Chapter I: Observations on the Mer De Glace 339 (13)

In very clear and very deep sea-water this condition is approximately fulfilled, and hence the extraordinary darkness of such water. The indigo, to which I have already referred, is, I believe, to be ascribed in part to the suspended matter, which is never absent, even in the purest natural water, and in part to the slight reflection of the light from the limiting surfaces of strata of different densities. A modicum of light is thus thrown back to the eye before the depth necessary to absolute extinction has been attained. An effect precisely similar occurs under the moraines of the Swiss glaciers. The ice here is exceptionally compact, and, owing to the absence of the internal scattering common in bubbled ice, the light plunges into the mass, is extinguished, and the perfectly clear ice presents an appearance of pitchy blackness.[50]

[50] I learn from a correspondent that certain Welsh tarns, which are reputed bottomless, have this inky hue.

The green colour of the sea when it contains matter in a state of mechanical suspension has now to be accounted for, and here, again, let us fall back upon the sure basis of experiment. A strong white dinner-plate was surrounded securely by cord, and had a lead weight fastened to it. Fifty or sixty yards of strong hempen line were attached to the plate. With it in his hand, my assistant, Thorogood, occupied a boat fastened as usual to the davits of the ‘Urgent,’ while I occupied a second boat nearer to the stern of the ship. He cast the plate as a mariner heaves the lead, and by the time it had reached me it had sunk a considerable depth in the water. In all cases the hue of this plate was green: even when the sea was of the darkest indigo, the green was vivid and pronounced. I could notice the gradual deepening of the colour as the plate sank, but at its greatest depth in indigo water the colour was still a blue-green.[51]

[51] In no case, of course, is the green pure, but a mixture of green and blue.

Other observations confirmed this one. The ‘Urgent’ is a screw steamer, and right over the blades of the screw was an orifice called the screw-well, through which one could look from the poop down upon the screw. The surface glimmer which so pesters the eye was here in a great measure removed. Midway down a plank crossed the screw-well from side to side, and on this I used to place myself to observe the action of the screw underneath. The eye was rendered sensitive by the moderation of the light, and, still further to remove all disturbing causes, Lieutenant Walton had a sail and tarpaulin thrown over the mouth of the well. Underneath this I perched myself and watched the screw. In an indigo sea the play of colour was indescribably beautiful, and the contrast between the water which had the screw-blades for a background, and that which had the bottom of the ocean as a background, was extraordinary. The one was of the most brilliant green, the other of the deepest ultramarine. The surface of the water above the screw-blade was always ruffled. Liquid lenses were thus formed, by which the coloured light was withdrawn from some places and concentrated upon others, the colour being thus caused to flash with metallic lustre. The screw-blades in this case played the part of the plate in the former case, and there were other instances of a similar kind. The white bellies of the porpoises showed the green hue, varying in intensity as the creatures swung to and fro between the surface and the deeper water. Foam, at a certain depth below the surface, is also green. In a rough sea the light which has penetrated the summit of a wave sometimes reaches the eye, a beautiful green cap being thus placed upon the wave even in indigo water.

But how is this colour to be connected philosophically with the suspended particles? Take the dinner-plate which showed so brilliant a green when thrown into indigo water. Suppose it to diminish in size until it reaches an almost microscopic magnitude. It would still behave substantially as the larger plate, sending to the eye its modicum of green light. If the plate, instead of being a large coherent mass, were ground to a powder sufficiently fine, and in this condition diffused through the clear sea-water, it would send green light to the eye. In fact, the suspended particles which the home examination reveals act in all essential particulars like the plate, or like the screw-blades, or like the foam, or like the bellies of the porpoises. Thus I think the greenness of the sea is physically connected with the matter which it holds in suspension.

We reached Portsmouth on the 5th of January 1871. There ended a voyage which, though its main object was not realised, has left behind it pleasant memories, both of the aspects of nature and the genial kindliness of men.

Transcriber's Note

Images have been moved next to the text which they illustrate.

The following apparent errors have been corrected:

p. 34 (note) "gehen is" changed to "gehen’ is"

p. 47 "probablity" changed to "probability"

p. 64 "corespondingly" changed to "correspondingly"

p. 65 "WATER" changed to "WATER."

p. 83 "layers" changed to "layers."

p. 115 "maximun" changed to "maximum"

p. 122 "Brueil" changed to "Breuil"

p. 141 "Shreckhorn" changed to "Schreckhorn"

p. 150 "ceased Bennen" changed to "ceased. Bennen"

p. 151 "Lys We" changed to "Lys. We"

p. 156 "aproached" changed to "approached"

p. 206 "XIX" changed to "XIX."

p. 209 (note) "p. 82" changed to "p. 82."

p. 217 "ever since" changed to "ever since."

p. 226 "Finisteraarschlucht" changed to "Finsteraarschlucht"

p. 255 "nobleness" changed to "nobleness."

p. 262 "Maggiore" changed to "Maggiore."

p. 264 "Miracles." changed to "Miracles.’"

p. 267 "Bauman" changed to "Baumann"

p. 278 "did them" changed to "did then"

p. 283 "barometic" changed to "barometric"

p. 283 "and myself" changed to "and myself."

p. 291 "equilibruim" changed to "equilibrium"

p. 300 "morning" changed to "morning."

p. 326 "uppose" changed to "suppose"

p. 327 (note) "suprised" changed to "surprised"

p. 335 "crevasess" changed to "crevasses"

p. 339 "explanation" changed to "explanation."

p. 345 "truth" changed to "truth."

p. 388 "ice," changed to "ice,’"

p. 389 "liquefied" changed to "liquefied."

p. 394 "0°C" changed to "0° C"

p. 398 "unclouded ice" changed to "unclouded ice."

p. 403 "filled" changed to "filled."

p. 406 "vapour" changed to "vapour."

p. 415 "domes" changed to "domes."

p. 422 "late snow" changed to "late snow."

p. 426 "the wind" changed to "the wind."

p. 443 "wierd" changed to "weird"

p. 450 "enchroachment" changed to "encroachment"

p. 469 "follow green" changed to "follow green."

The following are used inconsistently in the text:

cowbells and cow-bells

FIG, FIG., and Fig.

frostbitten and frost-bitten

icefall and ice-fall

Lotschsattel and Lötschsattel

Nicol’s prism and Nicol prism

selfsame and self-same

sérac and _sérac_

snowbeds and snow-beds

subglacial and sub-glacial

Theodule and Théodule

thunderstorm and thunder-storm

End of Project Gutenberg's Hours of Exercise in the Alps, by John Tyndall