Crystals
do not ordinarily attain large dimensions. For the greater number of
minerals, crystals of 2 inches are almost gigantic: few indeed exceed
4 inches in height. Quartz, however, forms an exception to this rule.
Specimens are brought from Madagascar more than 12 inches in length,
and remarkably pure and transparent, notwithstanding their great size.
The rock crystal of this island is used for the object-glasses of
astronomical telescopes. Magnificent crystals have also been found in
the Alps; one of these Alpine crystals, taken in Italy by the French,
was borne in triumph to Paris in 1797. There is a beautiful specimen in
the Museum of Natural History at Paris which measures 3 feet every way,
and weighs nearly 800 pounds. It was found at Fischbach in the Valais.
At
the French Exhibition of 1866, in the sections of Japan and of Brazil,
there were some wonderful crystals. One crystal brought from Brazil
weighs 212 pounds, is 2-1/2 feet high, and 1 foot in diameter, and is a
perfect six-sided prism.
A
remarkable phenomenon in quartz is exhibited by the fluid drops which
are contained in many specimens. Sir David Brewster ascertained that
the fluid is not water, but of an oleaginous nature, one part volatile
at twenty-seven degrees, and the other a fixed oil. Prof. Dana has
named the former cryptoline, and the latter brewsterine.
