stream into another into which a number of large pines had been blown by a winter storm, or fallen from some other cause; in this lake its colour was blue-green. In a third lake, in which there were not only pines and their branches, but likewise other decaying vegetable matter, it had a tint of faded grass-green; and these changes had occurred in a space not much more than a mile in length. These observations I made in 1815; on returning to the same spot twelve years after, in August and September, I found the character of the lakes entirely changed. The pine wood washed into the second lake had disappeared; a large quantity of stones and gravel, washed down by torrents, or detached by an avalanche, supplied their place; there was no perceptible difference of tint in the two upper lakes; but the lower one, where there was still some vegetable matter, seemed to possess a greener hue. The same principle will apply to the Scotch and Irish rivers, which, when they rise or issue from pure rocky sources, are blue or bluish green; and when fed from peat bogs, or alluvial countries, yellow, or amber-coloured or brown, even after they have deposited a part of their impurities in great lakes. Sometimes, though rarely, mineral impregnations give colour to water; small streams are sometimes green or yellow from ferruginous depositions. Calcareous matters seldom affect their colour, but often their transparency when deposited, as is the case with the Velino at Terni, and the Anio at Tivoli; but I doubt if pure saline matters, which are in themselves white, ever change the tint of water.