300 C H E M 
it, and add more water; and repeat the operation till all 
the pafte is reduced to a fine black powder. Throw this 
powder into boiling water; and, having left it to fettle, 
draw off the water, and dry the precipitate. If, inftead 
of iron filings, iron already oxydated be ufed, a fingle 
operation is fufficient to convert the whole into Ethiops. 
For thedecompofitionof water by iron, fee page 207. If 
concentrated fulphuricacid be poured overiron-fil ings,ful- 
phureous acid gas is obtained. When this mixture is 
diftilled to drynefs, the retort is found to contain flowers 
of fulphur fublimed, and a white mafs offulphatof iron, 
partly foluble in water, which, however, does not afford 
cryftals. If this acid, diluted with two parts of water, 
be mixed with half its weight of iron-filings, it difiolves 
the metal very readily. The folution is attended with 
the dif'engagement of a large quantity of hydrogen gas, 
. which may be made to detonate with a confiderable noife, 
by applying a lighted candle to the aperture of the veffel, 
after having clofed it for a flrort time with the hand. In 
proportion as the diluted fulphuric acid adls on the iron, 
a portion of the metal is precipitated in a black powder, 
which Stahl fuppofed to be fulphur, but Monnet found, 
on examination, to be martial PEthiops. This portion 
of black oxyd of iron, fuperabundant to the faturation, 
frequently contains carbure of iron. The fulphuric 
acid diflblves more than half its weight of iron, and the 
folution filtered and evaporated, affords, by cooling, a 
tranfparent fait, of a green colour, cryftallized in rhom¬ 
boids, called martial vitriol, ok green copperas-, we term 
l it fulphat of iron. This fait is not made in the direft 
way, becaufe it is abundantly afforded by nature, and is 
eafily extracted by art from martial pyrites. 
Sulphat of iron is of an emerald green colour, and has 
a very ftrong aftringent tafle; its cryftals contain, ac¬ 
cording to Kunckel and .Monnet, more than half their 
weight of water; if'it be heated brifkly, it liquifies like 
all falts, which are more foluble in hot than in cold wa¬ 
ter; it becomes of a whitifh grey by drying, it was call¬ 
ed 'vitriol calcined to whitenefs. Diftilled in a retort 
placed in a reverberatory furnace, this fait affords firft, 
water flightly acid, called dew of 'vitriol . The receiver 
is to be changed, in order to obtain feparately the con¬ 
centrated fulphuric acid, which, when the heat is very 
ftrong, paffes over of a black colour, and exhaling a 
fuffocating fmell of volatile fulphureous acid. Thefe 
characters depend on its being deprived of a part of its 
oxygen, which is fixed in the iron, according to the doc¬ 
trine of the gafes; towards the end of the operation, 
the acid which comes over takes a concrete and cryftal- 
line form, and is diftinguifhed by the name of glacial 
fulphuric acid. When glacial fulphuric acid is diftilled in 
a finall retort, it gives out fulphureous gas, and comes 
over white and fluid; its concrete ltate is therefore 
owing to the prefence of this gas ; it unites with water 
with noife and heat, fulphureous gas being at the fame 
time difengaged. The fuming oil of vitriol of Noor- 
thauffen is of this kind, and the concrete fait obtained 
from it by a gentle heat, of which Fourcroy gave an 
analyfis in a memoir publifhed among thofe of the Aca¬ 
demy for the year 1785. 
The refill ue of fulphat of iron, after diftillation, is red. 
Colcothar is fulphat of iron, calcined to rednefs : Put 
fulphat of iron into a crucible ; cover the crucible, place 
it in a furnace, and make it red-hot; keep it in this ftate 
for an hour, then let it cool, and it will be of a red co¬ 
lour. When walhed with water, a white fait little 
known, and named fait of colcothar, or fixed fait of 'vi¬ 
triol, isfeparated; a red inlipid earth, which is a pure 
oxyd of iron, and is called fweet earth of 'vitriol, remains 
behind. 
Sulphat of iron expofed to the air becomes yellowifli, 
and covered with ruft; oxygen being gradually abforbed, 
oxydates the iron more and more, lb that it cannot re¬ 
main united with the fulphuric acid. Thus the green 
fulphat is changed to red. The fame thing may be done 
1 
I S V R Y. 
by diflolving this fait in aerated water, by agitating its 
folution in the air, by the aftion of nitric acid, or of 
oxygenated muriatic acid; in fliort, by all thofe methods. 
which communicate oxygen, and which may increafethe 
quantity of 0-27 of oxygen, which it contained at firft, to 
o - 48, the proportion neceflary to conftitute the red oxyd. 
From thefe experiments, firft made by Prouft, it ap¬ 
pears, that there are two lulphats of iron, very different 
from each other. The properties which that chemift dis¬ 
covered in each areas follow: That the green fulphat was 
cryftallizable, and infoluble in alcohol, of a fea-green 
colour, effiorefcing and turning yellow by expofureto the 
air, not altered by the gallic acid, yielding no Prufiian 
blue with the alkaline prufliats; it gives with cauftic al¬ 
kalis a dark green precipitate which grows black under 
water when deprived of contact with air, and contain¬ 
ing o - 27 of oxygen, and 0^73 of iron. The red fulphat 
of iron does not cryftallize; it is foluble in alcohol, deli- 
quefcent, gives a black precipitate with the gallic acid, 
and blue with the alkaline prufliats, but with pure alkalis 
a reddilh yellow precipitate, no longer oxydable, and 
containing 0-48 of oxygen, and 0-52 of iron. Hydro- 
fulphurated water, or fulphureous water, decompofes 
and precipitates the red fulphat of iron, nitrat of iron, 
&c. but does not affeCt the green fulphat: the red oxyd 
burns the hydrogen, and the fulphur. Thus the mo¬ 
ther-water of fulphat of iron, or this fait fuper-oxygena- 
ted, may be reftored to the ftate of a green and cryltalli- 
zable fait. When this laft is precipitated by fulphureous 
water, the precipitate is brown, which is occafioned by 
a fulphat of iron, being often mixed with this fait for 
fale. The prefence of copper in this fulphat will be evi¬ 
dent upon plunging a bar of iron into the folution, or by 
means of ammoniac. 
Sulphat of iron is decompofed by lime. Lime-water 
poured into'a folution of this fait, forms a precipitate in 
flocks, of a deep olive green ; a portion of this precipi¬ 
tate is re-diflolved in the lime-water, and communicates 
to it a reddifti colour. Potafh, faturated with the car¬ 
bonic acid, or the carbonat of potafli, forms a precipi¬ 
tate of a. greenifh white colour, not foluble in the alkali; 
this difference arifes from the prefence of the carbonic 
acid, which feizes the iron, in proportion as it is itfelf fe- 
parated from the alkali by the fulphuric acid. Pure or 
cauftic ammoniac feparates from the folution of fulphat 
of iron a precipitate of lo deep a green, that it appears 
almoft black; it is trot foluble in the ammoniac. 
Vegetable aftringent matters, fuch as nut-galls, fu- 
nuch, rind of pomegranates, hulks of unts, quinquina, 
cyprefs nuts, logwood, tea, &c. have the property of 
precipitating fulphat of a black colour; this precipitate, 
which cannot be miftaken for iron, is fo extremely divi¬ 
ded, that it remains fufpended in the fluid ; the addition 
of gum arabic to the mixture, caufes the iron to be per¬ 
manently fufpended, and forms the black fluid, called 
ink. We call this fait the gallic acid, and fhall give an 
account of it in the vegetable clals. 
Thedecompofitionof fulphat of iron, by analkali calcin¬ 
ed with bullock’s blood, is a phenomenon ftill more diffi¬ 
cult to be underftood than the aCtion of the nut-gall on this 
fait; the precipitate obtained is of a beautiful blue co¬ 
lour, and infoluble in acids. This precipitate is called 
P ruffian or Berlin blue, from the place of its dilcovery. 
To form Pruffian blue, four ounces of nitre fixed by 
tartar, are mixed with an, equal weight of dried ox’s 
blood; this mixture is calcined in a crucible tillitrefem- 
bles coal, and no longer produces any flame ; a fuffici¬ 
ent quantity of water is then added to diffolve all the fa- 
line matter, which is called pblogifiicated alkali, or colour¬ 
ing lixi'vium, or pruffiat of potajh with excefs of alkali, 
and is concentrated by evaporation ; two ounces of ful¬ 
phat of iron, and fourounces of fulphat of alumine, are 
afterwards diffolved in a pint of water; the folution of 
thefe falts is mixed with the alkaline lixivium, a green¬ 
ifh precipitate foils down, whichis feparated by the filter, 
