ISLAND, or crystal, a body famous among the writers of optics, for its property of a double refraction; hut improperly called by that name, as it has none of the distinguishing characters of crystal, and is plainly a body of another class. Dr. Hill has reduced it to its pro per class, and determined it to be of a genus of spars, which he has called, from their figure, parallelopipedia, and of which he has described several species, all of which, as well as some other bodies of a different genus, have the same pro perties. Bartholine, Huygens, and Sir Isaac Newton, have described the body at large, but have accounted it either a crys tal or a talc ; errors which could not have happened, had the criterions of fossils been at that time fixed; since Sir Isaac Newton has recorded its property of mak ing an ebullition with aquafortis, which alone must prove, that it is neither talc nor crystal, both those bodies being wholly unaffected by that menstruum.
See CRYSTAL, and TALC.
It is always found in form of an oblique parallelopiped, with six sides, and is found of various sizes, from a quarter of an inch to three inches or more in diameter. It is pellucid, and not much less bright than the purest crystal, and its planes are all tolerably smooth, though, when nicely viewed, they are found to be waved with crooked lines, made by the edges of im perfect plates.
What appears very singular in the structure of this body is, that all the faces are placed in the same manner, and consequently it will split off into thin plates, either horizontally or larly; but this is found, on a miscroscopic esamination,to be owing to the regularity of figure, smoothness of surface, and nice joining of the several small parallelopiped concretions, of which the whole is com- posed ; and to the same cause is probably its ts remarkable property in refrac tion. See OPTICS, and REFRACTION.
It is very soft, and easily scratched with the point of a pin ; it will not give fire on being struck against steel, and ferments, and is perfectly dissolved in aquafortis. It is found in Iceland, from whence it has its name ; and in France, Germany, and many other places. In England, fragments of other spars are very often mistaken for it, many of them having in some degree the same property.