TOR 
TOR 
ranged under one species by Mr. Rom6 de 
Lisle. The word topaz, derived from an 
island in the Bed Sea, where the antients 
used to find topazes, was applied by them to 
a mineral very different from ours. _ One 
variety ot our topaz they denominated 
chrysolite. _ . 
The topaz is found in Saxony, Bohemia, 
Siberia, and Brazil, mixed with other minerals 
in granite rocks. . . 
It is commonly chrystallized. The primi- 
tive form of its crystals is a prism whose sides 
are rectangles, and bases rhombs, having their 
greatest angles 124" 22', and the internal mo- 
lecule has the same form ; and the height of 
the prism is to a side of the rhomboidal 
bases as 3 to 2. The different varieties of 
topaz crystals hitherto observed, amount to 
6. Five of these are eight-sided prisms, ter- 
minated by four-sided pyramids, or wedge- 
shaped summits, or by irregular figures of 7, 
13, or 15 sides: the last variety is a twelve- 
sided prism, terminated by six-sided pyramids 
wanting the apex. For an accurate descrip- 
tion and figure of these varieties, the reader 
is referred to Mr. Id any. 
The texture of the topaz is foliated. It 
causes a double refraction. Specific gravity 
from 3.46 to 3.56. The Siberian and Brazil 
topazes, when heated, become positively 
electrified on one side, and negatively on the 
other. It is infusible by the blowpipe. The 
yellow topaz of Brazil becomes red when ex- 
posed to a strong heat in a crucible; that of 
Saxony becomes white by the same process. 
This shews us that the colouring matter of 
these two stones is different. 
The colour of the topaz is various, which 
has induced mineralogists to divide it into the 
following varieties: 
] . Red topaz, of a red colour, inclining 
to yellow ; called Brazilian or occidental 
ruby. 
2. Yellow^ topaz, of a golden-yellow co- 
lour, and sometimes also nearly white; called 
occidental or Brazil topaz. The powder of 
this and the following variety, causes syrup 
of violets to assume a green colour. 
3. Saxon topaz. It is of a pale wine yel- 
low colour, and sometimes greyish white. 
4. Aigue marini, of a bluish or pale-green 
colour. 
5. Occidental sapphire, of a blue colour, 
and sometimes white. 
A specimen ot white Saxon topaz, analyzed 
by Vauquelin, contained 
68 alumina 
31 siiica 
99. 
TOPOGRAPHY, a description or draught 
of some particular place, or small tract of 
land, as that of a city or town, manor or 
tenement, field, garden, house, castle, &c. 
such as surveyors set out in their plots, or 
make draughts of, for the information and 
satisfaction of the proprietors. 
TORDYLIUM, hart-wort, in botany, a 
genus of plants of the class pentandria, and 
order digynia, and in the natural system 
arranged under the 45th order, umbel I at*. 
The corollets are radiated, and all herma- 
phrodite ; the fruit is roundish, and crenated 
on the margin; the involucra long and un- 
divided. There are seven species ; of which 
two are British, the maximum and officinale. 
3 
1. The maximum, or knotted parsley, has, 
sinipie sessile umbels, the exterior seeds be- 
ing rough. It grows in the borders of the 
corn-fields, and in dry stony places. 2. The 
officinale, officinal hart-wort, has partial in- 
volucra, as long as the flowers ; leaflets oval 
and jagged ; the seeds are large and flat, and 
their edges notched. 
TORMENTILLA, Tormentil, a genus 
of plants of the class icosandria, and order 
poiygynia, arid in the natural system ranging 
undeAhe 35t'n order, senticos* The calyx 
is octolid ; the petals are four ; the seeds 
round, naked, and affixed to a juiceless re- 
ceptacle. There are two species, the erecta 
and repens, both indigenous. The erecta, 
common tormentil, or septoil, has a stalk 
somewhat erect, and sessile leaves. The roots 
consist of thick tubercles, an inch or more in 
diameter, replete with a red juice of an as- 
tringent quality. They are used in most of 
the Western Islands. 
TORNADO, or Turnado, a sudden and 
vehement gust of wind from all pomts ot the 
compass, frequent on the coast of Guinea. 
A tornado seems to partake much of the 
nature of a whirlwind or perhaps of a water- 
spout, but is more violent in its effects. It 
commences very suddenly, several clouds 
being previously drawn together, when a 
spout of wind, proceeding from them, strikes 
the ground, in a round spot of a few rods or 
perches diameter, in the course of the wind 
of the day, and proceeds thus half a mile or 
a mile. The proneness of its descent makes 
it rebound from the earth, throwing such 
things as are moveable before it, but some 
sideways or in a lateral direction from it. A 
vapour, mist, or rain descends with it, by 
which the path of it is marked with wet. 
The gentleman who furnishes the above 
general description, gives an account of one 
which happened a few years since at Leices- 
ter, about fifty miles from Boston, in New 
England. “ It happened in July, on a hot 
day, about four o’clock in the afternoon. A 
few clouds having gathered westward, and 
coming overhead, a sudden motion of their 
running together in a point being observed, 
immediately a spout of wind struck the 
ground at the west end of a house, and in- 
stantly carried it away with a negro man in 
it, who was afterwards found de d in the 
path of it. Two men and a woman, by the 
breach of the floor, fell into the cellar ; and 
one man was driven forcibly up into the 
chimney-corner. These were preserved, 
though much bruised ; they were wet with a 
vapour or mist, as were the remains of the 
floor, and the whole path of the spout. This 
wind raised boards, timbers, &c. A joist was- 
found on one end, driven near three teet into 
the ground. The spout probably took it in 
its elevated state, and drove it forcibly down. 
The tornado moved with the celerity of a 
middling wind, and constantly declined in 
strength till it entirely ceased.” 
TORPEDO. See Raia, and Elec- 
tricity. 
TORRICELLIAN experiment, a fa- 
mous experiment made by Torricelli, a dis- 
ciple of the great Galileo, which has been 
already explained under Barometer. 
TORRID zone, among geographers, de- 
notes that tract of the earth lying upon the 
equator, and on each side as tar as the two 
TOU 80 ? 
tropics, or 23° 30' of north -and south lati- 
tude. 
TORTOISE-shell, the shell of the testa- 
ceous animal called a tortoise ; used in inlay- 
ing, and in various other works, a? for snuff- 
boxes, combs, &c. Mr. Catesby observes, 
that the hard strong covering which incloses 
ail sorts of tortoises, is very improperly call- 
ed a shell ; being of a perfect bony con- 
texture, but covered on the outside with 
scales, or rather plates of a horny substance ; 
which are what workmen call tortoise-shell. 
See Horn. - 
There are two general kinds of tortoises, . 
viz. the land and sea tortoise, testudo terres- 
tris and marina. The sea-tortoise, again, is 
of several kinds ; but it is the testudo iinbri- 
cata of Linnaeus, alone which furnishes that 
beautiful shell so much admired in Europe. 
See Testudo. 
The whole spoils of the tortoise consist in 
thirteen leaves or scales, eight of them flat, 
and live a little bent. Of the flat ones, there- 
are four large ones, sometimes a foot long, 
and seven inches broad. The best tortoise- 
shell is thick, clear, transparent, of the co- 
lour of antimony, sprinkled with brown and 
white. When used in marquetry, &c. the 
workmen give it what colour they please by 
means of coloured leaves, which they put 
underneath it. 
Working and joining of tortoise-shell . — - 
Tortoise-shell and horn become soft in a mo- 
derate heat, as that of boiling water, so as to 
be pressed, in a mould, into any form, the - 
shell or horn being previously cut into plates 
of a proper size. Plunder informs us, in his 
Art de Tourner, that two plates are likewise - 
united into one by healing and pressing 
them ; the edges being thoroughly cleaned, 
and made to fit close to one another. The 
tortoise-shell is conveniently heated for this 
purpose by applying a hot iron above and 
beneath the juncture, with the interposition 
of a wet cloth to prevent the shell from being 
scorched by the irons : these irons should be 
pretty thick, that they may not lose their 
heat before the union is effected. Both tor- 
toise-shell and horns may be stained of a va- ■ 
riety of colours, by means of the colouring 
drugs commonly used in dyeing, and by cer- 
tain metallic solutions. . 
TOUCAN. See Ramphastos. 
Toucan, in astronomy, a constellation of 
the southern hemisphere, consisting of eight 
small stars, and otherwise called anser ameri- 
canus. See Astronomy. 
TOUCH-needle, among assavers, refin- 
ers, See. little bars of gold, silver, and copper, 
combined together in all the different pro- 
portions and degrees of mixture ; the use of 
which is to discover the degree of purity ot 
any piece of gold or silver, by comparing the 
mark it leaves on the touchstone, with those 
of the bars. . The metals usually tried by the- 
touchstone, are gold, silver, and copper, 
either pure, or mixed with one another in 
different degrees and proportions, by fusion. 
In order to -find out the purity or quantity 
of baser metal in these various* admixtures, 
when they are to be examined, they are 
compared with these needles, which are mix- 
ed in a known proportion, and prepared for 
this use. The metals of these needles, both 
pure and mixed, are all made into laminae or 
plates, one-twelfth of an inch broad, and., a. 
