Sir David Brewster having broken in two a crystal of quartz of a smoky
colour, found both surfaces of the fracture absolutely black; and the
blackness appeared at first sight to be owing to a thin film of opaque
matter which had insinuated itself into the crevice. This opinion,
however, was untenable, as every part of the surface was black, and
the two halves of the crystals could not have stuck together had the
crevice extended across the whole section. Upon further examination Sir
David found that the surface was perfectly transparent by transmitted
light, and that the blackness of the surfaces arose from their being
entirely composed of _a fine down of quartz_, or of short and slender
filaments, whose diameter was so exceedingly small that they were
incapable of reflecting a single ray of the strongest light; and they
could not exceed the _one third of the millionth part of an inch_. This
curious specimen is in the cabinet of her grace the Duchess of Gordon.