Chapter XXIV: Note 9 ^{bis}). If there were more such (2)

If a solution of potassium hydroxide be added to a solution of a cobalt salt, a blue precipitate of the basic salt will be formed. If a solution of a cobalt salt be heated almost to the boiling-point, and the solution be then mixed with a boiling solution of an alkali hydroxide, a _pink precipitate of cobaltous hydroxide_, CoH_{2}O_{2}, will be formed. If air be not completely excluded during the precipitation by boiling, the precipitate will also contain brown cobaltic hydroxide formed by the further oxidation of the cobaltous oxide.[34] Under similar circumstances nickel salts form _a green precipitate of nickelous hydroxide_, the formation of which is not hindered by the presence of ammonium salts, but in that case only requires more alkali to completely separate the nickel. The nickelous oxide obtained by heating the hydroxide, or from the carbonate or nitrate, is a grey powder, easily soluble in acids and easily reduced, but the same substance may be obtained in the crystalline form as an ordinary product from the ores; it crystallises in regular octahedra, with a metallic lustre, and is of a grey colour. In this state the nickelous oxide almost resists the action of acids.[34 bis]

[34] Hydrated suboxide of cobalt (de Schulten, 1889) is obtained in the
following manner. A solution of 10 grams of CoCl_{2}6H_{2}O in 60
c.c. of water is heated in a flask with 250 grams of caustic
potash and a stream of coal gas is passed through the solution.
When heated the hydrate of the suboxide of cobalt which separates
out, dissolves in the caustic potash and forms a dark blue
solution. This solution is allowed to stand for 24 hours in an
atmosphere of coal gas (in order to prevent oxidation). The
crystalline mass which separates out has a composition Co(OH)_{2},
and to the naked eye appears as a violet powder, which is seen to
be crystalline under the microscope. The specific gravity of this
hydrate is 3·597 at 15°. It does not undergo change in the air;
warm acetic acid dissolves it, but it is insoluble in warm and
cold solutions of ammonia and sal-ammoniac.

[34 bis] The following reaction may be added to those of the cobaltous
and nickelous salts: potassium cyanide forms a precipitate with
cobalt salts which is soluble in an excess of the reagent and
forms a green solution. On heating this and adding a certain
quantity of acid, a double _cobalt cyanide_ is formed which
corresponds with potassium ferricyanide. Its formation is
accompanied with the evolution of hydrogen, and is founded upon
the property which cobalt has of oxidising in an alkaline
solution, the development of which has been observed in such a
considerable measure in the cobaltamine salts. The process which
goes on here may be expressed by the following equation;
CoC_{2}N_{2} + 4KCN first forms CoK_{4}C_{6}N_{6}, which salt with
water, H_{2}O, forms potassium hydroxide, KHO, hydrogen, H, and
the salt, K_{3}CoC_{6}N_{6}. Here naturally the presence of the
acid is indispensable in consequence of its being required to
combine with the alkali. From aqueous solutions this salt
crystallises in transparent, hexagonal prisms of a yellow colour,
easily soluble in water. The reactions of double decomposition,
and even the formation of the corresponding acid, are here
completely the same as in the case of the ferricyanide. If a
nickelous salt be treated in precisely the same manner as that
just described for a salt of cobalt, decomposition will occur.

It is interesting to note _the relation_ of the cobaltous and nickelous hydroxides _to ammonia_; aqueous ammonia dissolves the precipitate of cobaltous and nickelous hydroxide. The blue ammoniacal solution of nickel resembles the same solution of cupric oxide, but has a somewhat reddish tint. It is characterised by the fact that it dissolves silk in the same way as the ammoniacal cupric oxide dissolves cellulose. Ammonia likewise dissolves the precipitate of cobaltous hydroxide, forming a brownish liquid, which becomes darker in air and finally assumes a bright red hue, absorbing oxygen. The admixture of ammonium chloride prevents the precipitation of cobalt salts by ammonia; when the ammonia is added, a brown solution is obtained from which, as in the case of the preceding solution, potassium hydroxide does not separate the cobaltous oxide. Peculiar compounds are produced in this solution; they are comparatively stable, containing ammonia and an excess of oxygen; they bear the name cobaltoamine and cobaltiamine salts. They have been principally investigated by Genth, Frémy, Jörgenson and others. Genth found that when a cobalt salt, mixed with an excess of ammonium chloride, is treated with ammonia and exposed to the air, after a certain lapse of time, on adding hydrochloric acid and boiling, a red powder is precipitated and the remaining solution contains an orange salt. The study of these compounds led to the discovery of a whole series of similar salts, some of which correspond with particular higher degrees of oxidation of cobalt, which are described later.[35] Nickel does not possess this property of absorbing the oxygen of the air when in an ammoniacal solution. In order to understand this distinction, and in general the relation of nickel, it is important to observe that cobalt more easily forms a higher degree of oxidation--namely, _sesquioxide of cobalt_, _cobaltic oxide_, Co_{2}O_{3}--than nickel, especially in the presence of hypochlorous acid. If a solution of a cobalt salt be mixed with barium carbonate and an excess of hypochlorous acid be added, or chlorine gas be passed through it, then at the ordinary temperature on shaking, the whole of the cobalt will be separated in the form of black cobaltic oxide: 2CoSO_{4} + ClHO + 2BaCO_{3} = Co_{2}O_{3} + 2BaSO_{4} + HCl + 2CO_{2}. Under these circumstances nickelous oxide does not immediately form black sesquioxide, but after a considerable space of time it also separates in the form of sesquioxide, Ni_{2}O_{3}, but always later than cobalt. This is due to the relative difficulty of further oxidation of the nickelous oxide. It is, however, possible to oxidise it; if, for instance, the hydroxide NiH_{2}O_{2} be shaken in water and chlorine gas be passed through it, then nickel chloride will be formed, which is soluble in water, and insoluble nickelic oxide in the form of a black precipitate: 3NiH_{2}O_{2} + Cl_{2} = NiCl_{2} + Ni_{2}O_{3},3H_{2}O. Nickelic oxide may also be obtained by adding sodium hypochlorite mixed with alkali to a solution of a nickel salt. Nickelic and cobaltic hydrates are black. Nickelic oxide evolves oxygen with all acids, and in consequence of this it is not separated as a precipitate in the presence of acids; thus it evolves chlorine with hydrochloric acid, exactly like manganese dioxide. When nickelic oxide is dissolved in aqueous ammonia it liberates nitrogen, and an ammoniacal solution of nickelous oxide is formed. When heated, nickelic oxide loses oxygen, forming nickelous oxide. Cobaltic oxide, Co_{2}O_{3}, exhibits more stability than nickelic oxide, and shows feeble basic properties; thus it is dissolved in acetic acid without the evolution of oxygen.[35 bis] But ordinary acids, especially on heating, evolve oxygen, forming a solution of a cobaltous salt. The presence of a cobaltic salt in a solution of a cobaltous salt may be detected by the brown colour of the solution and the black precipitate formed by the addition of alkali, and also from the fact that such solutions evolve chlorine when heated with hydrochloric acid. Cobaltic oxide may not only be prepared by the above-mentioned methods, but also by heating cobalt nitrate, after which a steel-coloured mass remains which retains traces of nitric acid, but when heated further to incandescence evolves oxygen, leaving a compound of cobaltic and cobaltous oxides, similar to magnetic ironstone. Cobalt (but not nickel) undoubtedly forms besides Co_{2}O_{3} a _dioxide_ CoO_{2}. This is obtained[36] when the cobaltous oxide is oxidised by iodine or peroxide of barium.[37]

[35] The cobalt salts may be divided into at least the following
classes, which repeat themselves for Cr, Ir, Rh (we shall not stop
to consider the latter, particularly as they closely resemble the
cobalt salts):--

(_a_) _Ammonium cobalt salts_, which are simply direct compounds
of the cobaltous salts CoX_{2} with ammonia, similar to various
other compounds of the salts of silver, copper, and even calcium
and magnesium, with ammonia. They are easily crystallised from an
ammoniacal solution, and have a pink colour. Thus, for instance,
when cobaltous chloride in solution is mixed with sufficient
ammonia to redissolve the precipitate first formed, octahedral
crystals are deposited which have a composition
CoCl_{2},H_{2}O,6NH_{3}. These salts are nothing else but
combinations with ammonia of crystallisation--if it may be so
termed--likening them in this way to combinations with water of
crystallisation. This similarity is evident both from their
composition and from their capability of giving off ammonia at
various temperatures. The most important point to observe is that
all these salts contain 6 molecules of ammonia to 1 atom of
cobalt, and this ammonia is held in fairly stable connection.
Water decomposes these salts. (Nickel behaves similarly without
forming other compounds corresponding to the true cobaltic.)

(_b_) The solutions of the above-mentioned salts are rendered
turbid by the action of the air; they absorb oxygen and become
covered with a crust of _oxycobaltamine salts_. The latter are
sparingly soluble in aqueous ammonia, have a brown colour, and are
characterised by the fact that with warm water _they evolve
oxygen_, forming salts of the following category: The nitrate may
be taken as an example of this kind of salt; its composition is
CoN_{2}O_{7},5NH_{3},H_{2}O. It differs from cobaltous nitrate,
Co(NO_{3})_{2}, in containing an extra atom of oxygen--that is, it
corresponds with cobalt dioxide, CoO_{2}, in the same way that the
first salts correspond with cobaltous oxide; they contain 5, and
not 6, molecules of ammonia, as if NH_{3} had been replaced by O,
but we shall afterwards meet compounds containing either 5NH_{3}
or 6NH_{3} to each atom of cobalt.

(_c_) _The luteocobaltic salts_ are thus called because they have
a yellow (luteus) colour. They are obtained from the salts of the
first kind by submitting them in dilute solution to the action of
the air; in this case salts of the second kind are not formed,
because they are decomposed by an excess of water, with the
evolution of oxygen and the formation of luteocobaltic salts. By
the action of ammonia the salts of the fifth kind (roseocobaltic)
are also converted into luteocobaltic salts. These last-named
salts generally crystallise readily, and have a yellow colour;
they are comparatively much more stable than the preceding ones,
and even for a certain time resist the action of boiling water.
Boiling aqueous potash liberates ammonia and precipitates hydrated
cobaltic oxide, Co_{2}O_{3},3H_{2}O, from them. This shows that
the luteocobaltic salts correspond with cobaltic oxide,
Co_{2}O_{3}, and those of the second kind with the dioxide. When a
solution of luteocobaltic sulphate,
Co_{2}(SO_{4})_{3},12NH_{3},4H_{2}O, is treated with baryta,
barium sulphate is precipitated, and the solution contains
luteocobaltic hydroxide, Co(OH)_{3},6NH_{3}, which is soluble in
water, is powerfully alkaline, absorbs the oxygen of the air, and
when heated is decomposed with the evolution of ammonia. This
compound therefore corresponds to a solution of cobaltic hydroxide
in ammonia. The luteocobaltic salts contain 2 atoms of cobalt and
12 molecules of ammonia--that is, 6NH_{3} to each atom of cobalt,
like the salts of the first kind. The CoX_{2} salts have a
metallic taste, whilst those of luteocobalt and others have a
purely saline taste, like the salts of the alkali metals. In the
luteo-salts all the X's react (are ionised, as some chemists say)
as in ordinary salts--for instance, all the Cl_{2} is precipitated
by a solution of AgNO_{3}; all the (SO_{4})_{3} gives a
precipitate with BaX_{2}, &c. The double salt formed with PtCl_{4}
is composed in the same manner as the potassium salt,
K_{2}PtCl_{4} = 2KCl + PtCl_{4}, that is, contains
(CoCl_{3},6NH_{3})_{2},3PtCl_{4}, or the amount of chlorine in the
PtCl_{4} is double that in the alkaline salt. In the rosepentamine
(_e_), and rosetetramine (_f_), salts, also all the X's react or
are ionised, but in the (_g_) and (_h_) salts only a portion of
the X's react, and they are equal to the (_e_) and (_f_) salts
minus water; this means that although the water dissolves them it
is not combined with them, as PHO_{3} differs from PH_{3}O_{3};
phenomena of this class correspond exactly to what has been
already (Chapter XXI., Note 7) mentioned respecting the green and
violet salts of oxide of chromium.

(_d_) _The fuscocobaltic salts._ An ammoniacal solution of cobalt
salts acquires a brown colour in the air, due to the formation of
these salts. They are also produced by the decomposition of salts
of the second kind; they crystallise badly, and are separated from
their solutions by addition of alcohol or an excess of ammonia.
When boiled they give up the ammonia and cobaltic oxide which they
contain. Hydrochloric and nitric acids give a yellow precipitate
with these salts, which turns red when boiled, forming salts of
the next category. The following is an example of the composition
of two of the fuscocobaltic salts,
Co_{2}O(SO_{4})_{2},8NH_{3},4H_{2}O and
Co_{2}OCl_{4},8NH_{3},3H_{2}O. It is evident that the
fuscocobaltic salts are ammoniacal compounds of basic cobaltic
salts. The normal cobaltic sulphate ought to have the composition
Co_{2}(SO_{4})_{3} = Co_{2}O_{3},3SO_{3}; the simplest basic salts
will be Co_{2}O(SO_{4})_{2} = Co_{2}O_{3})2SO_{3}, and
Co_{2}O_{2}(SO_{4}) = Co_{2}O_{3},SO_{3}. The fuscocobaltic salts
correspond with the first type of basic salts. They are changed
(in concentrated solutions) into oxycobaltamine salts by
absorption of one atom of oxygen, Co_{2}O_{2}(SO_{4})_{2}. The
whole process of oxidation will be as follows: first of all
Co_{2}X_{4}, a cobaltous salt, is in the solution (X a univalent
haloid, 2 molecules of the salt being taken), then Co_{2}OX_{4},
the basic cobaltic salt (4th series), then Co_{2}O_{2}X_{4}, the
salt of the dioxide (2nd series). The series of basic salts with
an acid, 2HX, forms water and a normal salt, Co_{2}X_{6} (in 3, 5,
6 series). These salts are combined with various amounts of water
and ammonia. Under many conditions the salts of fuscocobalt are
easily transformed into salts of the next series. The salts of the
series that has just been described contain 4 molecules of ammonia
to 1 atom of cobalt.

(_e_) _The roseocobaltic_ (or rosepentamine),
CoX_{2}H_{2}O,5NH_{3}, _salts_, like the luteocobaltic, correspond
with the normal cobaltic salts, but contain less ammonia, and an
extra molecule of water. Thus the sulphate is obtained from
cobaltous sulphate dissolved in ammonia and left exposed to the
air until transformed into a brown solution of the fuscocobaltic
salt; when this is treated with sulphuric acid a crystalline
powder of the roseocobaltic salt,
Co_{2}(SO_{4})_{3},10NH_{3},5H_{2}O, separates. The formation of
this salt is easily understood: cobaltous sulphate in the presence
of ammonia absorbs oxygen, and the solution of the fuscocobaltic
salt will therefore contain, like cobaltous sulphate, one part of
sulphuric acid to every part of cobalt, so that the whole process
of formation may be expressed by the equation: 10NH_{3} +
2CoSO_{4} + H_{2}SO_{4} + 4H_{2}O + O =
Co_{2}(SO_{4})_{3},10NH_{3},5H_{2}O. This salt forms tetragonal
crystals of a red colour, slightly soluble in cold, but readily
soluble in warm water. When the sulphate is treated with baryta,
roseocobaltic hydroxide is formed in the solution, which absorbs
the carbonic anhydride of the air. It is obtained from the next
series by the action of alkalis.

(_f_) The _rosetetramine cobaltic salts_ CoCl_{2},2H_{2}O,4NH_{3}
were obtained by Jörgenson, and belong to the type of the
luteo-salts, only with the substitution of 2NH_{3} for H_{2}O.
Like the luteo- and roseo-salts they give double salts with
PtCl_{4}, similar to the alkaline double salts, for instance
(Co_{2}H_{2}O,4NH_{3})2(SO_{4})_{2}Cl_{2}PtCl_{4}. They are darker
in colour than the preceding, but also crystallise well. They are
formed by dissolving CoCO_{3} in sulphuric acid (of a given
strength), and after NH_{3} and carbonate of ammonium have been
added, air is passed through the solution (for oxidation) until
the latter turns red. It is then evaporated with lumps of
carbonate of ammonium, filtered from the precipitate and
crystallised. A salt of the composition
Co_{2}(CO_{3})_{2}(SO_{4}),(2H_{2}O,4NH_{3})_{2} is thus obtained,
from which the other salts may be easily prepared.

(_g_) The _purpureocobaltic salts_, CoX_{3},5NH_{3}, are also
products of the direct oxidation of ammoniacal solutions of cobalt
salts. They are easily obtained by heating the roseocobaltic and
luteo-salts with strong acids. They are to all effects the same as
the roseocobaltic salts, only anhydrous. Thus, for instance, the
purpureocobaltic chloride, Co_{2}Cl_{6},10NH_{3}, or
CoCl_{3},5NH_{3}, is obtained by boiling the oxycobaltamine salts
with ammonia. There is the same distinction between these salts
and the preceding ones as between the various compounds of
cobaltous chloride with water. In the purpureocobaltic only X_{2}
out of the X_{3} react (are ionised). To the rosetetramine salts
(_f_) there correspond the _purpureotetramine_ salts,
CoX_{3}H_{2}O,4NH_{3}. The corresponding chromium
purpureopentamine salt, CrCl_{3},5NH_{3} is obtained with
particular ease (Christensen, 1893). Dry anhydrous chromium
chloride is treated with anhydrous liquid ammonia in a freezing
mixture composed of liquid CO_{2} and chlorine, and after some
time the mixture is taken out of the freezing mixture, so that the
excess of NH_{3} boils away; the violet crystals then immediately
acquire the red colour of the salt, CrCl_{3},5NH_{3}, which is
formed. The product is washed with water (to extract the
luteo-salt, CrCl_{3},6NH_{3}), which does not dissolve the salt,
and it is then recrystallised from a hot solution of hydrochloric
acid.

(_h_) The _praseocobaltic salts_, CoX_{3},4NH_{3}, are green, and
form, with respect to the rosetetramine salts (_f_), the products
of ultimate dehydration (for example, like metaphosphoric acid
with respect to orthophosphoric acid, but in dissolving in water
they give neither rosetetramine nor tetramine salts. (In my
opinion one should expect salts with a still smaller amount of
NH_{3}, of the blue colour proper to the low hydrated compounds of
cobalt; the green colour of the prazeo-salts already forms a step
towards the blue.) Jörgenson obtained salts for ethylene-diamine,
N_{2}H_{4}C_{2}H_{4} which replaces 2NH_{3}. After being kept a
long time in aqueous solution they give rosetetramine salts, just
as metaphosphoric acid gives orthophosphoric acid, while the
rosetetramine salts are converted into prazeo-salts by Ag_{2}O and
NaHO. Here only one X is ionised out of the X_{3}. There are also
basic salts of the same type; but the best known is the chromium
salt called the rhodozochromic salt,
Cr_{2}(OH)_{3}Cl_{3},6NH_{3},2H_{2}O, which is formed by the
prolonged action of water upon the corresponding roseo-salt.

The cobaltamine compounds differ essentially but little from the
ammoniacal compounds of other metals. The only difference is that
here the cobaltic oxide is obtained from the cobaltous oxide in
the presence of ammonia. In any case it is a simpler question than
that of the double cyanides. Those forces in virtue of which such
a considerable number of ammonia molecules are united with a
molecule of a cobalt compound, appertain naturally to the series
of those slightly investigated forces which exist even in the
highest degrees of combination of the majority of elements. They
are the same forces which lead to the formation of compounds
containing water of crystallisation, double salts, isomorphous
mixtures and complex acids (Chapter XXI., Note 8 bis). The
simplest conception, according to my opinion, of cobalt compounds
(much more so than by assuming special complex radicles, with
Schiff, Weltzien, Claus, and others), may be formed by comparing
them with other ammoniacal products. Ammonia, like water, combines
in various proportions with a multitude of molecules. Silver
chloride and calcium chloride, just like cobalt chloride, absorb
ammonia, forming compounds which are sometimes slightly stable,
and easily dissociated, sometimes more stable, in exactly the same
way as water combines with certain substances, forming fairly
stable compounds called hydroxides or hydrates, or less stable
compounds which are called compounds with water of
crystallisation. Naturally the difference in the properties in
both cases depends on the properties of those elements which enter
into the composition of the given substance, and on those kinds of
affinity towards which chemists have not as yet turned their
attention. If boron fluoride, silicon fluoride, &c., combine with
hydrofluoric acid, if platinic chloride, and even cadmium
chloride, combine with hydrochloric acid, these compounds may be
regarded as double salts, because acids are salts of hydrogen. But
evidently water and ammonia have the same saline faculty, more
especially as they, like haloid acids, contain hydrogen, and are
both capable of further combination--for instance, ammonia with
hydrochloric acid. Hence it is simpler to compare complex
ammoniacal with double salts, hydrates, and similar compounds, but
_the ammonio-metallic salts_ present a most complete qualitative
and quantitative resemblance to _the hydrated salts of metals_.
The composition of the latter is MX_{_n_}_m_H_{2}O, where M =
metal, X = the haloid, simple or complex, and _n_ and _m_ the
quantities of the haloid and so-called water of crystallisation
respectively. The composition of the ammoniacal salts of metals is
MX_{_n_}_m_NH_{3}. The water of crystallisation is held by the
salt with more or less stability, and some salts even do not
retain it at all; some part with water easily when exposed to the
air, others when heated, and then with difficulty. In the case of
some metals all the salts combine with water, whilst with others
only a few, and the water so combined may then be easily
disengaged. All this applies equally well to the ammoniacal salts,
and therefore the combination of ammonia may be termed _the
ammonia of crystallisation_. Just as the water which is combined
with a salt is held by it with different degrees of force, so it
is with ammonia. In combining with 2NH_{3},PtCl_{2} evolves 31,000
cals.; while CaCl_{2} only evolves 14,000 cals.; and the former
compound parts with its NH_{3} (together with HCl in this case)
with more difficulty, only above 200°, while the latter disengages
ammonia at 180°. ZnCl_{2},2NH_{3} in forming ZnCl_{2},4NH_{3}
evolves only 11,000 cals., and splits up again into its components
at 80°. The amount of combined ammonia is as variable as the
amount of water of crystallisation--for instance, SnI_{4}8NH_{3},
CrCl_{2}8NH_{3}, CrCl_{3}6NH_{3},
CrCl_{3}5NH_{3},PtCl_{2},4NH_{3}, &c. are known. Very often NH_{3}
is replaceable by OH_{2} and conversely. A colourless, anhydrous
cupric salt--for instance, cupric sulphate--when combined with
water forms blue and green salts, and violet when combined with
ammonia. If steam be passed through anhydrous copper sulphate the
salt absorbs water and becomes heated; if ammonia be substituted
for the water the heating becomes much more intense, and the salt
breaks up into a fine violet powder. With water CuSO_{4},5H_{2}O
is formed, and with ammonia CuSO_{4},5NH_{3}, the number of water
and ammonia molecules retained by the salt being the same in each
case, and as a proof of this, and that it is not an isolated
coincidence, the remarkable fact must be borne in mind that water
and ammonia consecutively, molecule for molecule, are capable of
supplanting each other, and forming the compounds
CuSO_{4},5H_{2}O, CuSO_{4},4H_{2}O,NH_{3};
CuSO_{4},3H_{2}O,2NH_{3}; CuSO_{4},2H_{2}O,3NH_{3};
CuSO_{4},H_{2}O,4NH_{3}, and CuSO_{4},5NH_{3}. The last of these
compounds was obtained by Henry Rose, and my experiments have
shown that more ammonia than this cannot be retained. By adding to
a strong solution of cupric sulphate sufficient ammonia to
dissolve the whole of the oxide precipitated, and then adding
alcohol, Berzelius obtained the compound CuSO_{4},H_{2}O,4NH_{3},
&c. The law of substitution also assists in rendering these
phenomena clearer, because a compound of ammonia with water forms
ammonium hydroxide, NH_{4}HO, and therefore these molecules
combining with one another may also interchange, as being of equal
value. In general, those salts form stable ammoniacal compounds
which are capable of forming stable compounds with water of
crystallisation; and as ammonia is capable of combining with
acids, and as some of the salts formed by slightly energetic bases
in their properties more closely resemble acids (that is, salts of
hydrogen) than those salts containing more energetic bases, we
might expect to find more stable and more easily-formed
ammonio-metallic salts with metals and their oxides having weaker
basic properties than with those which form energetic bases. This
explains why the salts of potassium, barium, &c., do not form
ammonio-metallic salts, whilst the salts of silver, copper, zinc,
&c., easily form them, and the salts RX_{3} still more easily and
with greater stability. This consideration also accounts for the
great stability of the ammoniacal compounds of cupric oxide
compared with those of silver oxide, since the former is displaced
by the latter. It also enables us to see clearly the distinction
which exists in the stability of the cobaltamine salts containing
salts corresponding with cobaltous oxide, and those corresponding
with higher oxides of cobalt, for the latter are weaker bases than
cobaltous oxides. _The nature of the forces and quality of the
phenomena occurring during the formation of the most stable
substances, and of such compounds as crystallisable compounds, are
one and the same, although perhaps exhibited in a different
degree._ This, in my opinion, may be best confirmed by examining
the compounds of carbon, because for this element the nature of
the forces acting during the formation of its compounds is well
known. Let us take as an example two unstable compounds of carbon.
Acetic acid, C_{2}H_{4}O_{2} (specific gravity 1·06), with water
forms the hydrate, C_{2}H_{4}O_{2},H_{2}O, denser (1·07) than
either of the components, but unstable and easily decomposed,
generally simply referred to as a solution. Such also is the
crystalline compound of oxalic acid, C_{2}H_{2}O_{4}, with water,
C_{2}H_{2}O_{4},2H_{2}O. Their formation might be predicted as
starting from the hydrocarbon C_{2}H_{6}, in which, as in any
other, the hydrogen may be exchanged for chlorine, the water
residue (hydroxyl), &c. The first substitution product with
hydroxyl, C_{2}H_{5}(HO), is stable; it can be distilled without
alteration, resists a temperature higher than 100°, and then does
not give off water. This is ordinary alcohol. The second,
C_{2}H_{4}(HO)_{2}, can also be distilled without change, but can
be decomposed into water and C_{2}H_{4}O (ethylene oxide or
aldehyde); it boils at about 197°, whilst the first hydrate boils
at 78°, a difference of about 100°. The compound
C_{2}H_{3}(HO)_{3} will be the third product of such substitution;
it ought to boil at about 300°, but does not resist this
temperature--it decomposes into H_{2}O and C_{2}H_{4}O_{2}, where
only one hydroxyl group remains, and the other atom of oxygen is
left in the same condition as in ethylene oxide, C_{2}H_{4}O.
There is a proof of this. Glycol, C_{2}H_{4}(HO)_{2}, boils at
197°, and forms water and ethylene oxide, which boils at 13°
(aldehyde, its isomeride, boils at 21°); therefore the product
disengaged by the splitting up of the hydrate boils at 184° lower
than the hydrate C_{2}H_{4}(HO)_{2}. Thus the hydrate
C_{2}H_{3}(HO)_{3}, which ought to boil at about 300°, splits up
in exactly the same way into water and the product
C_{2}H_{4}O_{2}, which boils at 117°--that is, nearly 183° lower
than the hydrate, C_{2}H_{3}(HO)_{3}. But this hydrate splits up
before distillation. The above-mentioned hydrate of acetic acid is
such a decomposable hydrate--that is to say, what is called a
solution. Still less stability may be expected from the following
hydrates. C_{2}H_{2}(HO)_{4} also splits up into water and a
hydrate (it contains two hydroxyl groups) called glycolic acid,
C_{2}H_{2}O(HO)_{2} = C_{2}H_{4}O_{3}. The next product of
substitution will be C_{2}H(HO)_{5}; it splits up into water,
H_{2}O, and glyoxylic acid, C_{2}H_{4}O_{4} (three hydroxyl
groups). The last hydrate which ought to be obtained from
C_{2}H_{6}, and ought to contain C_{2}(HO)_{6}, is the crystalline
compound of oxalic acid, C_{2}H_{2}O_{4} (two hydroxyl groups),
and water, 2H_{2}O, which has been already mentioned. The hydrate
C_{2}(HO)_{6} = C_{2}H_{2}O_{4},2H_{2}O, ought, according to the
foregoing reasoning, to boil at about 600° (because the hydrate,
C_{2}H_{4}(HO)_{2}, boils at about 200°, and the substitution of 4
hydroxyl groups for 4 atoms of hydrogen will raise the
boiling-point 400°). It does not resist this temperature, but at a
much lower point splits up into water, 2H_{2}O, and the hydrate
C_{2}O_{2}(HO)_{2}, which is also capable of yielding water.
Without going into further discussion of this subject, it may be
observed that the formation of the hydrates, or compounds with
water of crystallisation, of acetic and oxalic acids has thus
received an accurate explanation, illustrating the point we
desired to prove in affirming that compounds with water of
crystallisation are held together by the same forces as those
which act in the formation of other complex substances, and that
the easy displaceability of the water of crystallisation is only a
peculiarity of a local character, and not a radical point of
distinction. All the above-mentioned hydrates, C_{2}X_{6}, or
products of their destruction, are actually obtained by the
oxidation of the first hydrate, C_{2}H_{3}(HO), or common alcohol,
by nitric acid (Sokoloff and others). Hence the forces which
induce salts to combine with _n_H_{2}O or with NH_{3} are
undoubtedly of the same order as the forces which govern the
formation of ordinary 'atomic' and saline compounds. (A great
impediment in the study of the former was caused by the conviction
which reigned in the sixties and seventies, that 'atomic' were
essentially different from 'molecular' compounds like
crystallohydrates, in which it was assumed that there was a
combination of entire molecules, as though without the
participation of the atomic forces.) If the bond between chlorine
and different metals is not equally strong, so also the bond
uniting _n_H_{2}O and _n_NH_{3} is exceeding variable; there is
nothing very surprising in this. And in the fact that the
combination of different amounts of NH_{3} and H_{2}O alters the
capacity of the haloids X of the salts RX_{2} for reaction (for
instance, in the luteo-salts all the X_{3}, while in the purpureo,
only 2 out of the 3, and in the prazeo-salts only 1 of the 3 X's
reacts), we should see in the first place a phenomenon similar to
what we met with in Cr_{2}Cl_{6} (Chapter XXI., Note 7 bis), for
in both instances the essence of the difference lies in the
removal of water; a molecule RCl_{3},6H_{2}O or RCl_{3},6NH_{3}
contains the halogen in a perfectly mobile (ionised) state, while
in the molecule RCl_{3},5H_{2}O or RCl_{3},5NH_{3} a portion of
the halogen has almost lost its faculty for reacting with
AgNO_{3}, just as metalepsical chlorine has lost this faculty
which is fully developed in the chloranhydride. Until the reason
of this difference be clear, we cannot expect that ordinary points
of view and generalisation can give a clear answer. However, we
may assume that here the explanation lies in the nature and kind
of motion of the atoms in the molecules, although as yet it is not
clear how. Nevertheless, I think it well to call attention again
(Chapter I.) to the fact that the combination of water, and hence,
also, of any other element, leads to most diverse consequences;
the water in the gelatinous hydrate of alumina or in the
decahydrated Glauber salt is very mobile, and easily reacts like
water in a free state; but the same water combined with oxide of
calcium, or C_{2}H_{4} (for instance, in C_{2}H_{6}O and in
C_{4}H_{10}O), or with P_{2}O_{5}, has become quite different, and
no longer acts like water in a free state. We see the same
phenomenon in many other cases--for example, the chlorine in
chlorates no longer gives a precipitate of chloride of silver with
AgNO_{3}. Thus, although the instance which is found in the
difference between the roseo- and purpureo-salts deserves to be
fully studied on account of its simplicity, still it is far from
being exceptional, and we cannot expect it to be thoroughly
explained unless a mass of similar instances, which are
exceedingly common among chemical compounds, be conjointly
explained. (Among the researches which add to our knowledge
respecting the complex ammoniacal compounds, I think it
indispensable to call the reader's attention to Prof. Kournakoff's
dissertation 'On complex metallic bases,' 1893.)

Kournakoff (1894) showed that the solubility of the luteo-salt,
CoCl_{3},6NH_{3}, at 0° = 4·30 (per 100 of water), at 20° = 7·7,
that in passing into the roseo-salt, CoCl_{3}H_{2}O_{5}NH_{3}, the
solubility rises considerably, and at 0° = 16·4, and at 20° =
about 27, whilst the passage into the purpureo-salt,
CoCl_{3},5NH_{3}, is accompanied by a great fall in the
solubility, namely, at 0° = 0·23, and at 20° = about 0·5. And as
crystallohydrates with a smaller amount of water are usually more
soluble than the higher crystallohydrates (Le Chatelier), whilst
here we find that the solubility falls (in the purpureo-salt) with
a loss of water, that water which is contained in the roseo-salt
cannot be compared with the water of crystallisation. Kournakoff,
therefore, connects the fall in solubility (in the passage of the
roseo- into the purpureo-salts) with the accompanying loss in the
reactive capacity of the chlorine.

In conclusion, it may be observed that the elements of the eighth
group--that is, the analogues of iron and platinum--according to
my opinion, will yield most fruitful results when studied as to
combinations with whole molecules, as already shown by the
examples of complex ammoniacal, cyanogen, nitro-, and other
compounds, which are easily formed in this eighth group, and are
remarkable for their stability. This faculty of the elements of
the eighth group for forming the complex compounds alluded to, is
in all probability connected with the position which the eighth
group occupies with regard to the others. Following the seventh,
which forms the type RX_{7}, it might be expected to contain the
most complex type, RX_{8}. This is met with in OsO_{4}. The other
elements of the eighth group, however, only form the lower types
RX_{2}, RX_{3}, RX_{4} ... and these accordingly should be
expected to aggregate themselves into the higher types, which is
accomplished in the formation of the above-mentioned complex
compounds.

[35 bis] Marshall (1891) obtained cobaltic sulphate,
Co_{2}(SO_{4})_{3},18H_{2}O, by the action of an electric current
upon a strong solution of CoSO_{4}.

[36] The action of an alkaline hypochlorite or hypobromite upon a
boiling solution of cobaltous salts, according to Schroederer
(1889), produces oxides, whose composition varies between
Co_{3}O_{5} (Rose's compound) and Co_{2}O_{3}, and also between
Co_{5}O_{8} and Co_{12}O_{19}. If caustic potash and then bromine
be added to the liquid, only Co_{2}O_{3} is formed. The action of
alkaline hypochlorites or hypo-bromites, or of iodine, upon
cobaltic salts, gives a highly-coloured precipitate which has a
different colour to the hydrate of the oxide Co_{2}(OH)_{6}.
According to Carnot the precipitate produced by the hypochlorites
has a composition Co_{10}O_{16}, whilst that given by iodine in
the presence of an alkali contains a larger amount of oxygen.
Fortmann (1891) reinvestigated the composition of the higher
oxygen oxide obtained by iodine in the presence of alkali, and
found that the greenish precipitate (which disengages oxygen when
heated to 100°) corresponds to the formula CoO_{2}. The reaction
must be expressed by the equation: CoX_{2} + I_{2} + 4KHO =
CoO_{2} + 2KX + 2KI + 2H_{2}O.

[37] Prior to Fortmann, Rousseau (1889) endeavoured to solve the
question as to whether CoO_{2} was able to combine with bases. He
succeeded in obtaining a barium compound corresponding to this
oxide. Fifteen grams of BaCl_{2} or BaBr_{2} are triturated with
5-6 grams of oxide of barium, and the mixture heated to redness in
a closed platinum crucible; 1 gram of oxide of cobalt is then
gradually added to the fused mass. Each addition of oxide is
accompanied by a violent disengagement of oxygen. After a short
time, however, the mass fuses quietly, and a salt settles at the
bottom of the crucible, which, when freed from the residue,
appears as black hexagonal, very brilliant crystals. In dissolving
in water this substance evolves chlorine; its composition
corresponds to the formula 2(CoO_{2})BaO. If the original mass be
heated for a long time (40 hours), the amount of dioxide in the
resultant mass decreases. The author obtained a neutral salt
having the composition CoO_{2}BaO (this compound = BaO_{2}CoO) by
breaking up the mass as it agglomerates together, and bringing the
pieces into contact with the more heated surface of the crucible.
This salt is formed between the somewhat narrow limits of
temperature 1,000°-1,100°; above and below these limits compounds
richer or poorer in CoO_{2} are formed. The formation of CoO_{2}
by the action of BaO_{2}, and the easy decomposition of CoO_{2}
with the evolution of oxygen, give reason for thinking that it
belongs to the class of peroxides (like Cr_{2}O_{7}, CaO_{2},
&c.); it is not yet known whether they give peroxide of hydrogen
like the true peroxides. The fact that it is obtained by means of
iodine (probably through HIO), and its great resemblance to
MnO_{2}, leads rather to the supposition that CoO_{2} is a very
feeble saline oxide. The form CoO_{2} is repeated in the cobaltic
compounds (Note 35), and the existence of CoO_{2} should have long
ago been recognised upon this basis.

Nickel alloys possess qualities which render them valuable for technical purposes, the alloy of nickel with iron being particularly remarkable. This alloy is met with in nature as _meteoric iron_. The Pallasoffsky mass of meteoric iron, preserved in the St. Petersburg Academy, fell in Siberia in the last century; it weighs about 15 cwt. and contains 88 p.c. of iron and about 10 p.c. of nickel, with a small admixture of other metals. In the arts _German silver_ is most extensively used; it is an alloy containing nickel, copper, and zinc in various proportions. It generally consists of about 50 parts of copper, 25 parts of zinc, and 25 parts of nickel. This alloy is characterised by its white colour resembling that of silver, and, like this latter metal, it does not rust, and therefore furnishes an excellent substitute for silver in the majority of cases where it is used. Alloys which contain silver in addition to nickel show the properties of silver to a still greater extent. Alloys of nickel are used for currency, and if rich deposits of nickel are discovered a wide field of application lies before it, not only in a pure state (because it is a beautiful metal and does not rust) but also for use in alloys. Steel vessels (pressed or forged out of sheet steel) covered with nickel have such practical merits that their manufacture, which has not long commenced, will most probably be rapidly developed, whilst nickel steel, which exceeds ordinary steel in its tenacity, has already proved its excellent qualities for many purposes (for instance, for armour plate).

Until 1890 no compound of cobalt or nickel was known of sufficient volatility to determine the molecular weights of the compounds of these metals; but in 1890 Mr. L. Mond, in conducting (together with Langer and Quincke) his researches on the action of nickel upon carbonic oxide (Chapter IX., Note 24 bis), observed that nickel gradually volatilises in a stream of carbonic oxide; this only takes place at low temperatures, and is seen by the coloration of the flame of the carbonic oxide. This observation led to the discovery of a remarkable volatile _compound of nickel and carbonic oxide_, having as molecular composition Ni(CO)_{4},[38] as determined by the vapour density and depression of the freezing point. Cobalt and many other metals do not form volatile compounds under these conditions, but iron gives a similar product (Note 26 bis). Ni(CO)_{4} is prepared by taking finely divided Ni (obtained by reducing NiO by heating it in a stream of hydrogen, or by igniting the oxalate NiC_{2}O_{4})[39] and passing (at a temperature below 50°, for even at 60° decomposition may take place and an explosion) a stream of CO over it; the latter carries over the vapour of the compound, which condenses (in a well-cooled receiver) into a perfectly colourless extremely mobile liquid, boiling without decomposition at 43°, and crystallising in needles at -25° (Mond and Nasini, 1891). Liquid Ni(CO)_{4} has a sp. gr. 1·356 at 0°, is insoluble in water, dissolves in alcohol and benzene, and burns with a very smoky flame due to the liberation of Ni. The vapour when passed through a tube heated to 180° and above deposits a brilliant coating of metal, and disengages CO. If the tube be strongly heated the decomposition is accompanied by an explosion. If Ni(CO)_{4} as vapour be passed through a solution of CuCl_{2}, it reduces the latter to metal; it has the same action upon an ammoniacal solution of AgCl, strong nitric acid oxidises Ni(CO)_{4}, dilute solutions of acids have no action; if the vapour be passed through strong sulphuric acid, CO is liberated, chlorine gives NiCl and COCl_{2}; no simple reactions of double decomposition are yet known for Ni(CO)_{4}, however, so that its connection with other carbon compounds is not clear. Probably the formation of this compound could be applied for extracting nickel from its ores.[40]

[38] This compound is known as nickel tetra-carbonyl. It appears to me
yet premature to judge of the structure of such an extraordinary
compound as Ni(CO)_{4}. It has long been known that potassium
combines with CO forming K_{_n_}(CO)_{_n_} (Chapter IX., Note 31),
but this substance is apparently saline and non-volatile, and has
as little in common with Ni(CO)_{4} as Na_{2}H has with SbH_{3}.
However, Berthelot observed that when NiC_{4}O_{4} is kept in air,
it oxidises and gives a colourless compound,
Ni_{3}C_{2}O_{3},10H_{2}O, having apparently saline properties. We
may add that Schützenberger, on reducing NiCl_{2} by heating it in
a current of hydrogen, observed that a nickel compound partially
volatilises with the HCl and gives metallic nickel when heated
again. The platinum compound, PtCl_{2}(CO)_{3} (Chapter XXIII.,
Note 11), offers the greatest analogy to Ni(CO)_{4}. This compound
was obtained as a volatile substance by Schützenberger by
moderately heating (to 235°) metallic platinum in a mixture of
chlorine and carbonic oxide. If we designate CO by Y, and an atom
of chlorine by X, then taking into account that, according to the
periodic system, Ni is an analogue of Pt, a certain degree of
correspondence is seen in the composition NiY_{4} and
PtX_{2}Y_{2}. It would be interesting to compare the reactions of
the two compounds.

[39] According to its empirical formula oxalate of nickel also contains
nickel and carbonic oxide.

[40] The following are the thermo-chemical data (according to Thomsen,
and referred to gram weights expressed by the formula, in large
calories or thousand units of heat) for the formation of
corresponding compounds of Mn, Fe, Co, Ni, and Cu (+ Aq signifies
that the reaction proceeds in an excess of water):

R = Mn Fe Co Ni Cu
R + Cl_{2} + Aq 128 100 95 94 63
R + Br_{2} + Aq 106 78 73 72 41
R + I_{2} + Aq 76 48 43 41 32
R + O + H_{2}O 95 68 63 61 38
R + O_{2} + SO_{2} + _n_H_{2}O 193 169 163 163 130
RCl_{2} + Aq +16 18 18 19 11

These examples show that for analogous reactions the amount of
heat evolved in passing from Mn to Fe, Co, Ni, and Cu varies in
regular sequences as the atomic weight increases. A similar
difference is to be found in other groups and series, and proves
that thermo-chemical phenomena are subject to the periodic law.