CAMEO CUT IN A CASSIS.
It consists of the interior layers of many species of shells, principally Bivalves; but some among the Univalves have an interior brilliantly nacreous. The Top-shells (Trochus), several species of which occur on our own shores, are very rich in this respect, and the widely-gaping Ear-shells (Haliotis) are most gorgeous.
The elegant articles called cameos, so much used as clasps and brooches, are formed out of the substance of large shells: the ponderous Helmets (Cassis) of the West Indies are chiefly used for this purpose. A great excellence in the art consists in the careful cutting away of the material so that the ground shall display one colour, commonly a dark hue, while the design is carved in another, commonly the pure white, which overlays the brown.
Shells, being composed of carbonate of lime, are capable of being burned to a quick-lime, having all the essential properties of that made from stone. On some parts of our own shores where limestone is scarce, shell-lime is extensively burned; but in other countries, as Holland and the United States of America, scarcely any other is used, either for building purposes or for manuring land.
The Tyrian purple, the most celebrated manufacture of that famous crowning city whose merchants were princes, was the juice of a shell-fish. Several species were employed to communicate various tints, but the principal was the Murex trunculus, one of the commonest shells of the Mediterranean, which may be compared for size and general appearance to our familiar Whelk.
Place your linen in the light of the sun, and look at it again in half-an-hour, or, if you please, watch its changes. The marks have by this time passed from yellow into pea-green, 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
