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phous sulphide is formed also by the reduction of ferric oxide
or its salts in presence of sulphates and decomposing organic
matter. This is the cause of the black deposit found in drains,
as well as in the excrement, when iron is used as a medicine.

Iron Sesquisulphide, or Ferric Sulphide, Fe 2 S 3 , does not occur
pure in the mineral kingdom, but probably forms a constituent of
magnetic pyrites and copper pyrites. It is obtained by the
action of ammonium sulphide solution on a solution of a ferric
salt, so long as alkali is in excess, 2 but if the ferric salt be in ex-

1 Treitschke and Tammann, Ze.it. anorg. chem., 1906, 49, 320. Compare
Allen, Crenshaw, Johnston, and Larseri, Amer. J. Sci., 1912, 33, 169.

2 See Stokes, J. Amer. Chem. 8oc., 1907, 29, 304: Malfatti, Zeit. anal.
Chem., 1908, 49, 133.

VOL. II 4 K



1234



IRON



cess a mixture of ferrous sulphide and sulphur is formed. It
produced also by the action of sulphuretted hydrogen on ferric
hydroxide in presence of traces of ammonia, and is therefore the
chief product formed in the purification of coal-gas from sul-
phuretted hydrogen (Vol. I., p. 871), as this gas always contains
small quantities of ammonia. 1 In the dry way it is formed by
gently heating sulphur and iron together, as well as by the
action of sulphuretted hydrogen on ferric oxide at a temperature
not above 100. The sulphide obtained at a red heat forms a
yellow non-magnetic mass which has a specific gravity of 4'4,
and is decomposed by dilute hydrochloric acid into sulphuretted
hydrogen, ferrous sulphate, and iron disulphide. Iron sesqui-
sulphide forms compounds with the other sulphides. 2

Potassium Ferric Sulphide, K 2 Fe 2 S 4 , is obtained when iron
filings, sulphur, and potassium carbonate are heated together,
and the residue is extracted with water. The purple-coloured
glistening needle-shaped crystals thus obtained have a specific
gravity of 2*863, and burn when heated in the air, but
when ignited in a current of hydrogen are converted without
change of form into the black compound, K 2 Fe 2 S 3 .

Sodium Ferric Sulphide, Na 2 Fe 2 S 4 ,4H 2 O, is obtained in a
similar way and forms brown microscopic needles. It is found
in the " black ash " liquors obtained in the manufacture of soda
by the Leblanc process (p. 297).

Silver Ferric Sulphide, Ag 2 Fe 2 S 4 , is a dark brownish-black
crystalline powder obtained by the action of silver nitrate solu-
tion on the potassium compound.

Cuprous Ferric Sulphide, Cu 2 S,Fe 2 S 3 , occurs as the mineral
copper pyrites.

Magnetic pyrites maybe regarded as a compound of the mono-
and sesqui-sulphides. Its composition varies between 5FeS,
Fe 2 S 3 and 6FeS,Fe 2 S 3 . It crystallises in hexagonal plates,
usually, however, occurring in the massive state, having a
brownish-yellow or brassy colour; it is attracted by the
magnet, sometimes being itself magnetic. Its specific gravity
varies from 4*4 to 47, and it frequently contains as much as
5 '5 per cent, of nickel, the latter metal being obtained in
Canada from this source in considerable quantities.

Iron, Disulphide, FeS 2 , occurs very widely distributed as iron

1 Gedel, J.fiir Gaftbdeuchtung, 1905, 48, 400 : Stokes, loc. cit.
a Schneider, Pogg. Ann., 1869, 136, 4(50; IVeis, J. pr. Ch<m., 1S<><), 107,
10; Malfatti, /.'it. !. cAn., 1909,48, XV-'.



IRON PYRITES 1235



pyrites. This mineral was known in early times, but was not
distinguished from copper pyrites, both being known under the
name of irvpiTrjs. Agricola considered these as two varieties of
the same mineral.

Iron pyrites is found in all geological formations ; it crystallises
usually in cubes or pentagonal dodecahedra, 1 but occurs also
in many other forms and combinations of the regular system,
no fewer than sixty-nine different forms having been described.
It is frequently found in spherical, botryoidal, or stalactitic
masses, being formed by the action of organic matter on water
which con tains iron sulphate in solution. Hence it is frequently
found in peat and coal in crystalline masses often possessing the
form of the original organic matter, such as wood, roots, &c.
It is likewise found in chalk cliffs, in similar concretionary
forms.

In the pure state iron disulphide has a brass-yellow colour, and
a specific gravity of 5'185. It is very hard, giving sparks when
struck with steel, for which purpose it was formerly employed.
Iron disulphide occurs also as radiated pyrites and marcasite
forming bright brass-coloured rhombic crystals with a specific
gravity of 4'68 to 4*85. This also is widely diffused, and occurs
in various forms, especially in lignite beds. Iron disulphide may
be obtained artificially by gently heating the monosulphide with
sulphur, or by passing sulphuretted hydrogen over the oxides
or chlorides of iron heated to redness. When an intimate
mixture of ferric oxide, sulphur, and sal-ammoniac is heated
slowly above the temperature at which the latter compound
volatilises, the disulphide is obtained in small brass-yellow
octahedra and cubes (Wohler). Crystalline pyrites is formed also
when carbon disulphide vapour acts upon heated ferric oxide
(Schlagdenhauffen), and when ferric chloride is heated with
phosphorus pentasulphide. 2 Iron pyrites and marcasite may be
produced artificially also by the action of hydrogen sulphide on
ferric sulphate solution. 3 Iron disulphide is not magnetic and
is not attacked by dilute acids or sulphuric acid, but readily
dissolves in nitric acid with separation of sulphur. There is
some evidence showing that the iron in iron pyrites is in the
ferrous condition. 4

1 See Poschl, Ze.it. Kryst. Min., 1911, 48, 572

2 Glatzel, Btr., 1890, 23, 37.

3 Allen, J. Washington Acad. Sci., 1911, 1, 170.

4 Benedek, Zeit. Kryst. Min., 1910, 48, 447; Plummer, /. Amer. Chem.
Soc., 1911, 33, 1487.

4 K 2



1236



IRON



Iron Subsulphide, Fe 4 S 3 , is formed when iron is heated in the
vapour of carbon disulphide, as a crystalline mass of specific
gravity 6'95*7. It dissolves in dilute acids with evolution of
hydrogen and sulphuretted hydrogen. 1

Ferrous Sulphite, FeSO 3 . When iron is dissolved in aqueous
sulphurous acid in absence of air, no gas is evolved, and the
solution contains ferrous sulphite and ferrous thiosulphate :

2Fe + 3H 2 S0 3 = FeSO 3 + FeS 2 3 + 3H 2 O.

The latter salt is a very soluble one ; the first, however, is only
slightly soluble, so that after a short time it is deposited in
colourless or greenish crystals. When freshly precipitated
ferric hydroxide is dissolved in sulphurous acid, a red solution is
obtained, which quickly becomes decolorised with formation of
ferrous sulphite, whilst, on the other hand, a solution of ferrous
sulphite becomes red on exposure to the air. 2

Ferrous Sulphate, or Green Vitriol, FeS0 4 ,7H 2 O, was probably
used by Geber. Agricola, in his discourse De re Metallica,
mentions two kinds of pyrites. The one, such as coal-brasses,
decomposes spontaneously and yields a vitriol ; whilst the other,
as the ordinary Mason's pyrites, does so only when it is roasted.
Its preparation by dissolving iron in sulphuric acid was described
by " Basil Valentine " in his 2 J reatise on Natural and Super-
natural Things: "Take oleum vitrioli; dissolve therein mars,
and prepare a vitriol from it." In his last volume he describes
the method for preparing sulphide of iron and for obtaining
vitriol from it : " Limaturam ferri and sulphur ana calcined
in a potter's furnace until it becomes tinted purple; then
pour upon this distilled water, when a fine green liquid is
formed. Draw this off ad tertias, allow it to deposit, and thus
obtain an artificial vitriol." Green vitriol occurs as the mineral
melanterite, either crystalline or in fibrous stalactitic forms,
but generally massive and pulverulent. It is usually derived
from the decomposition of pyrites or marcasite. Ferrous
sulphate is likewise frequently found in solution in drainage
water from mines, and it is manufactured on a large scale from
this source. Large quantities of green vitriol (about 100 tons
per week) are manufactured in South Lancashire from the
pyrites occurring in the coal measures. These are piled up in
heaps and exposed to the atmosphere. The soluble ferrous

1 (Sautier and Hallopeau, Compt. rend., 1889, 108, 800.

<J See also Seubert and Elten, Zeit. anory. Chem., 1893, 4, 44.



FERROUS SULPHATE 1237

sulphate, together with the excess of sulphuric acid formed, runs
into underground tanks, where the excess of acid is removed
by means of scrap iron. On evaporating the liquor large
crystals of ferrous sulphate are obtained. Ferrous sulphate is
formed also as a by-product in the manufacture of copper
sulphate or blue vitriol (p. 435). The commercial salt not
infrequently contains traces of copper sulphate, and this may be
detected and separated, as was pointed out by Vigani, so long
ago as 1683, by leaving the solution in contact with metallic
iron until the whole of the copper is precipitated. Another
common impurity is ferric sulphate ; this may be removed by
recrystallisation, but zinc sulphate, manganese sulphate, and
other salts cannot thus be got rid of. Hence when chemically
pure ferrous sulphate is needed it is best to treat an excess of
iron wire with dilute sulphuric acid. When the evolution of
hydrogen has ceased, the liquid is boiled together with the un-
dissolved portion of the wire, filtered, and evaporated to crys-
tallisation.

Ferrous sulphate forms well-defined monoclinic crystals with
a specific gravity of 1'889 at 4 (Joule and Play fair), but is
dimorphous, often crystallising in rhombic prisms, which are
isomorphous with zinc sulphate. These are obtained when a
crystal of zinc sulphate is thrown into a supersaturated solution
of the ferrous salt, but, on the other hand, if a crystal of copper
sulphate be employed, triclinic crystals having the composition
FeS0 4 ,5H 2 are obtained (Lecoq de Boisbaudran). When
ferrous sulphate is heated in a vacuum to 140, it yields a white
powder of the monohydrate, FeS0 4 ,H 2 O, and this, when gently
heated in absence of air, yields the anhydrous salt, FeSO 4 .
Exposed to the air, the heptahydrate gradually loses water and
becomes converted into basic ferric sulphate.

Up to 100 there are three hydrates which are successively in
stable equilibrium with the aqueous solution, namely, the
hepbahydrate, stable from 1'82 (the eutectic point) to 56'6,
the tetrahydrate, from 56'6 to 64'4, and the monohydrate,
above the latter temperature. 1 The solubility at various tem-
peratures, in grams of anhydrous ferrous sulphate per 100 grams
of saturated solution, is as follows :

10' 20 40 54 64 77 90

13-5 17-0 21-0 287 34'3 357 31'5 27-2,

1 Franckel, Zeit. anorg. Chem., 1907, 55, 223.



In addition to those already mentioned, hydrates with 6H 2 O,
3H 2 0, and 2H 2 O have been described.

Ferrous sulphate is insoluble in concentrated sulphuric acid
and absolute alcohol, and the addition of strong sulphuric acid
to a saturated solution of the salt causes the precipitation of
the monohydrate. 1 A solution of ferrous sulphate, like the
chloride, absorbs nitric oxide. The dark brown saturated solu-
tion, which probably contains the compound FeS0 4 ,NO (p. 1241),
gives off the gas in a vacuum as well as when heated ; in the
latter case small quantities of nitrogen monoxide and ferric
sulphate are formed. When the brown solution is mixed with
strong sulphuric acid, care being taken to keep the mixture
cool, it becomes of a purple-red colour : and upon this reaction
the well-known test for nitric acid and the nitrates depends, as
well as the method of detecting the presence of nitrous fumes
in sulphuric acid.

Green vitriol is largely used in the arts and manufactures for
the preparation of iron mordants, inks, Prussian blue, &c.

Ferrous sulphate enters into the composition of various
double salts. There are, for instance, the red and yellow sub-
stances formed when sulphuric acid acts on a concentrated
mixed solution of ferrous sulphate and copper sulphate. 2 Again,
ferrous sulphate, like the sulphates of the metals of the magne-
sium group, copper, and manganese, forms crystalline double
salts, with the sulphates of the alkali metals; of these the
following is the most important.

Ferrous Ammonium Sulphate, FeSO 4 ,(NH 4 ) 2 S0 4 ,6H 2 O, is
obtained when the calculated quantities of ammonium sulphate
and green vitriol are dissolved in the minimum quantity of hot
water, and the filtered solution is allowed to crystallise or is pre-
cipitated with alcohol. It forms clear, hard, bluish-green mono-
clinic crystals, which have a specific gravity of 1*81 3. One
hundred parts of water dissolve (Tobler) :

At 20 30 60 75"

FeS0 4 ,(NH 4 ) 2 SO 4 12'2 21'6 281 44'6 567.

This salt is a very stable one, and does not readily undergo
alteration in the air, being much less easily oxidised than green
vitriol itself. Hence it is largely used instead of the latter salt
for the purposes of volumetric analysis.

1 Compare Kenrick, J. Physical Chem., 1908, 12, 693.

8 6tard, Compt. rend., 1878, 87, 602; Scott, Journ. Chem. Soc., 1897, 71,
564 ; Allmand, Zeit. anorg. Chem., 1909, 61, 202.



FERRIC SULPHATE 1239

Ferrous Disulphate, FeS 9 7 , separates out as a white powder
when a concentrated solution of ferrous sulphate is mixed with
several times its volume of concentrated sulphuric acid. It
forms microscopic prisms, and is decomposed by water into
sulphuric acid and green vitriol. 1

Ferric Sulphate, Fe 2 (S0 4 ) 3 , is obtained by the action of nitric
acid on a hot solution of green vitriol, to which the requisite
quantity of sulphuric acid has been added :

6FeS0 4 + 3H 2 SO 4 + 2HN0 3 = 3Fe 2 (SO 4 ) 3 + 2NO + 4H 2 O.

The yellowish-brown solution gives a syrupy liquid when com-
centrated, from which colourless crystals are deposited on
standing. When these are heated, or when sulphuric acid is
added to the concentrated solution, the anhydrous salt is formed
as a white powder, which dissolves slowly in water, whilst by the
action of green vitriol on boiling sulphuric acid the same salt is
deposited in small crystalline scales or rhombic prisms :

2FeS0 4 + 2H 2 S0 4 = Fe 2 (SO 4 ) 3 + SO 2 + 2H 2 0.

Various hydrates also have been described. 2

When a dilute solution of ferric sulphate is boiled or incom-
pletely precipitated with alkalis, or when a solution of green
vitriol is allowed to oxidise in the air, various basic ferric sulphates
are formed. 3 An iron mordant obtained by oxidising green
vitriol with nitric acid deposits on standing large transparent
crystals, which are probably monclinic, having the composition
FeS0 4 (OH),7H 2 ; these are decomposed by water with
formation of the insoluble salt, Fe 2 SO 4 (OH) 4 ,5H 2 O. 4 Various
other basic ferric sulphates occur as minerals, 5 being formed by
the oxidation of the sulphides of iron. Amongst these may
be mentioned vitriol ochre, FeS0 4 (OH) 4 ,2Fe(OH) 3 ,H 2 O. This
frequently occurs in long brown, green, or ochreous yellow stal-
actites; whilst copiapite, Fe 2 (SO 4 ) 3 ,Fe 2 (SO 4 ) 2 (OH) 2 ,10H 2 0, occurs
in sulphur-yellow tablets or crystalline scales, and nbro-ferrite,
2Fe 2 (SO 4 ) 2 (OH) 2 ,Fe 2 SO 4 (OH) 4 ,24H 2 O, forms a pale yellow or
nearly white pearly or silky mass.

Anhydrous ferric sulphate, when heated, dissociates into ferric

1 Bolas, Journ. Chem. Soc., 1874, 27, 212.

' J Recoura, Compt. rend., 1905, 141, 108 ; 1907, 144, 1427.

3 Compare Cameron and Robinson, /. Physical Chem., 1907, 11, 641.

4 Ber., 1875, 8, 771.

5 See Scharizer, Zeit. Kryst. Min., 1907, 43, 113 ; 1909, 46, 427.



1240



[RON



oxide and sulphur trioxide. This fact is of importance in con-
nection with the use of ferric oxide as a catalyst in the contact
process for manufacturing sulphuric acid. 1

Ferroso-ferric Sulphates. The two sulphates of iron form
various double salts, of which some are found in the mineral
kingdom. Amongst these is roemerite, Fe 3 (SO 4 ) 4 ,12H 2 O,
yellow monoclinic crystals, occurring at the Rammelsberg
mine, near Goslar, together with another similar mineral termed
voltaite, in which a part of the iron is replaced by isomorphous
metals.

Ferric Potassium Sulphate or Iron Alum, Fe 2 (SO 4 ) 3 ,K 2 S0 4 ,
24H 2 O, is obtained when the proper quantity of potassium
sulphate is added to a solution of ferric sulphate and the con-
centrated solution allowed to stand for some days about 0.
The salt forms bright-violet octahedra, and dissolves in about
five parts of cold water. If caustic potash is added to the
solution and the dark liquid allowed to evaporate, transparent
yellowish-brown hexagonal crystals separate out which have
the composition 5K 2 SO 4 ,2Fe 2 (SO 4 ) 2 (OH) 2 ,16H 2 O, and possess
the peculiar optical properties of tourmaline. This salt easily
decomposes into iron alum and an insoluble basic ferric salt.

Ferric Ammonium Alum, Fe 2 (S0 4 ) 3 ,(NH 4 ) 2 S0 4 ,24H 2 O, closely
resembles the potassium derivative, and is sometimes termed
" iron alum." Several other double salts of ferric and ammonium
sulphates have been described. 2



IRON AND THE ELEMENTS OF THE NITROGEN GROUP.

566 Iron Nitrides. When nitrogen is passed over heated iron,
the metal is rendered brittle, probably owing to the alternate
formation and decomposition of an iron nitride, but the latter
compound cannot be prepared in this manner. The nitride
may, however, be obtained by heating the metal in ammonia,
an observation first made by Berthelot, and confirmed by
Stahlschmidt. 3

If ammonia in excess is allowed to act on anhydrous ferrous
chloride or bromide, or on finely divided reduced iron, or on

1 Keppeler and D'Ans, Zeit. physilcal Chem., 1908, 62, 89 ; Wohler,
Pliiddemann, and Wohler, Ber., 1908, 41, 703 ; Zeit. physical. Chem., 1908,
62, 641 ; Bodenstein and Suzuki, Zeit. Elektrochem. , 1910, 16, 912.

2 Lachoud and Lepierre, Compt. rentl., 1892, 114, 915.

3 Fogy. 4ftft., !*;.=), 125, 37.



FERROUS NITRATE 1241

iron amalgam at a temperature of about 420, iron nitride
is obtained as a dull grey powder. 1 In the compact state it
is best obtained by heating iron wire or rod to a bright red
heat with a large excess of ammonia, excess being necessary
inasmuch as the hydrogen formed reduces iron nitride at the
same temperature as that at which it is produced. 2

The substance has a composition corresponding with the
empirical formula Fe 2 N, and is so brittle that it may be
readily powdered in a mortar ; it is somewhat magnetic, and has
a specific gravity of 6'0-6'5. It readily oxidises when heated
in the air, and ignites when warmed in chlorine. It is
dissolved by dilute hydrochloric and sulphuric acids with
evolution of hydrogen, and formation of ferrous and ammonium
salts.

According to Guntz, 3 ferrous nitride, Fe 3 N 2 , and ferric nitride,
FeN, both of which are black powders and differ from the fore-
going, are formed by heating lithium nitride with ferrous
potassium chloride and ferric potassium chloride respectively.
A nitride of the formula Fe 5 N 2 was found by Silvestri 4 as a
lustrous metallic deposit on the Etna lavas. This is, however,
possibly a mixture or solid solution of iron and the nitride,
Fe 2 N.

Ferrous Nitrate, Fe(N0 3 ) 2 ,6H 2 O, is best obtained by the
decomposition of green vitriol with barium nitrate, the filtrate
being evaporated in a vacuum over sulphuric acid. It is
very soluble in water and very unstable, easily passing into
ferric nitrate. When iron is dissolved in cold dilute nitric
acid, the following reaction takes place (Berzelius) :

8Fe + 20HN0 3 = 8Fe(NO 3 ) 2 + 2NH 4 NO 3 + 6H 2 O,

but the reaction varies greatly with the concentration of the
acid and the temperature. 5

Ferric Nitrate, Fe(NO 3 ) 3 , is formed by dissolving iron in
nitric acid. The brown concentrated solution deposits, on
addition of nitric acid, according to the acidity and the con-
centration of the solution, colourless cubes of Fe(N0 3 ) 3 ,6H 2 O,
or colourless monoclinic crystals containing 9 molecules of

1 Fowler, Journ. Chem. Soc., 1901, 79, 285; White and Kirschbraun,
J. Amer. Chem. Soc., 1906, 28, 1343; Girardet, Bull. Soc. chim., 1910, 7,
1028.

2 Beilby and Henderson, Journ. Chem. Soc., 1901, 79, 1249.

9 Compt. rend., 1902, 135, 738. 4 Pogg. Ann., 1876, 157, 165.

3 Montemartini, Journ. Chem. Soc., 1892, 62, 1278.



1242



IRON



water. 1 These are very deliquescent, and dissolve in water
forming a brown liquid, which becomes colourless when concen-
trated nitric acid is added to it. Ferric nitrate is used as a
mordant in dyeing and calico-printing.

Several soluble and several insoluble basic ferric nitrates are
known.

Nitroso-compounds of Iron. As already mentioned, nitric
oxide is readily absorbed by solutions of ferrous salts with
formation of dark olive-green to black solutions, which con-
tain unstable compounds of nitric oxide with the ferrous salt.
The maximum absorption for solutions of ferrous salts in water,
alcohol, and other neutral solvents, is found to be in the pro-
portion of 1 mol. NO to 1 mol. ferrous salt. The reaction,
however, is reversible, the degree of dissociation not only vary-
ing with different ferrous salts, but being dependent also on the
pressure, the temperature, the concentration of ferrous salt, the
solvent, and the presence of other dissolved substances. 2 When
dissolved in strong hydrochloric acid, ferrous chloride absorbs
about twice as much nitric oxide as when dissolved in the same
quantity of water. 3

Anhydrous ferric chloride also absorbs nitric oxide, forming
the compounds 2FeCl 3 ,NO and 4FeCl 3 ,NO, which are reddish-
brown non-crystalline hygroscopic powders. When nitric oxide
is passed into an ethereal solution of ferric chloride, nitrosyl
chloride is formed, and on evaporation over sulphuric acid,
black needles of the composition FeCl 2 ,NO,2H 2 O are obtained,
whilst at 60 the anhydrous compound FeCl 2 ,NO is formed
crystallising in yellow needles. 4

In addition to the foregoing unstable nitroso-derivatives
others of a more stable nature have been prepared. These were
discovered by Roussin, 5 who prepared them by the action of
ferrous sulphate on mixed solutions of the nitrites and sulphides
of the alkalis, and they have since been investigated by many
other chemists. 6 Their exact constitution has not yet been

1 See Cameron and Robinson, J. Physical Chtm., 1909, 13, 251.

2 Manchot and Zechentmayer, Annalen, 1906, 350, 368 ; Manchot and
Hiittner, ibid., 1910, 372, 153.

3 Kohlschiitter and Kutscheroff, Ber., 1907, 40, 873.

4 Thomas, Compt. rend., 1895, 120, 447.

5 Roussin, Compt. rend., 1858, 46, 224.

6 Porczinsky, Annalen, 1863, 125, 302; Rosenberg, Ber., 1879, 3, 312;
I'avi'l, ibid., 1882, 15, 2600 ; Marchlewski and Sachs, Zeit. anorg. Chem., 1892,
2, 175 ; Hofmann and Wiede, ibid., 1895, 8, 318 ; 1895, 9, 295 ; 1896, 11, 281 ;
Mari6 and Marquis, Compt. rend., 1896, 122, 137.



POTASSIUM FERRODINITROSOSULPHIDE 1243

determined, but they are closely allied to the ferrocyanides and
similar compounds, and consist of salts of complex acids con-
taining both the iron and nitroso-groups in the acid radical.
Two classes of salts have been prepared, viz., the ferrodinitroso-
derivatives, such as the salt K[Fe(NO) 2 S], and the ferrohepta-
TuYroso-derivatives, such as the salt K[Fe 4 (NO) 7 S 3 ]. The salts
of the first named series have possibly a molecular formula
double that given above.

Potassium Ferrodinitrososulphide, K[Fe(NO) 2 S],2H 2 0, is ob-
tained by the action of potash on the heptanitrososulphide. It
forms dark-red crystals, insoluble in water, and decomposes
violently when heated, yielding, among other products, potassium
and ammonium sulphates. By the action of sulphuric acid it
yields the free acid, H[Fe(NO) 2 S], which slowly decomposes in
the cold into sulphuretted hydrogen, nitrogen, nitrous oxide, and
the heptanitroso-acid.

The corresponding salts of the other alkalis, and in addition
the crystalline ethyl and phenyl derivatives, C 2 H 5 [Fe(NO) 2 S]
and C 6 H 5 [Fe(NO) 2 S], are known. Potassium, ammonium, and
sodium ferrodinitrosothiosulphates have also been prepared
(Hofrnann and Wiede).

Potassium Ferroheptanitrososulphide, K[Fe 4 (NO) 7 S 3 ],H 2 O, is
the most stable of these salts, and is obtained by adding a solu-
tion of ferrous sulphate to one of potassium nitrite and sulphide.
It is formed also by boiling a solution of the ferrodinitrosothio-
sulphate, sulphur dioxide being evolved and ferric hydroxide
precipitated. It forms dark monoclinic crystals with a diamond



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