With reference to these last—the sky-effects—a few words may not be out of place here.
The superfine “ultra-microscopic” dust, which was blown by the volcano in quantities so enormous to such unusual heights, was, after dropping its heavier particles back to earth, caught by the breezes which always blow in the higher regions from east to west, and carried by them for many months round and round the world. The dust was thickly and not widely spread at first, but as time went on it gradually extended itself on either side, becoming visible to more and more of earth’s inhabitants, and at the same time becoming necessarily less dense.
Through this medium the sun’s rays had to penetrate. In so far as the dust-particles were opaque they would obscure these rays; where they were transparent or polished they would refract and reflect them. That the material of which those dust-particles was composed was very various has been ascertained, proved, and recorded by the Krakatoa Committee. The attempt to expound this matter would probably overtax the endurance of the average reader, yet it may interest all to know that this dust-cloud travelled westward within the tropics at the rate of about double the speed of an express train—say 120 miles an hour; crossed the Indian Ocean and Africa in three days, the Atlantic in two, America in two, and, in short, put a girdle round the world in thirteen days. Moreover, the cloud of dust was so big that it took two or three days to pass any given point. During its second circumnavigation it was considerably spread and thinned, and the third time still more so, having expanded enough to include Europe and the greater part of North America. It had thinned away altogether and disappeared in the spring of 1884.
Who has not seen—at least read or heard of—the gorgeous skies of the autumn of 1883? Not only in Britain, but in all parts of the world, these same skies were seen, admired, and commented on as marvellous. And so they were. One of the chief peculiarities about them, besides their splendour, was the fact that they consisted chiefly of “afterglows”—that is, an increase of light and splendour after the setting of the sun, when, in an ordinary state of things, the grey shadows of evening would have descended on the world. Greenish-blue suns; pink clouds; bright yellow, orange, and crimson afterglows; gorgeous, magnificent, blood-red skies—the commentators seemed unable to find language adequately to describe them. Listen to a German observer’s remarks on the subject:—
“The display of November 29th was the grandest and most manifold. I give a description as exactly as possible, for its overwhelming magnificence still presents itself to me as if it had been yesterday. When the sun had set about a quarter of an hour there was not much afterglow, but I had observed a remarkably yellow bow in the south, about 10 degrees above the horizon. In about ten minutes more this arc rose pretty quickly, extended itself all over the east and up to and beyond the zenith. The sailors declared, ‘Sir, that is the Northern Lights.’ I thought I had never seen Northern Lights in greater splendour. After five minutes more the light had faded, though not vanished, in the east and south, and the finest purple-red rose up in the south-west; one could imagine one’s-self in Fairyland.”
All this, and a great deal more, was caused by the dust of Krakatoa!
“But how—how—why?” exclaims an impatient and puzzled reader.
“Ay—there’s the rub.” Rubbing, by the way, may have had something to do with it. At all events we are safe to say that whatever there was of electricity in the matter resulted from friction.
Here is what the men of science say—as far as we can gather and condense.
The fine dust blown out of Krakatoa was found, under the microscope, to consist of excessively thin, transparent plates or irregular specks of pumice—which inconceivably minute fragments were caused by enormous steam pressure in the interior and the sudden expansion of the masses blown out into the atmosphere. Of this glassy dust, that which was blown into the regions beyond the clouds must have been much finer even than that which was examined. These glass fragments were said by Dr Flügel to contain either innumerable air-bubbles or minute needle-like crystals, or both. Small though these vesicles were when ejected from the volcano, they would become still smaller by bursting when they suddenly reached a much lower pressure of atmosphere at a great height. Some of them, however, owing to tenacity of material and other causes, might have failed to burst and would remain floating in the upper air as perfect microscopic glass balloons. Thus the dust was a mass of particles of every conceivable shape, and so fine that no watches, boxes, or instruments were tight enough to exclude from their interior even that portion of the dust which was heavy enough to remain on earth!