Earths. 
MINERALOGY. 
Earths. 
453 
yellow, or pale liver-colour, rarely diluted flefh-colour, 
with fometimes black fpots or ftirub-like ramifications ; 
furface fmooth and polilhed like agate 5 frafture conchoi- 
dal, with angular fragments. Its conftituent parts, as 
given by different experimenters, are— 
Klaproth. 
Bergman. 
Silex - - - 
- - 66 
47 
Alumine - - 
- - 14.50 
19 
Magnefia - - 
- C25 
6 
Lime - 
- - 0’25 
5’4 
Soda - - - 
- - 3 ’ 5 ° 
•- 
Oxyd of Iron • 
. - - 6 
— 
Water - - - 
- - 8 
17 
The Lemnian earth affords a confulerable revenue to the 
Turkifh government. It is dug but once a-year, on the 
1 5 th of Atlguft, in the prefence of the clergy and magif- 
trates of the ifiand, after the reading of prayers. The 
native ochre is walhed in order to feparate the fund, See. 
and is then formed into flat cylindrical pieces of an ounce 
weight, and each of them is afterwards flumped with a 
fealf having on it the Turkifh name of the mineral, and 
are thus fold under the name of terra figillata, or fealed 
earth. It is in great repute in the Eafl, as an antidote 
ao-ainft poifon and the plague ; and even in the time of Ho¬ 
mer was valued on the fame account. It was then damped 
as at prefent, and none but priefts were allowed to handle 
it. Severe punifhments were alfo inflidled on tliofe vyho 
prefumed to dig for it at any other but the ftated period. 
Its ufe with us is now principally confined to painting. 
7. Argilla fapo, or foap-clay : grealy, lightifh, very loft, 
fliining by friftion, adhering to the tongue, brownilh 
black. Found near Olhutfch in Poland ; of an earthy tex¬ 
ture. 
8. Argilla communis, or common brick-clay: very Ant, 
greafy, adhering to the tongue, plaftic, flaming the fingers 
a little, of an earthy texture, growing red in the fire, and t 
before the blow-pipe melting into a greenifh glafs. 
The ufual colour of this clay, which is met with in 
great abundance in various parts of England, is brown 
or llightly reddifh-brown ; which depends upon the pre¬ 
fence of oxyd of iron. The red colour peculiar to 
common bricks and tiles depends upon the alteration 
of the oxyd of iron during the procefs of burning; and 
this probably is eftedfted by the decompoiition of the wa¬ 
ter, with which the clay was tempered to give it the re- 
quilite degree of tenacity ; its oxygen contributing to the 
farther oxydation of the iron, and confequently produc¬ 
ing the ufual change of colour. If the proportion of 
oxyd of iron contained in brick-clay be greater than'or¬ 
dinary, and efpecially if at the fame time the proportion 
of lime is alfo conliderable, the bricks are often fuperfi- 
ciaily vitrified; and this affords an explanation of an ap¬ 
pearance very common in brick-kilns, where the furfaces 
of fome of the bricks are glazed, and of a green colour ; 
the oxyd of iron being difpofed to vitrify with earthy fub- 
flances, particularly lime, and poflefling the property of 
communicating a green colour to glafs. A variation in 
the proportions of lime and oxyd of iron is thus feen to 
affedt the chemical characters of bricks. Their mecha¬ 
nical characters are alfo affeCted by a variation in the 
proportions of the filex and alumine: if the.filex predo¬ 
minates in too great a degree, the clay is not fufficiently 
duCtile; if the alumine, the bricks made from it will be 
rifty. The belt proportions are, lilex 86, alumine 14. 
Thefe proportions are very nearly the fame as in the clay 
of the neighbourhood of London ; and, where the pro¬ 
portion ofalumine predominates to this extent, the com¬ 
pound is called loam. 
Stourbridge Clay is a natural clay, containing very little 
lime or iron ; and hence it is neither much coloured nor 
difpofed to vitrify in the higher degrees of heat; on which 
account it is employed in the conftruCIion of furnaces ; 
Von. XV. No. 1056. 
from external injury during that procefs. Thofe fire¬ 
bricks which are made at Stourbridge very readily pro¬ 
duce lparks when flruck by fleel; and their internal ap¬ 
pearance is altogether not very unlike the foft white gras 
nite of Cornwall, excepting that they are of a yellow co¬ 
lour. The granite of Cornwall is fometimes indeed em¬ 
ployed as a clay in the fabrication of crucibles, a manu¬ 
factory of which kind is carried on at Truro. From an 
examination of the fubflance of thefe crucibles it appears, 
that the granite has been Amply pounded ; and tempered 
with juft a fufftcient proportion of clay to give it the re- 
quifite degree of tenacity ; fince the conftituent parts of 
the granite may be eafily recognized by the eye. Thefe 
crucibles are nearly if not quite as refraftory in the fire 
as the Helfian crucibles; the Truro crucibles probably 
do not contain fo great a proportion of oxyd of iron. 
9. Argilla indurata, or indurated clay: foft, a little 
greafy, adhering flightly to the tongue, crumbling and 
loftening in water. ( 3 . Granular when dry. Found in 
every part of the globe ; lightifh, yellowifh, bluilh, or 
greenifh-grey, reddifh-brown, dull rofy, or greenifh, or 
varied with rounded fpots; of an earthy texture, and dif- 
covers but little duClility: falls to powder, but does not 
diffufe itlelf in water, and is fometimes fo hard as to ferve 
the purpoles of building-ftones. 
10. Argilla fiflilis, or fhiftol'e clay: very foft, rather 
meagre and dry to the touch, adhering a little to the 
tongue, of a flaty texture, breaking into dilcoid fragments. 
Found in Britain and various parts of Europe, in large 
layers, generally over and under veins of coal, and 
frequently penetrated with bitumen. Colour black or 
grey, rarely blue, fometimes yellowifh, reddifli, or brown ; 
when it contains much bitumen, it is of a blackifh-brown 
colour, appears like bad coal, and burns with a weak 
flame anti fulphureous fmell; frequently bears the im- 
preflion of plants, efpecially thofe of the equifetum, adi- 
. anthum, and fern, tribes; gives a whitifh or grey ftreak, 
moulders gradually in water. 
11. Argilla fterilis, or barren clay: fomewhat meagre, 
lamellar, white when dry, growing reddifli and hardening 
a little in the fire, melting in a greater degree of heat. 
Found in the barren plains of Sudermannia in Sweden, 
efpecially where birch-trees flourifh, and forms entire 
ltrata alternating with beds of fand. 
12. Argilla bullofa, or bubble-clay : greafy, foft, Alining 
by friction, adhering a little to the tongue, plaftic, grow¬ 
ing reddifli and blillery in the fire. Found in Tranfyl- 
vania. 
1 3. Argilla bolus, or bole : greafy, Alining by fri&ion, 
difl'olving in the mouth, crumbling into powder imme¬ 
diately in water, growing reddifli and eafily difl'olving in 
the fire, of a conchoidal texture. Found in Armenia, 
Italy, France, Portugal, and Germany : and is frequently 
produced from decayed lavas. Colour generally dull red 
or brown, fometimes yellow, flefh-colour, cinereous, and 
in innumerable varieties. Near Idria in Carniola, it is 
found mixed with cinnabar; and near Kafnick in Hun¬ 
gary, combined with filver. 
( 3 . Strigonienfis terra, or earth of Strigonium, probably 
a variety of this, is found about the gold-mines of Strigo¬ 
nium in Hungary, and ufed in Ibme places as an aftrin- 
gent and l’udorilic. Itis of a coarfe and impure texture ; in 
colour it is of a ftrong but dull red ; and is of a tolerably 
fmooth furface. It is apt to crumble to pieces between 
the fingers, and ftains the fkin in handling. It melts 
freely in the mouth, and has a remarkable lmoothnefs, 
but very little aftringency in its tafte, and leaves a fenli- 
ble grittinefs between the teeth. It is fometimas veined 
and ipo.tted with finall moleculas of an earth, like the 
whitifh variegations of the red French bole. It makes a 
flight effervefcence with aqua fortis, or any other acid 
menftruum, and ftiffers no change of colour by burning. 
mu. 
5 Z 14. ArgilJa. 
