LITMIR.BIZ

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and are now of a full grass-green; under the influence of the light the change proceeds rapidly, the yellow element gradually disappearing, and the blue element becoming more and more prominent, until through the stages of deep-green, sea-green, and greenish-blue, the colour at length appears a full indigo. The red element now begins to be apparent, and rapidly increases in intensity until the hue is a dull, reddish purple. In my own experiments this was the ultimate tint obtained; a tint perfectly indelible as long as the texture of the material remained, neither light, nor time, nor washing, nor the application of chemical agents having now the least influence either in changing its hue or causing it to fade. I have seen it stated that if the cloth be washed in scalding water and soap, it comes out from the lather changed from the reddish purple hue to a fair bright crimson; with me, however, the soap and the hot water had no appreciable ​influence in brightening the colour. My experiments were performed in winter, and I will not affirm that the intensity of a summer's sun would not in some degree have modified the result. There appears to me one objection to this material ever having been used to dye large surfaces of uniform colour; for from the admixture of mucus with the colouring matter, when any quantity of the latter is collected, the hue is found to imbue the cloth in a mottled or blotched manner, some parts being much darker than others. What method the ancients had of avoiding this appearance I do not know.

      I have seen it repeatedly stated that the slimy liquor remaining in the shell of the common snail, when the animal is crushed, is an admirable cement for glass or china, resisting both heat and moisture. I have tried it both simple, and mixed, as sometimes directed, with quick-lime in powder, but am compelled to confess, that I found it utterly worthless, the adhesion being in every case no greater than if I had used spittle for the purpose.

      Let us now see what rank the Mollusca can assume among those creatures which inflict direct injury upon man. The ravages committed by various species of snails and slugs are often annoying, and sometimes serious. There are probably few of my gentle readers who have a garden at their disposal, who have not been disappointed of their crops of spring flowers by the nightly depredations of these pests. The border has been well dug and smoothed, the seed has been carefully sown, and the spot has been eagerly watched from day to day; but no sooner have the tender seed-leaves appeared above ground in a slender green ​line or circle, than night after night they are gnawed away, until nothing remains but the brown earth, and the label which tells where the seed had been. But to the farmer the consequences are often much more important. In wet seasons the slugs increase with such rapidity in the fields, that a wheat-crop after one of clover, tares, or beans, is very uncertain, and may be said generally to fail. The damage annually done to corn, clover, and turnips, by these apparently insignificant creatures, is very great. In France and the South of Europe, the vineyards are subject to similar attacks from the vine snail (Helix pomatia). The buds and opening leaves of the vine are gnawed off by them as they appear, and the hopes of the autumnal vintage are often blasted.

But much more lamentable than any of these are the injuries inflicted upon shipping, and the piers and defences of maritime towns, by the ship-worm (Teredo navalis). Ranging over extensive seas from the tropics to the shores of Northern Europe, this boring worm, or rather Mollusk with a worm-like form, is incessantly engaged in devouring and destroying all kinds of woodwork that is immersed in the sea. Linnæus long ago styled it the calamity of ships, and there is no maritime nation which has not confessed the formidable power of this subtle enemy. In the years 1731 and 1732, the United Provinces were under a dreadful alarm; for it was discovered that these mollusks had made such depredations on the piles which support the banks of Zealand and Freisland, as to threaten them with total destruction, reclaiming from man what he had with unexampled labour wrested from the ocean. A few ​ TEREDO.years after they fortunately abandoned the dikes; but fearful of the return of an enemy more powerful than even the Grand Turk, who boasted that he would exterminate them with a host armed with spades and shovels, the Dutch offered a reward of value, to any one who should discover a remedy to ward off their attacks, and ointments, varnishes, and poisonous liquors, were recommended by the hundred. The exact amount of the damage done at this visitation—which Sellius, unable to discover any natural cause for it, says was sent by the Deity to punish the growing pride of the Hollanders—I have not been able to ascertain. Writers in general speak of it as "very great;" and Dr. Job Baster mentions the Teredo as an animal "which has done so many millions damage to these countries." In our own country it has done, and continues to do, extensive mischief. The soundest and hardest oak cannot resist these noxious creatures; but in the course of four or five years, they will so drill it as to render its ​removal necessary, as has repeatedly happened in the dockyard of Plymouth. To preserve the timbers used there, and exposed to them, the plan now adopted is to cover the parts under water with short broad-headed nails, which, in salt water, soon invests the whole with a strong coating of rust impenetrable by their augers. The plan appears to have proved effectual, for, in the harbours of Plymouth and Falmouth, where the Teredo was once abundant, it is now rare or not to be found; but in other parts it has still a residence, and within these few years it has materially injured or destroyed many of the piles used in the construction of the pier at Port Patrick, on the coast of Ayrshire; and the Limnoria terebrans, a crustaceous insect, co-operating with it, the result of their united efforts can hardly fail to be the utter and speedy destruction of all the timber in the pier."^[13]

Another kind of injury is dependent on the fact that certain species, which are generally eatable and even wholesome, become at certain times highly poisonous. Some foreign species are liable to this fatality, particularly oysters, both in the East and West Indies. But we need not go to distant countries for cases in point. The Mussels of our own rocks, though generally sold and eaten by many persons without fear, are well known to be fickle in their qualities, and many cases are on record in which their use has proved fatal. One of these, well authenticated, and investigated by scientific medical men, occurred at Leith in June 1827. Many of the poor of this town were poisoned by eating mussels which had been collected in the docks. ​ MUSSELS. "The town," says Dr. Combe, "was in a ferment, and the magistrates with great propriety issued a warning against the use of the mussels. Many deaths were reported, and hundreds of individuals were stated to be suffering under the effects of the poison. Luckily, matters were not so deplorable; but we ascertained that in addition to the man mentioned before, the companion of our patient, an elderly woman, had died. In all about thirty cases occurred, with great uniformity of symptoms, but varying very much in severity; but none, so far as I know, have left any permanent bad effects."^[14]

      The cause of this occasional liability to become poisonous seems involved in almost total obscurity. Dr. Johnston, who discusses at some length the many loose and vague conjectures that have been hazarded on the subject, has shown, that not one of them is tenable, unless it be that in some cases the poisonous principle proceeds from some particular food which, not fatal to the Mollusks, yet ​generates a diseased condition of the body, deadly to other creatures. The Leith mussels, he adds, were living in a dock, where we may presume they were nurtured and fattened amid putrescent matters; and Dr. Coldstream, than whom no one is better qualified to decide the point, gave it as his opinion that the liver was larger, darker, and more brittle than in the wholesome fish, and satisfied Dr, Christison that there was a difference of the kind. It must be confessed, however, that these observations leave the question pretty nearly where it was before.

      Some peculiar secretions of the Mollusca remain to be noticed. And first the black liquor or ink of the Cuttles and Squids, which has already been mentioned as useful to man, but which is doubtless much more useful to the animals themselves. These animals, when in danger, are known to pour forth from a funnel-like orifice a liquor of a blackish-brown colour in considerable abundance; this fluid, readily diffusing itself and mingling with the surrounding water, produces such a cloud of obscurity as frequently enables the crafty animal to escape, enveloped in the mist of its own making—as the deities in Homer are represented as concealing their favourite heroes.

      Somewhat analogous to this is a secretion of a rich purple hue produced and poured forth under excitement, by those large and naked mollusks, the Aplysiæ. I have found one of these animals, on

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