we % 3 FE: L 
way ‘be lengthened in the whole, and is ~3_ of the whole an inftrument or machine for fhewing when fhe fun arrived at 
length XZ when-fhorteit. ~The reafon is, this arm is the tropics and the:equinoétial line. » This name was alf@ 
always to ircreafe and decreafe in proportion to the fecant ufed for a fun-dial in gener: oda 
i HELIoTROPE, in Mineralogy, Silex heliotropius, Wern 
but the radius is to the fecant of 23° 30’, (the fun’s greate Jafpis variegata heliotropius: Wall. -Heliotrope, Broch, Helios 
‘declination,) as 10 : 4411, or as 100 to fog. tropium, Kirw. Eljotropio, Napion. vuls. Blood-ftone, Engl. 
Now the reafon of this conftru@tion of the arm X Z is to Jafpe oriental, Jafpe fanguin, &c. Fr. Griiner jafpis, Oriens 
find for any given day the diftance of the centre of the fpe- thes jafpis, Germ. 
culum S from the top / of the ftyle /N, which muft ever “~The colour of this mineral is green, generally between 
be equal to the fecant of the fun’s declination ; for it muft grafs and leek-green; the depth of the colour is proportion. 
always be equal to the diftance of the top of the faid ftyle 1d ote to the degree of tranfparency : the varieties that are but 
from the centre of the cylinder R in the fork T, and that is little tranflucent are fometimes blackith-green ; it alfo occurs 
ae : of a colour between feladon and blueifhe reen, and fometimes 
For fince the flyle 7N and the fork 'T’ are In @pofition hibits a mixture of feveral of Rt unequally diftri- 
parallel to each other, therefore the middle hole in the fides buted. ‘The red and yellow fpots and veins with which it is 
of the fork being (as ‘they muft be of the fame height often marked, are produced by diffeminated Particles of 
jafper of thofe colours. ei ee 
Itis found maffive and in rounded and angular pieces, with | 
now and then {mall botroidal ‘furface ; a fpecimen | fore us é 
is furnifhed with many minute globular excrefcences, coated. 
fh 
fame ftyle will fall exaétly on the faid hole, with a yellow ochrey crult, 
oe | 
. 
“ which the rays make with this firft or middle ray, that tranflucidity, from nearly tranfparent to feebly tranflucent 
pa eR : 
~_ Now it may be demonttrated, that on any day of the year Specific gravity 2.633, Blumenbach ; 2.620—2.700, Kire. 
wan. . 
be exactly in the meridian, and that the pofition of Rinthe — ‘The heliotrope is infufible before the blow- ipe without 
fork'be fuch, that the fun’s rays go directly throughit, and addition; it only becomes r more brittle; its green 
the thadow of the ftyle’s top fall juft upon the hole 3 more- colour is changed into greyifh-white,. or reddifh-brown, the 
over, if the diftance of the centre of the fpeculum § from red fpots difappear, leaving {mall vacuities, with a lofs in 
the top of the ftyle / be made equal (by the pofitor) to the weight of 0.01. 
diftance of the central point R therefrom ; and laftly, the Its native places are not well afcertained. It is well 
tail of the {peculum DE pafling through R ; if then the known that the epithet oriental, applied to ftones held in elti- 
ock be be into motion, the index NO fhall carry about mation, conveys no “other meaning than that of fuperior 
lit beauty. It ; é t 
of the day when the fun can come upon the {peculum it will i forces arti ve Welter fia! Siescially in Perfia; 
reflect the rays conftantly in one and the fame pofition and jf, jin Egypt, in fome” parts of Italy, efpecially Sicily; 
direction the time without variation, . (where it likewife occurs with yellow veins,) and in Ice- 
The machine thus conftituted is placed in a box or cafe, Jand. 
and fet in a window with one fide open, expofed to the fun, 
and all the other parts clofe ; fo that when the room is made 
dark, and the folar microfcope fixed to the fore-part of the 
The heliotrope is u/ed for ornamental purpofes, and for 
{nuff boxes, {mall veffels, &c.; nor is it neglecte 
glyptic artift. See Gems, engraved. 
of the fpeculum to receive the reflexed horizontal beam, all It is probable, that the heliotrope of Pliny, which is 
the experiments of the darkened room are then performed as defcribed by this author as a leek-green ftone traverfed by 
ufual.” This is a very ingenious conftrudtion of ‘a folar mi- bloed-red veins, is the fame with our blood-ftone. It ree 
crofcope apparatus, and full of art, but, it is to be feared, ceived the Latin name from its fuppofed quale of . reflect. 
too expenfive and troublefome for common ufe. However, mg the folar rays blood-red, when immer ed in water. 
it is eafy to fee that this machine is capable of being greatly The eer bafe of blood-ftone, (the gras. of red jafper 
aNee, pie tte well are accidental, ) is; timat binat chale 5 oy aga 
without a clock ; alfo the fpeculum may be glafs initead of green earth, not with chlorite. The more tran{parent varies 
metal,’ aidall fixed on one foot or pedeltal. ties refemble much the plafma, and fome varieties of prafe. 
For a farther account of this inftrument, the principles _ Of its geognoftic fituation we know but little ; accords 
of its tion, and its ufe, fee S'Gravefande’s Phyfices ing’to the accounts of fome mineralogifts it occurs, in.a fim} 
Element. Mathematica, &c. tom. ii. p. 715, &c. ed. 3tia. lar manner as ch 8 i Ww 
r. Accum given the defcription and analyfis of am 
tothe Egyptian heliotrope, which, in ccaitantel and phyfical cha- 
vebeen racters, agtees retty well with the European and Afiatic 
varieties already known. By ignition it c its original 
colour for chocolate brown, with white yellow veins 
and loft rather more than ;!. ofits weight. The a 
grains analyfed by the above chemilt, confilted of “Silex 
sS 
a 
at i 
