Note 1 — Chapter I

: DARWIN, in his admirable account of the coral formations of the Pacific and Indian oceans, has propounded a theory as to the abundance of fresh water in the atolls and islands on coral reefs, furnished by wells which ebb and flow with the tides. Assuming it to be impossible to separate salt from sea water by filtration, he suggests that the porous coral rock being permeated by salt water, the rain which falls on the surface must sink to the level of the surrounding sea, "and must accumulate there, displacing an equal bulk of sea water—and as the portion of the latter in the lower part of the great sponge-like mass rises and falls with the tides, so will the fresh water near the surface."—Naturalist's Journal, ch. xx. But subsequent experiments have demonstrated that the idea of separating the salt by filtration is not altogether imaginary; as Darwin seems to have then supposed; and Mr. WITT, in a remarkable paper On a peculiar power possessed by Porous Media of removing matters from solution in water, has since succeeded in showing that "water containing considerable quantities of saline matter in solution may, by merely percolating through great masses of porous strata during long periods, be gradually deprived of its salts to such an extent as probably to render even sea-water fresh."—Philos. Mag., 1856. Divesting the subject therefore of this difficulty, other doubts would appear to suggest themselves as to the applicability of Darwin's theory to coral formations in general. For instance, it might be supposed that rain falling on a substance already saturated with moisture, would flow off instead of sinking into it; and that being of less specific gravity than salt water, it would fail to "displace an equal bulk" of the latter. There are some extraordinary but well attested statements of a thin layer of fresh water being found on the surface of the sea, after heavy rains in the Bay of Bengal. (Journ. Asiat. Soc. Beng. vol. v. p. 239.) Besides, I fancy that in the majority of atolls and coral islands the quantity of rain which so small an area is calculated to intercept would be insufficient of itself to account for the extraordinary abundance of fresh water daily drawn from the wells. For instance, the superficial extent of each of the Laccadives is but two or three square miles, the surface soil resting on a crust of coral, beneath which is a stratum of sand; and yet on reaching the latter, fresh water flows in such profusion, that wells and large tanks for soaking coco-nut fibre are formed in any place by merely "breaking through the crust and taking out the sand."—Madras Journal, vol. xiv. It is curious that the abundant supply of water in these wells should have attracted the attention of the early navigators, and Cosmas Indicoplenstes, writing in the sixth century, speaks of the numerous small islands off the coast of Taprobane, with abundance of fresh water and coco-nut palms, although these islands rest on a bed of sand. (Cosmas Ind. ed. Thevenot, vol. i. p. 3, 20). It is remarkable that in the little island of Ramisseram, one of the chain which connects Adam's Bridge with the Indian continent, fresh water is found freely on sinking for it in the sand. But this is not the case in the adjacent island of Manaar, which participates in the geologic character of the interior of Ceylon. The fresh water in the Laccadive wells always fluctuates with the rise and fall of the tides. In some rare instances, as on the little island of Bitra, which is the smallest inhabited spot in the group, the water, though abundant, is brackish, but this is susceptible of an explanation quite consistent with the experiments of Mr. Witt, which require that the process of percolation shall be continued "during long periods and through great masses of porous strata;" Darwin equally concedes that to keep the rain fresh when banked in, as he assumes, by the sea, the mass of madrepore must be "sufficiently thick to prevent mechanical admixture; and where the land consists of loose blocks of coral with open interstices, the water, if a well be dug, is brackish." Conditions analogous to all these particularised, present themselves at Jaffna, and seem to indicate that the extent to which fresh water is found there, is directly connected with percolation from the sea. The quantity of rain which annually falls is less than in England, being but thirty inches; whilst the average heat is highest in Ceylon, and the evaporation great in proportion. Throughout the peninsula, I am informed by Mr. Byrne, the Government surveyor of the district, that as a general rule "all the wells are below the sea level." It would be useless to sink them in the higher ground, where they could only catch surface water. The November rains fill them at once to the brim, but the water quickly subsides as the season becomes dry, and "sinks to the uniform level, at which it remains fixed for the next nine or ten months, unless when slightly affected by showers." "No well below the sea level becomes dry of itself," even in seasons of extreme and continued drought. But the contents do not vary with the tides, the rise of which is so trifling that the distance from the ocean, and the slowness of filtration, renders its fluctuations imperceptible.

On the other hand, the well of Potoor, the phenomena of which indicate its direct connection with the sea, by means of a fissure or a channel beneath the arch of magnesian limestone, rises and falls a few inches in the course of every twelve hours. Another well at Navokeiry, a short distance from it, does the same, whilst the well at Tillipalli is entirely unaffected as to its level by any rains, and exhibits no alteration of its depths on either monsoon. ADMIRAL FITZROY, in his Narrative of the Surveying Voyages of the Adventure and Beagle, the expedition to which Mr. Darwin was attached, adverts to the phenomenon in connection with the fresh water found in the Coral Islands, and the rise and fall of the wells, and the flow and ebb of the tide. He advances the theory propounded by Darwin of the retention of the river-water, which he says, "does not mix with the salt water which surrounds it except at the edges of the land. The flowing tide pushes on every side, the mixed soil being very porous, and causes the water to rise: when the tide falls, the fresh water sinks also. A sponge full of fresh water placed gently in a basin of salt water, will not part with its contents for a length of time if left untouched, and the water in the middle of the sponge will be found untainted by salt for many days: perhaps much longer if tried."—Vol. i. p. 365. In a perfectly motionless medium the experiment of the sponge may no doubt be successful to the extent mentioned by Admiral Fitzroy; and so the rain-water imbibed by a coral rock might for a length of time remain fresh where it came into no contact with the salt. But the disturbance caused by the tides, and the partial intermixture admitted by Admiral Fitzroy, must by reiterated occurrence tend in time to taint the fresh water which is affected by the movement: and this is demonstrable even by the test of the sponge; for I find that on charging one with coloured fluid, and immersing it in a vessel containing water perfectly pure, no intermixture takes place so long as the pure water is undisturbed; but on causing an artificial tide, by gradually withdrawing and as gradually replacing a portion of the surrounding contents of the basin, the tinted water in the sponge becomes displaced and disturbed, and in the course of a few ebbs and flows its escape is made manifest by the quantity of colour which it imparts to the surrounding fluid.