172 NATURAL HISTORY. [NORTH 
bules, and marked with concentrically disposed brown and 
yellow colours, is called toad's eye wood-tin. 
In the next Case begin the oxides of electro-negative 
bodies, and their various combinations. 
Case 19. Alumina and Aluminates . To the former 
belongs the corundum , comprehending the precious stones, 
commonly called oriental gems (the sapphire, ruby, orien¬ 
tal amethyst, oriental topaz, oriental emerald), of the crys¬ 
tallized forms of which the principal modifications are 
here deposited; and the common or imperfect corundum 
from Bengal, Mysore, China, the Carnatic (Werner's dia¬ 
mond spar), Lapland, Piedmont, &c. As appendix to 
these are added, the fibrolite , (bournonite of Lucas,) one 
of the concomitant substances of the common corundum of 
the Carnatic;—the indianite of Bournon, being one of the 
matrices of the same corundum;—the emery , which owes its 
hardness and consequent usefulness in polishing to an ad¬ 
mixture of blue corundum.—The diaspore and the gibb~ 
site: both hydrates of alumina. 
Aluminate of magnesia— the spinel: among its varieties 
is the blue spinel of Aker in Siidermannia. The ceylon - 
ite or pleonaste, and the automolite (also called gahnite ), 
from Fahlun in Sweden and from Franklin in New Jersey, 
are, the former an aluminate of protoxide of iron and mag¬ 
nesia, the latter an aluminate of zinc. The mineral 
called gum-lead Aplomb gomme), from Hueigoet in Brit¬ 
tany : a hydrous aluminate of lead. 
The five following Cases contain the acid or oxide of 
silicium (silica, quartz), the numerous varieties of which, 
formerly considered as so many distinct species, are mostly 
indebted for their generally very striking external charac¬ 
ters to the admixture of matter foreign to the species, or 
to other casual circumstances that prevailed at their form¬ 
ation. 
Case 20. Amethyst quartz of various tints, in grouped 
crystals, &c.*— Rock crystal: various modifications of crys¬ 
talline forms: small dodecahedral and other crystals, 
known by the names of Gibraltar diamonds, Bristol dia¬ 
monds, &c.; varieties of colour, according to which this 
substance obtains the familiar denominations of smoky 
topaz or morion, cairngorm, citrine, &c.; specimens of rock 
