Chapter XXII: ) acidulated with acetic acid gives a salt which after (1)

drying at 100° has the composition
Co_{2}O_{3}10NH_{3}7MoO_{3}3H_{2}O. After ignition this salt leaves
a residue having the composition 2CoO_{7}MoO_{3}. An analogous
compound is also obtained for tungstic acid, having the composition
Co_{2}O_{3}10NH_{3}10WO_{3}9H_{2}O. In this case after ignition
there remains a salt of the composition CoO_{5}WO_{3} (Carnot,
1889). Professor Kournakoff, by treating a solution of potassium
and sodium molybdates, containing a certain amount of suboxide of
cobalt, with bromine obtained salts having the composition:
3K_{2}OCo_{2}O_{3}12MoO_{3}20H_{2}O (light green) and
3K_{2}OCo_{2}O_{3}10Mo_{3}10H_{2}O (dark green). Péchard (1893)
obtained salts of the four complex phosphotungstic acids by
evaporating equivalent mixtures of solutions of phosphoric acid and
metatungstic acid (_see_ further on): phosphotrimetatungstic acid
P_{2}O_{5}12WO_{3}48H_{2}O, phosphotetrametatungstic acid
P_{2}O_{5}16WO_{3}69H_{2}O, phosphopentametatungstic acid
P_{2}O_{5}20WO_{3}H_{2}O, and phosphohexametatungstic acid
P_{2}O_{5}24WO_{3}59H_{2}O. Kehrmann and Frankel described still
more complex salts, such as:
3Ag_{2}O_{4}BaOP_{2}O_{5}22WO_{3}H_{2}O,5BaO_{2}
K_{2}OP_{2}O_{3}22WO_{3}48H_{2}O.
Analogous double salts with 22WO_{3} were also obtained with KSr,
KHg, BaHg, and NH_{4}Pb. Kehrmann (1892) considers the possibility
of obtaining an unlimited number of such salts to be a general
characteristic of such compounds. Mahom and Friedheim (1892)
obtained compounds of similar complexity for molybdic and arsenic
acids.

For tungstic acid there are known: (1) Normal salts--for example,
K_{2}WO_{4}; (2) the so-called acid salts have a composition like
3K_{2}O,7WO_{3},6H_{2}O or K_{6}H_{8}(WO_{4})_{7},2H_{2}O; (3) the
tritungstates like Na_{3}O,3WO_{3},3H_{2}O =
Na_{2}H_{4}(WO_{4})_{3},H_{2}O. All these three classes of salts
are soluble in water, but are precipitated by barium chloride, and
with acids in solution give an insoluble hydrate of tungstic acid;
whilst those salts which are enumerated below do not give a
precipitate either with acids or with the salts of the heavy
metals, because they form soluble salts even with barium and lead.
They are generally called metatungstates. They all contain water
and a larger proportion of acid elements than the preceding salts;
(4) the tetratungstates, like Na_{2}O,4WO_{3},10H_{2}O and
BaO,4WO_{3},9H_{2}O for example; (5) the octatungstates--for
example, Na_{2}O,8WO_{3},24H_{2}O. Since the metatungstates lose so
much water at 100° that they leave salts whose composition
corresponds with an acid, 3H_{2}O,4WO_{3}--that is,
H_{6}W_{4}O_{15}--whilst in the meta salts only 2 hydrogens are
replaced by metals, it is assumed, although without much ground,
that these salts contain a particular soluble metatungstic acid of
the composition H_{6}W_{4}O_{15}.

As an example we will give a short description of the sodium salts.
The normal salt, Na_{2}WO_{4}, is obtained by heating a strong
solution of sodium carbonate with tungstic acid to a temperature of
80°; if the solution be filtered hot, it crystallises in rhombic
tabular crystals, having the composition Na_{2}WO_{4},2H_{2}O,
which remain unchanged in the air and are easily soluble in water.
When this salt is fused with a fresh quantity of tungstic acid, it
gives a ditungstate, which is soluble in water and separates from
its solution in crystals containing water. The same salt is
obtained by carefully adding hydrochloric acid to the solution of
the normal salt so long as a precipitate does not appear, and the
liquid still has an alkaline reaction. This salt was first supposed
to have the composition Na_{2}W_{2}O_{7},4H_{2}O, but it has since
been found to contain (at 100°) Na_{6}W_{7}O_{24},16H_{2}O--that
is, it corresponds with the similar salt of molybdic acid.

(If this salt be heated to a red heat in a stream of hydrogen, it
loses a portion of its oxygen, acquires a metallic lustre, and
turns a golden yellow colour, and, after being treated with water,
alkali, and acid, leaves golden yellow leaflets and cubes which are
very like gold. This very remarkable substance, discovered by
Wöhler, has, according to Malaguti's analysis, the composition
Na_{2}W_{3}O_{9}; that is, it, as it were, contains a double
tungstate of tungsten oxide, WO_{2}, and of sodium,
Na_{2}WO_{4},WO_{2}WO_{3}. The decomposition of the fused sodium
salt is best effected by finely-divided tin. This substance has a
sp. gr. 6·6; it conducts electricity like metals, and like them has
a metallic lustre. When brought into contact with zinc and
sulphuric acid it disengages hydrogen, and it becomes covered with
a coating of copper in a solution of copper sulphate in the
presence of zinc--that is, notwithstanding its complex composition
it presents to a certain extent the appearance and reactions of the
metals. It is not acted on by aqua regia or alkaline solutions, but
it is oxidised when ignited in air.)

The ditungstate mentioned above, deprived of water (having
undergone a modification similar to that of metaphosphoric acid),
after being treated with water, leaves an anhydrous, sparingly
soluble tetratungstate, Na_{2}WO_{4},3WO_{3}, which, when heated at
120° in a closed tube with water, passes into an easily soluble
metatungstate. It may therefore be said that the metatungstates are
hydrated compounds. On boiling a solution of the above-mentioned
salts of sodium with the yellow hydrate of tungstic acid they give
a solution of metatungstate, which is the hydrated tetratungstate.
Its crystals contain Na_{2}W_{4}O_{13},10H_{2}O. After the hydrate
of tungstic acid (obtained from the ordinary tungstates by
precipitation with an acid) has stood a long time in contact with a
solution (hot or cold) of sodium tungstate, it gives a solution
which is not precipitated by hydrochloric acid; this must be
filtered and evaporated over sulphuric acid in a desiccator (it is
decomposed by boiling). It first forms a very dense solution
(aluminium floats in it) of sp. gr. 3·0, and octahedral crystals of
_sodium metatungstate_, Na_{2}W_{4}O_{13},10H_{2}O, sp. gr. 3·85,
then separate. It effloresces and loses water, and at 100° only two
out of the ten equivalents of water remain, but the properties of
the salt remain unaltered. If the salt be deprived of water by
further heating, it becomes insoluble. At the ordinary temperature
one part of water dissolves ten parts of the metatungstate. The
other metatungstates are easily obtained from this salt. Thus a
strong and hot solution, mixed with a like solution of barium
chloride, gives on cooling crystals of barium metatungstate,
BaW_{4}O_{13},9H_{2}O. These crystals are dissolved without change
in water containing hydrochloric acid, and also in hot water, but
they are partially decomposed by cold water, with the formation of
a solution of metatungstic acid and of the normal barium salt
BaWO_{4}.

In order to explain the difference in the properties of the salts
of tungstic acid, we may add that a mixture of a solution of
tungstic acid with a solution of silicic acid does not coagulate
when heated, although the silicic acid alone would do so; this is
due to the formation of a silicotungstic acid, discovered by
Marignac, which presents a fresh example of a complex acid. A
solution of a tungstate dissolves gelatinous silica, just as it
does gelatinous tungstic acid, and when evaporated deposits a
crystalline salt of silicotungstic acid. This solution is not
precipitated either by acids (a clear analogy to the
metatungstates) or by sulphuretted hydrogen, and corresponds with a
series of salts. These salts contain one equivalent of silica and 8
equivalents of hydrogen or metals, in the same form as in salts, to
12 or 10 equivalents of tungstic anhydride; for example the
crystalline potassium salt has the composition
K_{8}W_{12}SiO_{42},14H_{2}O = 4K_{2}O,12WO_{3},SiO_{2},14H_{2}O.
Acid salts are also known in which half of the metal is replaced by
hydrogen. The complexity of the composition of such complex acids
(for example, of the phosphomolybdic acid) involuntarily leads to
the idea of polymerisation, which we were obliged to recognise for
silica, lead oxide, and other compounds. This polymerisation, it
seems to me, may be understood thus: a hydrate A (for example,
tungstic acid) is capable of combining with a hydrate B (for
example, silica or phosphoric acid, with or without the
disengagement of water), and by reason of this faculty it is
capable of polymerisation--that is, A combines with A--combines
with itself--just as aldehyde, C_{2}H_{4}O, or the cyanogen
compounds are able to combine with hydrogen, oxygen, &c., and are
liable to polymerisation. On this view the molecule of tungstic
acid is probably much more complex than we represent it; this
agrees with the easy volatility of such compounds as the
chloranhydrides, CrO_{2}Cl_{2}, MoO_{2}Cl_{2}, the analogues of the
volatile sulphuryl chloride, SO_{2}Cl_{2}, and with the
non-volatility, or difficult volatility, of chromic and molybdic
anhydrides, the analogues of the volatile sulphuric anhydride. Such
a view also finds a certain confirmation in the researches made by
Graham on the _colloidal_ state of tungstic acid, because colloidal
properties only appertain to compounds of a very complex
composition. The observations made by Graham on the colloidal state
of tungstic and molybdic acids introduced much new matter into the
history of these substances. When sodium tungstate, mixed in a
dilute solution with an equivalent quantity of dilute hydrochloric
acid, is placed in a dialyser, hydrochloric acid and sodium
chloride pass through the membrane, and a solution of tungstic acid
remains in the dialyser. Out of 100 parts of tungstic acid about 80
parts remain in the dialyser. The solution has a bitter, astringent
taste, and does not yield gelatinous tungstic acid (hydrogel)
either when heated or on the addition of acids or salts. It may
also be evaporated to dryness; it then forms a vitreous mass of the
_hydrosol_ of _tungstic acid_, which adheres strongly to the walls
of the vessel in which it has been evaporated, and is perfectly
soluble in water. It does not even lose its solubility after having
been heated to 200°, and only becomes insoluble when heated to a
red heat, when it loses about 2-1/2 p.c. of water. The dry acid,
dissolved in a small quantity of water, forms a gluey mass, just
like gum arabic, which is one of the representatives of the
hydrosols of colloidal substances. The solution, containing 5 p.c
of the anhydride, has a sp. gr. of 1·047; with 20 p.c., of 1·217;
with 50 p.c., of 1·80; and with 80 p.c., of 3·24. The presence of a
polymerised trioxide in the form of hydrate, H_{2}OW_{3}O_{9} or
H_{2}O_{4}WO_{3}, must then be recognised in the solution: this is
confirmed by Sabaneeff's cryoscopic determinations (1889). A
similar stable solution of molybdic acid is obtained by the
dialysis of a mixture of a strong solution of sodium molybdate with
hydrochloric acid (the precipitate which is formed is
re-dissolved). If MoCl_{4} be precipitated by ammonia and washed
with water, a point is reached at which perfect solution takes
place, and the molybdic acid forms a colloid solution which is
precipitated by the addition of ammonia (Muthmann). The addition of
alkali to the solutions of the hydrosols of tungstic and molybdic
acids immediately results in the re-formation of the ordinary
tungstates and molybdates. There appears to be no doubt but that
the same transformation is accomplished in the passage of the
ordinary tungstates into the metatungstates as takes place in the
passage of tungstic acid itself from an insoluble into a soluble
state; but this may be even actually proved to be the case, because
Scheibler obtained a solution of tungstic acid, before Graham, by
decomposing barium metatungstate (BaO_{4}WO_{3},9H_{2}O) with
sulphuric acid. By treating this salt with sulphuric acid in the
amount required for the precipitation of the baryta, Scheibler
obtained a solution of metatungstic acid which, when containing
43·75 p.c. of acid, had a sp. gr. of 1·634, and with 27·61 p.c. a
sp. gr. of 1·327--that is, specific gravities corresponding with
those found by Graham.

Péchard found that as much heat is evolved by neutralising
metatungstic acid as with sulphuric acid.

Questions connected with the metamorphoses or modifications of
tungstic and molybdic acids, and the polymerisation and colloidal
state of substances, as well as the formation of complex acids,
belong to that class of problems the solution of which will do much
towards attaining a true comprehension of the mechanism of a number
of chemical reactions. I think, moreover, that questions of this
kind stand in intimate connection with the theory of the formation
of solutions and alloys and other so-called indefinite compounds.

Hydrogen (which does not directly form compounds with Cr, Mo, and W) reduces molybdic and tungstic anhydride at a red heat; and this forms the means of obtaining metallic molybdenum and tungsten. _Both metals_ are infusible, and both under the action of heat form compounds with carbon and iron (the addition of tungsten to steel renders the latter ductile and hard).[9] Molybdenum forms a grey powder, which scarcely aggregates under a most powerful heat, and has a specific gravity of 8·5. It is not acted on by the air at the ordinary temperature, but when ignited it is first converted into a brown, and then into a blue oxide, and lastly into molybdic anhydride. Acids do not act on it--that is, it does not liberate hydrogen from them, not even from hydrochloric acid--but strong sulphuric acid disengages sulphurous anhydride, forming a brown mass, containing a lower oxide of molybdenum. Alkalis in solution do not act on molybdenum, but when fused with it hydrogen is given off, which shows, as does its whole character, the acid properties of the metal. The properties of tungsten are almost identical; it is infusible, has an iron-grey colour, is exceedingly hard, so that it even scratches glass. Its specific gravity is 19·1 (according to Roscoe), so that, like uranium, platinum, &c., it is one of the heaviest metals.[9 bis] Just as sulphur and chromium have their corresponding persulphuric and perchromic acids, H_{2}S_{2}O_{8} and H_{2}CrO_{8}, having the properties of peroxides, and corresponding to peroxide of hydrogen, so also molybdenum and tungsten are known to give _permolybdic_ and _pertungstic_ acids, H_{2}Mo_{2}O_{8} and H_{2}W_{2}O_{8}, which have the properties of true peroxides, _i.e._ easily disengage iodine from KI and chlorine from HCl, easily part with their oxygen, and are formed by the action of peroxide of hydrogen, into which they are readily reconverted (hence they may be regarded as compounds of H_{2}O_{2} with 2MoO_{3} and 2WO_{3}), &c. Their formation (Boerwald 1884, Kemmerer 1891) is at once seen in the coloration (not destroyed by boiling), which is obtained on mixing a solution of the salts with peroxide of hydrogen, and on treating, for instance, molybdic acid with a solution of peroxide of hydrogen (Péchard 1892). The acid then forms an orange-coloured solution, which after evaporation in vacuo leaves Mo_{2}H_{2}O_{8}4H_{2}O as a crystalline powder, and loses 4H_{2}O at 100°, beyond which it decomposes with the evolution of oxygen.[9 tri]

[9] Moissan (1893) studied the compounds of Mo and W formed with carbon
in the electrical furnace (they are extremely hard) from a mixture
of the anhydrides and carbon. Poleck and Grützner obtained definite
compounds FeW_{2} and FeW_{2}C_{3} for tungsten. Metallic W and Mo
displace Ag from its solutions but not Pb. There is reason for
believing that the sp. gr. of pure molybdenum is higher than that
(8·5) generally ascribed to it.

[9 bis] We may conclude our description of tungsten and molybdenum by
stating that their sulphur compounds have an acid character, like
carbon bisulphide or stannic sulphide. If sulphuretted hydrogen be
passed through a solution of a molybdate it does not give a
precipitate unless sulphuric acid be present, when a dark brown
precipitate of _molybdenum trisulphide_, MoS_{3}, is formed. When
this sulphide is ignited without access of air it gives the
bisulphide MoS_{2}; the latter is not able to combine with
potassium sulphide like the trisulphide MoS_{3}, which forms a
salt, K_{2}MoS_{4}, corresponding with K_{2}MoO_{4}. This is
soluble in water, and separates out from its solution in red
crystals, which have a metallic lustre and reflect a green light.
It is easily obtained by heating the native bisulphide, MoS_{2},
with potash, sulphur, and a small amount of charcoal, which serves
for deoxidising the oxygen compounds. Tungsten gives similar
compounds, R_{2}WS_{4}, where R = NH_{4}, K, Na. They are
decomposed by acids, with the separation of tungsten trisulphide,
WS_{3}, and molybdenum trisulphide, MoS_{3}. Rideal (1892) obtained
W_{2}N_{3} by heating WO_{3} in NH_{3}. This compound exhibited the
general properties of metallic nitrides.

[9 tri] When peroxide of hydrogen acts upon a solution of potassium
molybdate well-formed yellow crystals belonging to the triclinic
system separate out in the cold. When these crystals are heated in
vacuo they first lose water and then decompose, leaving a residue
composed of the salt originally taken. They are soluble in water
but insoluble in alcohol. Their composition is represented by the
formula K_{2}Mo_{2}O_{8}2H_{2}O. An ammonium salt is obtained by
evaporating peroxide of hydrogen with ammonium molybdate. The
following salts have also been obtained by the action of peroxide
of hydrogen upon the corresponding molybdates:
Na_{2}Mo_{2}O_{6}6H_{2}O--in yellow prismatic crystals;
MgMo_{2}O_{8}10H_{2}O--stellar needles; BaMoO_{8}2H_{2}O--in
microscopic yellow octahedra. A corresponding sodium pertungstate
has been obtained by Péchard by boiling sodium tungstate with a
solution of peroxide of hydrogen for several minutes. The solution
rapidly turns yellow, and no longer gives a precipitate of tungstic
anhydride when treated with nitric acid. When evaporated in vacuo
the solution leaves a thick syrupy liquid from which ray-like
crystals separate out; these crystals are more soluble in water
than the salt originally taken. When heated they also lose water
and oxygen. Their composition answers to the formula
M_{2}W_{2}O_{8}2H_{2}O, where M = Na, NH_{4}, &c. The permolybdates
and pertungstates have similar properties. When treated with oxygen
acids they give peroxide of hydrogen, and disengage chlorine and
iodine from hydrochloric acid and potassium iodide.

Piccini (1891) showed that peroxide of hydrogen not only combines
with the oxygen compounds of Mo and W, but also with their
fluo-compounds, among which ammonium fluo-molybdate
MoO_{2}F_{2}2NH_{4} and others have long been known. (A few new
salts of similar composition have been obtained by F. Moureu in
1893.) The action of peroxide of hydrogen upon these compounds
gives salts containing a larger amount of oxygen; for instance, a
solution of MoO_{2}F_{2}2KFH_{2}O with peroxide of hydrogen gives a
yellow solution which after cooling separates out yellow
crystalline flakes of MoO_{3}F_{2}2KFH_{2}O, resembling the salt
originally taken in their external appearance. By employing a
similar method Piccini also obtained:
MoO_{3}F_{2}2RbFH_{2}O--yellow monoclinic crystals;
MoO_{3}F_{2},2CsFH_{2}O,--yellow flakes, and the corresponding
tungstic compounds. All these salts react like peroxide of
hydrogen.

In speaking of these compounds I for my part think it may be well
to call attention to the fact that, in the first place, the
composition of Piccini's oxy-fluo compounds does not correspond to
that of permolybdic and pertungstic acid. If the latter be
expressed by formulæ with one equivalent of an element, they will
be HMoO_{4} and HWO_{4}, and the oxy-fluo form corresponding to
them should have the composition MoO_{3}F and WO_{3}F while it
contains MO_{3}F_{2} and WO_{3}F_{2}, _i.e._ answers as it were to
a higher degree of oxidation, MoH_{2}O_{3} and W_{3}HO_{3}. But if
permolybdic acid be regarded as 2MoO_{3} + H_{2}O_{2}, _i.e._ as
containing the elements of peroxide of hydrogen, then Piccini's
compound will also be found to contain the original salts + H_{2}O;
for example, from MoO_{2}F_{2}2KFH_{2}O there is obtained a
compound MoO_{2}F_{2}2KFH_{2}O_{2}, _i.e._ instead of H_{2}O they
contain H_{2}O_{2}. In the second place the capacity of the salts
of molybdenum and tungsten to retain a further amount of oxygen or
H_{2}O_{2} probably bears some relation to their property of giving
complex acids and of polymerising which has been considered in Note
8 bis. There is, however, a great chemical interest in the
accumulation of data respecting these high peroxide compounds
corresponding to molybdic and tungstic acids. With regard to the
peroxide form of uranium, _see_ Chapter XX., Note 66.

_Uranium_, U = 240, has the highest atomic weight of all the analogues of chromium, and indeed of all the elements yet known. Its highest salt-forming oxide, UO_{3}, shows very feeble acid properties. Although it gives sparingly-soluble yellow compounds with alkalis, which fully correspond with the dichromates--for example, Na_{2}U_{2}O_{7} = Na_{2}O,2UO_{3},[10]--yet it more frequently and easily reacts with acids, HX, forming fluorescent yellowish-green salts of the composition UO_{2}X_{2}, and in this respect uranic trioxide, UO_{3}, differs from chromic anhydride, CrO_{3}, although the latter is able to give the oxychloride, CrO_{2}Cl_{2}. In molybdenum and tungsten, however, we see a clear transition from chromium to uranium. Thus, for example, chromyl chloride, CrO_{2}Cl_{2}, is a brown liquid which volatilises without change, and is completely decomposed by water; molybdenum oxychloride, MoO_{2}Cl_{2}, is a crystalline substance of a yellow colour, which is volatile and soluble in water (Blomstrand), like many salts. Tungsten oxychloride, WO_{2}Cl_{2}, stands still nearer to uranyl chloride in its properties; it forms yellow scales on which water and alkalis act, as they do on many salts (zinc chloride, ferric chloride, aluminium chloride, stannic chloride, &c.), and perfectly corresponds with the difficultly volatile salt, UO_{2}Cl_{2} (obtained by Peligot by the action of chlorine on ignited uranium dioxide, UO_{2}), which is also yellow and gives a yellow solution with water, like all the salts UO_{2}X_{2}. The property of uranic oxide, UO_{3}, of forming salts UO_{2}X_{2} is shown in the fact that the hydrated oxide of uranium, UO_{2}(HO)_{2}, which is obtained from the nitrate, carbonate, and other salts by the loss of the elements of the acid, is easily soluble in acids, as well as in the fact that the lower grades of oxidation of uranium are able, when treated with nitric acid, to form an easily crystallisable uranyl nitrate, UO_{2}(NO_{3})_{2},6H_{2}O; this is the most commonly occurring uranium salt.[11]

[10] Uranium trioxide, or uranic oxide, shows its feeble basic and acid
properties in a great number of its reactions. (1) Solutions of
uranic salts give yellow precipitates with alkalis, but these
precipitates do not contain the hydrate of the oxide, but
compounds of it with bases; for example, 2UO_{2}(NO_{3})_{2} +
6KHO = 4KNO_{3} + 3H_{2}O + K_{2}U_{2}O_{7}. There are other
_urano-alkali compounds_ of the same constitution; for example,
(NH_{4})_{2}U_{2}O_{7} (known commercially as uranic oxide),
MgU_{2}O_{7}, BaU_{2}O_{7}. They are the analogues of the
dichromates. Sodium uranate is the most generally used under the
name of uranium yellow, Na_{2}U_{2}O_{7}. It is used for imparting
the characteristic yellow-green tint to glass and porcelain.
Neither heat nor water nor acids are able to extract the alkali
from sodium uranate, Na_{2}U_{2}O_{7}, and therefore it is a true
insoluble salt, of a yellow colour, and clearly indicates the acid
character (although feeble) of uranic oxide. (2) The carbonates of
the alkaline earths (for instance, barium carbonate) precipitate
uranic oxide from its salts, as they do all the salts of feeble
bases; for example, R_{2}O_{3}. (3) The _alkaline carbonates_,
when added to solutions of uranic salts, give a _precipitate,
which is soluble in_ _an excess of the reagent_, and particularly
so if the acid carbonates be taken. This is due to the fact that
(4) the uranyl salts _easily form double salts_ with the salts of
the alkali metals, including the salts of ammonium. Uranium, in
the form of these double salts, often gives salts of well-defined
crystalline form, although the simple salts are little prone to
appear in crystals. Such, for example, are the salts obtained by
dissolving potassium uranate, K_{2}U_{2}O_{7}, in acids, with the
addition of potassium salts of the same acids. Thus, with
hydrochloric acid and potassium chloride a well-formed crystalline
salt, K_{2}(UO_{2})Cl_{4},2H_{2}O, belonging to the monoclinic
system, is produced. This salt decomposes in dissolving in pure
water. Among these double salts we may mention the double
carbonate with the alkalis, R_{4}(UO_{2})(CO_{3})_{3} (equal to
2R_{2}CO_{3} + UO_{2}CO_{3}); the acetates,
R(UO_{2})(C_{2}H_{3}O_{2})_{3}--for instance, the sodium salt,
Na(UO_{2})(C_{2}H_{3}O_{2})_{3}, and the potassium salt,
K(UO_{2})(C_{2}H_{3}O_{2})_{3},H_{2}O; the sulphates,
R_{2}(UO_{2})(SO_{4})_{3},2H_{2}O, &c. In the preceding formula R
= K, Na, NH_{4}, or R_{2} = Mg, Ba, &c. _This property of giving
comparatively stable double salts indicates feebly developed basic
properties_, because double salts are mainly formed by salts of
distinctly basic metals (these form, as it were, the basic element
of a double salt) and salts of feebly energetic bases (these form
the acid element of a double salt), just as the former also give
acid salts; the acid of the acid salts is replaced in the double
salts by the salt of the feebly energetic base, which, like water,
belongs to the class of intermediate bases. For this reason barium
does not give double salts with alkalis as magnesium does, and
this is why double salts are more easily formed by potassium than
by lithium in the series of the alkali metals. (5) The most
remarkable property, proving the feeble energy of uranic oxide as
a base, is seen in the fact that when their composition is
compared with that of other salts those of uranic oxide _always
appear as basic salts_. It is well known that a normal salt,
R_{2}X_{6}, corresponds with the oxide R_{2}O_{3}, where X = Cl,
NO_{3}, &c., or X_{2} = SO_{4}, CO_{3}, &c.; but there also exist
basic salts of the same type where X = HO or X_{2} = O. We saw
salts of all kinds among the salts of aluminium, chromium, and
others. With uranic oxide no salts are known of the types UX_{6}
(UCl_{6}, U(SO_{4})_{3}, alums, &c., are not known), nor even
salts, U(HO)_{2}X_{4} or UOX_{4}, but it always forms salts of the
type U(HO)_{4}X_{2}, or UO_{2}X_{2}. Judging from the fact that
nearly all the salts of uranic oxide retain water in crystallising
from their solutions, and that this water is difficult to separate
from them, it may be thought to be water of hydration. This is
seen in part from the fact that the composition of many of the
salts of uranic oxide may then be expressed without the presence
of water of crystallisation; for instance, U(HO)_{4}K_{2}Cl_{4}
(and the salt of NH_{4}, U(HO)_{4}K_{2}(SO_{4})_{2},
U(HO)_{4}(C_{2}H_{3}O_{2})_{2}. Sodium uranyl acetate however does
not contain water.

[11] _Uranyl nitrate_, or uranium nitrate, UO_{2}(NO_{3})_{2},6H_{2}O,
crystallises from its solutions in transparent yellowish-green
prisms (from an acid solution), or in tabular crystals (from a
neutral solution), which effloresce in the air and are easily
soluble in water, alcohol, and ether, have a sp. gr. of 2·8, and
fuse when heated, losing nitric acid and water in the process. If
the salt itself (Berzelius) or its alcoholic solution (Malaguti)
be heated up to the temperature at which oxides of nitrogen are
evolved, there then remains a mass which, after being evaporated
with water, leaves uranyl hydroxide, UO_{2}(HO)_{2} (sp. gr.
5·93), whilst if the salt be ignited there remains the dioxide,
UO_{2}, as a brick-red powder, which on further heating loses
oxygen and forms the dark olive uranoso-uranic oxide, U_{3}O_{8}.
The solution of the nitrate obtained from the ore is purified in
the following manner: sulphurous anhydride is first passed through
it in order to reduce the arsenic acid present into arsenious
acid; the solution is then heated to 60°, and sulphuretted
hydrogen passed through it; this precipitates the lead, arsenic,
and tin, and certain other metals, as sulphides, insoluble in
water and dilute nitric acid. This liquid is then filtered and
evaporated with nitric acid to crystallisation, and the crystals
are dissolved in ether. Or else the solution is first treated with
chlorine in order to convert the ferrous chloride (produced by the
action of the hydrogen sulphide) into ferric chloride, the oxides
are then precipitated by ammonia, and the resultant precipitate,
containing the oxides Fe_{2}O_{3}, UO_{3}, and compounds of the
latter with potash, lime, ammonia, and other bases present in the
solution (the latter being due to the property of uranic oxide of
combining with bases), is washed and dissolved in a strong,
slightly-heated solution of ammonium carbonate, which dissolves
the uranic oxide but not the ferric oxide. The solution is
filtered, and on cooling deposits a well-crystallising _uranyl
ammonium carbonate_, UO_{2}(NH_{4})_{4}(CO_{3})_{3}, in brilliant
monoclinic crystals which on exposure to air slowly give off
water, carbonic anhydride, and ammonia; the same decomposition is
readily effected at 300°, the residue then consisting of uranic
oxide. This salt is not very soluble in water, but is readily so
in ammonium carbonate; it is obvious that it may readily be
converted into all the other salts of oxides of uranium. Uranium
salts are also purified in the form of _acetate_, which is very
sparingly soluble, and is therefore directly precipitated from a
strong solution of the nitrate by mixing it with acetic acid.

We may also mention the _uranyl phosphate_, HUPO_{6}, which must
be regarded as an orthophosphate in which two hydrogens are
replaced by the radicle uranyl, UO_{2}, _i.e._ as H(UO_{2})PO_{4}.
This salt is formed as a hydrated gelatinous yellow precipitate,
on mixing a solution of uranyl nitrate with disodium phosphate.
The precipitation occurs in the presence of acetic acid, but not
in the presence of hydrochloric acid. If moreover an excess of an
ammonium salt be present, the ammonia enters into the composition
of the bright yellow gelatinous precipitate formed, in the
proportion UO_{2}NH_{4}PO_{4}. This precipitate is not soluble in
water and acetic acid, and its solution in inorganic acids when
boiled entirely expels all the phosphoric acid. This fact is taken
advantage of for removing phosphoric acids from solutions--for
instance, from those containing salts of calcium and magnesium.

_Uranium_, which gives an oxide, UO_{3}, and the corresponding salt UO_{2}X_{2} and dioxide UO_{2}, to which the salts UX_{4} correspond, is rarely met with in nature. Uranite or the double orthophosphate of uranic oxide, R(UO_{2})H_{2}P_{2}O_{8},7H_{2}O, where R = Cu or Ca, uranium-vitriol U(SO_{4})_{2},H_{2}O, samarakite, and æschynite, are very rarely found, and then only in small quantities. Of more frequent and abundant occurrence is the non-crystalline, earthy brown uranium ore known as _pitchblende_ (sp. gr. 7·2), which is mainly composed of the intermediate oxide, U_{3}O_{8} = UO_{2},2UO_{3}. This ore is found at Joachimsthal in Bohemia and in Cornwall. It usually contains a number of different impurities, chiefly sulphides and arsenides of lead and iron, as well as lime and silica compounds. In order to expel the arsenic and sulphur it is roasted, ground, washed with dilute hydrochloric acid, which does not dissolve the uranoso-uranic oxide, U_{3}O_{8}, and the residue is dissolved in nitric acid, which transforms the uranium oxide into the nitrate, UO_{2}(NO_{3})_{2}.

It must be observed that the oxide of uranium, first distinguished by Klaproth (1789), was for a long time regarded as able to give metallic uranium under the action of charcoal and other reducing agents (with the aid of heat). But the substance thus obtained was only the _uranium dioxide_, UO_{2}. The compound nature of this dioxide,[12] or the presence of oxygen in it, was demonstrated by Peligot (1841), by igniting it with charcoal in a stream of chlorine. He thus obtained a volatile _uranium tetrachloride_, UCl_{4},[13] which, when heated with sodium, gave _metallic uranium_ as a grey metal, having a specific gravity of 18·7, and liberating hydrogen from acids, with the formation of green uranous salts, UX_{4}, which act as powerful reducing agents.[14]

[12] Uranium dioxide, or _uranyl_, UO_{2}, which is contained in the
salts UO_{2}X_{2}, has the appearance and many of the properties
of a metal. Uranic oxide may be regarded as uranyl oxide,
(UO_{2})O, its salts as salts of this uranyl; its hydroxide,
(UO_{2})H_{2}O_{2}, is constituted like CaH_{2}O_{2}. The green
oxide of uranium, uranoso-uranic oxide (easily formed from uranic
salts by the loss of oxygen), U_{3}O_{8} = UO_{2},2UO_{3}, when
ignited with charcoal or hydrogen (dry) gives a brilliant
crystalline substance of sp. gr. about 11·0 (Urlaub), whose
appearance resembles that of metals, and decomposes steam at a red
heat with the evolution of hydrogen; it does not, however,
decompose hydrochloric or sulphuric acid, but is oxidised by
nitric acid. The same substance (i.e. uranium dioxide UO_{2}) is
also obtained by igniting the compound (UO_{2})K_{2}Cl_{4} in a
stream of hydrogen, according to the equation UO_{2}K_{4}Cl_{4} +
H_{2} = UO_{2} + 2HCl + 2KCl. It was at first regarded as the
metal. In 1841 Peligot found that it contained oxygen, because
carbonic oxide and anhydride were evolved when it was ignited with
charcoal in a stream of chlorine, and from 272 parts of the
substance which was considered to be metal he obtained 382 parts
of a volatile product containing 142 parts of chlorine. From this
it was concluded that the substance taken contained an equivalent
amount of oxygen. As 142 parts of chlorine correspond with 32
parts of oxygen, it followed that 272 - 32 = 240 parts of metal
were combined in the substance with 32 parts of oxygen, and also
in the chlorine compound obtained with 142 parts of chlorine.
These calculations have been made for the now accepted atomic
weight of uranium (U = 240, _see_ Note 14). Peligot took another
atomic weight, but this does not alter the principle of the
argument.

[13] _Uranium tetrachloride_, uranous chloride, UCl_{4}, corresponds
with uranous oxide as a base. It was obtained by Peligot by
igniting uranic oxide mixed with charcoal in a stream of _dry_
chlorine: UO_{3} + 3C + 2Cl_{2} = UCl_{4} + 3CO. This green
volatile compound (Note 12) crystallises in regular octahedra, is
very hygroscopic, easily soluble in water, with the development of
a considerable amount of heat, and no longer separates out from
its solution in an anhydrous state, but disengages hydrochloric
acid when evaporated. The solution of uranous chloride in water is
green. It is also formed by the action of zinc and copper (forming
cuprous chloride) on a solution of uranyl chloride, UO_{2}Cl_{2},
especially in the presence of hydrochloric acid and sal-ammoniac.
Solutions of uranyl salts are converted into uranous salts by the
action of various reducing agents, and among others by organic
substances or by the action of light, whilst the salts UX_{4} are
converted into uranyl salts, UO_{2}X_{2}, by exposure to air or by
oxidising agents. Solutions of the green uranyl salts act as
powerful reducing agents, and give a brown precipitate of the
uranous hydroxide, UH_{4}O_{4}, with potash and other alkalis.
This hydroxide is easily soluble in acids but not in alkalis. On
ignition it does not form the oxide UO_{2}, because it decomposes
water, but when the higher oxides of uranium are ignited in a
stream of hydrogen or with charcoal they yield uranous oxide. Both
it and the chloride UCl_{4}, dissolve in strong sulphuric acid,
forming a green salt, U(SO_{4})_{2},2H_{2}O. The same salt,
together with uranyl sulphate, UO_{2}(SO_{4}), is formed when the
green oxide, U_{3}O_{8}, is dissolved in hot sulphuric acid. The
salts obtained in the latter instance may be separated by adding
alcohol to the solution, which is left exposed to the light; the
alcohol reduces the uranyl salt to uranous salt, an excess of acid
being required. An excess of water decomposes this salt, forming a
basic salt, which is also easily produced under other
circumstances, and contains UO(SO_{4}),2H_{2}O (which corresponds
to the uranic salt).

[14] The atomic weight of uranium was formerly taken as half the
present one, U = 120, and the oxides U_{2}O_{3}, suboxide UO, and
green oxide U_{3}O_{4}, were of the same types as the oxides of
iron. With a certain resemblance to the elements of the iron
group, uranium presents many points of distinction which do not
permit its being grouped with them. Thus uranium forms a very
stable oxide, U_{2}O_{3}(U = 120), but does not give the
corresponding chloride U_{2}Cl_{6} (Roscoe, however, in 1874
obtained UCl_{5}, like MoCl_{5} and WCl_{5}), and under those
circumstances (the ignition of oxide of uranium mixed with
charcoal, in a stream of chlorine), when the formation of this
compound might be expected, it gives (U = 120) the chloride
UCl_{2}, which is characterised by its volatility; this is not a
property, to such an extent, of any of the bichlorides, RCl_{2},
of the iron group.

The alteration or doubling of the atomic weight of uranium--_i.e._
the recognition of U = 240--was made for the first time in the
first (Russian) edition of this work (1871), and in my memoir of
the same year in Liebig's _Annalen_, on the ground that with an
atomic weight 120, uranium could not be placed in the periodic
system. I think it will not be superfluous to add the following
remarks on this subject: (1) In the other groups (K--Rb--Cs,
Ca--Sr--Ba, Cl--Br--I) the acid character of the oxides decreases
and their basic character increases with the rise of atomic
weight, and therefore we should expect to find the same in the
group Cr--Mo--W--U, and if CrO_{3}, MoO_{3}, WO_{3} be the
anhydrides of acids then we indeed find a decrease in their acid
character, and therefore uranium trioxide, UO_{3}, should be a
very feeble anhydride, but its basic properties should also be
very feeble. Uranic oxide does indeed show these properties, as
was pointed out above (Note 10). (2) Chromium and its analogues,
besides the oxides RO_{3}, also form lower grades of oxidation
RO_{2}, R_{2}O_{3}, and the same is seen in uranium; it forms
UO_{3}, UO_{2}, U_{2}O_{3} and their compounds. (3) Molybdenum and
tungsten, in being reduced from RO_{3}, easily and frequently give
an intermediate oxide of a blue colour, and uranium shows the same
property; giving the so-called green oxide which, according to
present views, must be regarded as U_{3}O_{8} = UO_{2}2UO_{3},
analogous to Mo_{3}O_{8}. (4) The higher chlorides, RCl_{6},
possible for the elements of this group, are either unstable
(WCl_{6}) or do not exist at all (Cr); but there is one single
lower volatile compound, which is decomposed by water, and liable
to further reduction into a non-volatile chlorine product and the
metal. The same is observed in uranium, which forms an easily
volatile chloride, UCl_{4}, decomposed by water. (5) The high sp.
gr. of uranium (18·6) is explained by its analogy to tungsten (sp.
gr. 19·1). (6) For uranium, as for chromium and tungsten, yellow
tints predominate in the form RO_{3}, whilst the lower forms are
green and blue. (7) Zimmermann (1881) determined the vapour
densities of uranous bromide, UBr_{4}, and chloride, UCl_{4} (19·4
and 13·2), and they were found to correspond to the formulæ given
above--that is, they confirmed the higher atomic weight U = 240.
Roscoe, a great authority on the metals of this group, was the
first to accept the proposed atomic weight of uranium, U = 240,
which since Zimmermann's work has been generally recognised.

As the salts of uranic oxide are reduced in the absence of organic matter by the action of light, and as they impart a characteristic coloration to glass,[15] they find a certain application in photography and glass work.

[15] Uranium glass, obtained by the addition of the yellow salt
K_{2}U_{2}O_{7} to glass, has a green yellow fluorescence, and is
sometimes employed for ornaments; it absorbs the violet rays, like
the other salts of uranic oxide--that is, it possesses an
absorption spectrum in which the violet rays are absent. The index
of refraction of the absorbed rays is altered, and they are given
out again as greenish-yellow rays; hence, compounds of uranic
acid, when placed in the violet portion of the spectrum, emit a
greenish-yellow light, and this forms one of the best examples
(another is found in a solution of quinine sulphate) of the
phenomenon of fluorescence. The rays of light which pass through
uranic compounds do not contain the rays which excite the
phenomena of fluorescence and of chemical transformation, as the
researches of Stokes prove.

If we compare together the highly acid elements, sulphur, selenium, and tellurium, of the uneven series, with chromium, molybdenum, tungsten, and uranium of the even series, we find that the resemblance of the properties of the higher form RO_{3} does not extend to the lower forms, and even entirely disappears in the elements, for there is not the smallest resemblance between sulphur and chromium and their analogues in a free state. In other words, this means that the small periods, like Na, Mg, Al, Si, P, S, Cl, containing seven elements, do not contain any near analogues of chromium, molybdenum, &c., and therefore their true position among the other elements must be looked for only in those large periods which contain two small periods, and whose type is seen in the period containing: K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br. These large periods contain Ca and Zn, giving RO, Sc, and Ga of the third group, Ti and Ge giving RO_{2}, V and As forming R_{2}O_{5}, Cr and Se of the sixth group, Mn and Br of the seventh group, and the remaining elements, Fe, Co, Ni, form connective members of the intermediate eighth group, to the description of the representatives of which we shall turn in the following chapters. We will now proceed to describe _manganese_, Mn = 55, as an element of the seventh group of the even series, directly following after Cr = 52, which corresponds with Br = 80 to the same degree that Cr does with Se = 79. For chromium, selenium, and bromine very close analogues are known, but for manganese as yet none have been obtained--that is, it is the only representative of the even series in the seventh group. In placing manganese with the halogens in one group, the periodic system of the elements only requires that it should bear an analogy to the halogens in the higher type of oxidation--_i.e._ in the salts and acids--whilst it requires that as great a difference should be expected in the lower types and elements as there exists between chromium or molybdenum and sulphur or selenium. And this is actually the case. The elements of the seventh group form a higher salt-forming oxide, R_{2}O_{7}, and its corresponding hydrate, HRO_{4}, and salts--for example, KClO_{4}. Manganese in the form of potassium permanganate, KMnO_{4}, actually presents a great analogy in many respects to potassium perchlorate, KClO_{4}. The analogy of the crystalline form of both salts was shown by Mitscherlich. The salts of permanganic acid are also nearly all soluble in water, like those of perchloric acid, and if the silver salt of the latter, AgClO_{4}, be sparingly soluble in water, so also is silver permanganate, AgMnO_{4}. The specific volume of potassium perchlorate is equal to 55, because its specific gravity = 2·54; the specific volume of potassium permanganate is equal to 58, because its specific gravity = 2·71. So that the volumes of equivalent quantities are in this instance approximately the same whilst the atomic volumes of chlorine (35·5/1·3 = 27) and manganese (55/7·5) are in the ratio 4 : 1. In a free state the higher acids HClO_{4} and HMnO_{4} are both soluble in water and volatile, both are powerful oxidisers--in a word, their analogy is still closer than that of chromic and sulphuric acids, and those points of distinction which they present also appear among the nearest analogues--for example, in sulphuric and telluric acids, in hydrochloric and hydriodic acids, &c. Besides Mn_{2}O_{7} manganese gives a lower grade of oxidation, MnO_{3}, analogous to sulphuric and chromic trioxides, and with it corresponds potassium manganate, K_{2}MnO_{4}, isomorphous with potassium sulphate.[16] In the still lower grades of oxidation, Mn_{2}O_{3} and MnO, there is hardly any similarity to chlorine, whilst every point of resemblance disappears when we come to the elements themselves--_i.e._ to manganese and chlorine--for manganese is a metal, like iron, which combines directly with chlorine to form a saline compound, MnCl_{2}, analogous to magnesium chloride.[17]

[16] The comparison of potassium permanganate with potassium
perchlorate, or of potassium manganate with potassium sulphate,
shows directly that many of the physical and chemical properties
of substances do not depend on the nature of the elements, but on
the atomic types in which they appear, on the kind of movements,
or on the positions in which the atoms forming the molecule occur.

[17] If, however, we compare the spectra (Vol. I. p. 565) of chlorine,
bromine, and iodine with that of manganese, a certain resemblance
or analogy is to be found connecting manganese both to iron and to
chlorine, bromine, and iodine.

Manganese belongs to the number of metals widely distributed in nature, especially in those localities where iron occurs, whose ores frequently contain compounds of manganous oxide, MnO, which presents a resemblance to ferrous oxide, FeO, and to magnesia. In many minerals magnesia and the oxides allied to it are replaced by manganous oxide; calcspars and magnesites--_i.e._ R´´CO_{3} in general--are frequently met with containing manganous carbonate, which also occurs in a separate state, although but rarely. The soil also and the ash of plants generally contain a small quantity of manganese. In the analysis of minerals it is generally found that manganese occurs together with magnesia, because, like it, manganous oxide remains in solution in the presence of ammoniacal salts, not being precipitated by reagents. The property of this manganous oxide, MnO, of passing into the higher grades of oxidation under the influence of heat, alkalis, and air, gives an easy means not only of discovering the presence of manganese in admixture with magnesia, but also of separating these two analogous bases. Magnesia is not able to give higher grades of oxidation, whilst manganese gives them with great facility. Thus, for instance, an _alkaline_ solution of sodium hypochlorite produces a precipitate of manganese dioxide in a solution of a manganous salt: MnCl_{2} + NaClO + 2NaHO = MnO_{2} + H_{2}O + 3NaCl; whilst magnesia is not changed under these circumstances, and remains in the form of MgCl_{2}. If the magnesia be precipitated owing to the presence of alkali, it may be dissolved in acetic acid, in which manganese dioxide is insoluble. The presence of small quantities of manganese may also be recognised by the green coloration which alkalis acquire when heated with manganese compounds in the air. This green coloration depends on the property of manganese of giving a green alkaline manganate: MnCl_{2} + 4KHO + O_{2} = K_{2}MnO_{4} + 2KCl + 2H_{2}O. Thus _the faculty of oxidising in the presence of alkalis_ forms an essential character of manganese. The higher grades of oxidation containing Mn_{2}O_{7} and MnO_{3} are quite unknown in nature, and even MnO_{2} is not so widely spread in nature as the ores composed of manganous compounds which are met with nearly everywhere. The most important ore of manganese is its dioxide, or so-called _peroxide_, MnO_{2}, which is known in mineralogy as _pyrolusite_. Manganese also occurs as an oxide corresponding with magnetic iron ore, MnO,Mn_{2}O_{3} = Mn_{3}O_{4}, forming the mineral known as _hausmannite_. The oxide Mn_{2}O_{3} also occurs in nature as the anhydrous mineral _braunite_, and in a hydrated form, Mn_{2}O_{3},H_{2}O, called _manganite_. Both of these often occur as an admixture in pyrolusite. Besides which, manganese is met with in nature as a rose-coloured mineral, _rhodonite_, or silicate, MnSiO_{3}. Very fine and rich deposits of manganese ores have been found in the Caucasus, the Urals, and along the Dnieper. Those at the Sharapansky district of the Government of Kutais and at Nicopol on the Dnieper are particularly rich. A large quantity of the ore (as much as 100,000 tons yearly) is exported from these localities.

Thus manganese gives oxides of the following forms: MnO, manganous oxide, and manganous salts, MnX_{2}, corresponding with the base, which resembles magnesia and ferrous oxide in many respects; Mn_{2}O_{3}, a very feeble base, giving salts, MnX_{3}, analogous to the aluminium and ferric salts, easily reduced to MnX_{2}; MnO_{2}, dioxide, generally called peroxide, an almost indifferent oxide, or feebly acid;[18] MnO_{3}, manganic anhydride, which forms salts resembling potassium sulphate;[18 bis] Mn_{2}O_{7}, permanganic anhydride, giving salts analogous to the perchlorates.

[18] The name 'peroxide' should only be retained for those _highest_
oxides (and MnO_{2} stands between MnO and MnO_{3}) which either
by a direct method of double decomposition are able to give
hydrogen peroxide or contain a larger proportion of oxygen than
the base or the acid, just as hydrogen peroxide contains more
oxygen than water. Their type will be H_{2}O_{2}, and they are
exemplified by barium peroxide, BaO_{2}, and sulphur peroxide,
S_{2}O_{7}, &c. Such a dioxide as MnO_{2} is, in all probability,
a salt--that is, a manganous manganate, MnO_{3}MnO, and also, as a
basic salt of a feeble base, capable of combining with alkalis and
acids. Hence the name of manganese peroxide should be abandoned,
and replaced by manganese dioxide. PbO_{2} is better termed lead
dioxide than peroxide. Bisulphide of manganese, MnS_{2},
corresponding to iron pyrites, FeS_{2}, sometimes occurs in nature
in fine octahedra (and cube combinations), for instance, in
Sicily; it is called Hauerite.

[18 bis] On comparing the manganates with the permanganates--for
example, K_{2}MnO_{4} with KMnO_{4}--we find that they differ in
composition by the abstraction of one equivalent of the metal.
Such a relation in composition produced by oxidation is of
frequent occurrence--for instance, K_{4}Fe(CN)_{6} in oxidising
gives K_{3}Fe(CN)_{6}; H_{2}SO_{4} in oxidising gives persulphuric
acid, HSO_{4}, or H_{2}S_{7}O_{8}; H_{2}O forms HO or
H_{2}O_{2}, &c.

_All the oxides of manganese when heated with acids give salts_, MnX_{2}, corresponding with the lower grade of oxidation, _manganous oxide_, MnO. Manganic oxide, Mn_{2}O_{3}, is a feebly energetic base; it is true that it dissolves in hydrochloric acid and gives a dark solution containing the salt MnCl_{3}, but the latter when heated evolves chlorine and gives a salt corresponding with manganous oxide MnCl_{2}--_i.e._ at first: Mn_{2}O_{3} + 6HCl = 3H_{2}O + Mn_{2}Cl_{6}, and then the Mn_{2}Cl_{6} decomposes into 2MnCl_{2} + Cl_{2}. None of the remaining higher grades of oxidation have a basic character, but _act as oxidising agents in the presence of acids_, disengaging oxygen and passing into salts of the lower grade of oxidation of manganese, MnO. Owing to this circumstance, _the manganous salts_ are often obtained; they are, for instance, left in the residue when the dioxide is used for the preparation of oxygen and chlorine.[19]

[19] In the preparation of oxygen from the dioxide by means of
H_{2}SO_{4}, MnSO_{4} is formed; in the preparation of chlorine
from HCl and MnO_{2}, MnCl_{2} is obtained. These two manganous
salts may be taken as examples of compounds MnX_{2}. Manganous
sulphate generally contains various impurities, and also a large
amount of iron salt (from the native MnO_{2}), from which it
cannot be freed by crystallisation. Their removal may, however, be
effected by mixing a portion of the liquid with a solution of
sodium carbonate; a precipitate of manganous carbonate is then
formed. This precipitate is collected and washed, and then added
to the remaining mass of the impure solution of manganous
sulphate; on heating the solution with this precipitate, the whole
of the iron is precipitated as oxide. This is due to the fact that
in the solution of the manganese dioxide in sulphuric acid the
whole of the iron is converted into the ferric state (because the
dioxide acts as an oxidising agent), which, as an exceedingly
feeble base precipitated by calcium carbonate and other kindred
salts, is also precipitated by manganous carbonate. After being
treated in this manner, the solution of manganous sulphate is
further purified by crystallisation. If it be a bright red colour,
it is due to the presence of higher grades of oxidation of
manganese; they may be destroyed by boiling the solution, when the
oxygen from the oxides of manganese is evolved and a very faintly
coloured solution of manganous sulphate is obtained. This salt is
remarkable for the facility with which it gives various
combinations with water. By evaporating the almost colourless
solution of _manganous sulphate_ at very low temperatures, and by
cooling the saturated solution at about 0°, crystals are obtained
containing 7 atoms of water of crystallisation, MnSO_{4},7H_{2}O,
which are isomorphous with cobaltous and ferrous sulphates. These
crystals, even at 10°, lose 5 p.c. of water, and completely
effloresce at 15°, losing about 20 p.c. of water. By evaporating a
solution of the salt at the ordinary temperature, but not above
20°, crystals are obtained containing 5 mol. H_{2}O, which are
isomorphous with copper sulphate; whilst if the crystallisation be
carried on between 20° and 30°, large transparent prismatic
crystals are formed containing 4 mol. H_{2}O (see Nickel). A
boiling solution also deposits these crystals together with
crystals containing 3 mol. H_{2}O, whilst the first salt, when
fused and boiled with alcohol, gives crystals containing 2 mol.
H_{2}O. Graham obtained a monohydrated salt by drying the salt at
about 200°. The last atom of water is eliminated with difficulty,
as is the case with all salts like MnSO_{4}nH_{2}O. The crystals
containing a considerable amount of water are rose-coloured, and
the anhydrous crystals are colourless. The solubility of
MnSO_{4},4H_{2}O (Chapter I., Note 24) per 100 parts of water is:
at 10°, 127 parts; at 37°·5, 149 parts; at 75°, 145 parts; and at
101°, 92 parts. Whence it is seen that at the boiling-point this
salt is less soluble than at lower temperatures, and therefore a
solution saturated at the ordinary temperature becomes turbid when
boiled. Manganous sulphate, being analogous to magnesium sulphate,
is decomposed, like the latter, when ignited, but it does not then
leave manganous oxide, but the intermediate oxide, Mn_{3}O_{4}. It
gives double salts with the alkali sulphates. With aluminium
sulphate it forms fine radiated crystals, whose composition
resembles that of the alums--namely,
MnAl_{2}(SO_{4})_{4},24H_{2}O. This salt is easily soluble in
water, and occurs in nature.