The explanation of these phenomena may be briefly stated. The fine particles, consisting for the most part of translucent crystals, or fragments of crystals, formed a canopy high up in the atmosphere, being gradually spread over both sides of the equator till it formed a broad belt, through which the rays of the sun and moon were refracted. Towards dawn and sunset they were refracted and reflected from the facets of the crystal, and thus underwent decomposition into the prismatic colours; as do the rays of the sun when refracted and reflected from the particles of moisture in a rain-cloud. The subject is one which cannot be fully dealt with here, and is rather outside the scope of this work.
(h.) Origin of the Eruption.—The ultimate cause of volcanic eruptions is treated in a subsequent chapter, nor is that of Krakatoa essentially different from others. It was remarkable, however, both for the magnitude of the forces evoked and the stupendous scale of the resulting phenomena. It is evident that water played an important part in these phenomena, though not as the prime mover;—any more than water in the boiler of a locomotive is the prime mover in the generation of the steam. Without the fuel in the furnace the steam would not be produced; and the amount of steam generated will be proportional to the quantity and heat of the fuel in the furnace and the quantity of water in the boiler. In the case of Krakatoa, both these elements were enormous and inexhaustible. The volcanic chimney (or system of chimneys), being situated on an island, was readily accessible to the waters of the ocean when fissures gave them access to the interior molten matter. That such fissures were opened we may well believe. The earthquakes which occurred at the beginning of May, and later on, on the 27th of that month, may indicate movements of the crust by which water gained access. It appears that in May the only crater in a state of activity was that of Perboewatan; in June another crater came into action, connected with Danan in the centre of the island, and in August a third burst forth. Thus there was progressive activity up to the commencement of the grand eruption of the 26th of that month.[11] During this last paroxysmal stage, the centre of the island gave way and sunk down, when the waters of the ocean gained free access, and meeting with the columns of molten matter rising from below originated the prodigious masses of steam which rose into the air.
(i.) Cause of the Detonations.—The detonations which accompanied the last great eruption are repeatedly referred to in all the accounts. These may have been due, not only to the sudden explosions of steam directly produced by the ocean water coming in contact with the molten lava, but by dissociation of the vapour of water at the critical point of temperature into its elements of oxygen and hydrogen; the reunion of these elements at the required temperature would also result in explosions.
The phenomena attending this great volcanic eruption, so carefully tabulated and critically examined, will henceforth be referred to as constituting an epoch in the history of volcanic action over the globe, and be of immense value for reference and comparison.
[1] The eruption of Krakatoa has been the subject of an elaborate Report published by the Royal Society, and is also described in a work by Chevalier R. D. M. Verbeek, Ingenieur en Chef des Mines, and published by order of the Governor-General of the Netherland Indies (1886). See also an Article by Sir R. S. Ball in the Contemporary Review for November, 1888.
[2] Verbeek, loc. cit., p. 4.
[3] The account of this eruption is a free translation from Verbeek.
[4] Verbeek, loc. cit., p. 160.
[5] Judd, Rep. R. S.
[6] A fuller account by Prof. Judd will be found in the Report of the Royal Society, p. 14. Several vessels at anchor were driven ashore on the straits owing to the strong wind which arose.