Chapter XX: , Note 20). Thus the forms PX{3} and PX{5} not only (2)

Saunders (1892) obtained 5RbCl,3SbCl_{3} and RbCl,SbCl_{3}. Ditte
and Metzner (1892) showed that Sb and Bi dissolve in hydrochloric
acid only owing to the participation of the oxygen of the air or
of that dissolved in the acid.

The heaviest analogue of nitrogen and phosphorus is _bismuth_, Bi = 208. Here, as in the other groups, the basic, metallic, properties increase with the atomic weight. Bismuth does not give any hydrogen compound and the highest oxide, Bi_{2}O_{5}, is a very feeble acid oxide. Bismuthous oxide, Bi_{2}O_{3}, is a base, and bismuth itself a perfect metal. To explain the other properties of bismuth it must further be remarked that in the eleventh series it follows mercury, thallium and lead, whose atomic weights are near to that of bismuth, and that therefore it resembles them and more especially its nearest neighbour, lead. Although PbO and PbO_{2}, represent types different from Bi_{2}O_{3} and Bi_{2}O_{5}, they resemble them in many respects, even in their external appearance, moreover the lower oxides both of Pb and Bi are basic and the higher acid, which easily evolve oxygen. But judging by the formula, Bi_{2}O_{3} is a more feeble base than PbO. They both easily give basic salts.

Bismuth forms compounds of two types, BiX_{3} and BiX_{5},[43] which entirely recall the two types we have already established for the compounds of lead. Just as in the case of lead, the type PbX_{2}, is basic, stable, easily formed, and passes with difficulty into the higher and lower types, which are unstable, so also in the case of bismuth the type of combination BiX_{3} is the usual basic form. The higher type of combination, BiX_{5},[44] in fact behaves toward this stable type, BiX_{3}, in exactly the same manner as lead dioxide does to the monoxide; and bismuthic acid is obtained by the action of chlorine on bismuth oxide suspended in water, in exactly the same way as lead dioxide is obtained from lead oxide. It is an oxidising agent like lead dioxide, and even the acid character in bismuthic acid is only slightly more developed than in lead dioxide. Here, as in the case of lead (minium), intermediate compounds are easily formed in which the bismuth of the lower oxide plays the part of a base combined with the acid which is formed by the higher form of the oxidation of bismuth.

[43] Metallic bismuth is very easily obtained when the compounds of the
oxide are reduced by powerful reducing agents, but when less
powerful reducing agents--for example, stannous oxide--are taken,
bismuth suboxide is formed as a black crystalline powder. It is a
compound of the type BiX_{2}, its composition being BiO; it is
decomposed by acids into the metal and oxide, which passes into
solution.

[44] The type BiX_{5} is represented by the pentoxide, Bi_{2}O_{5}, its
metahydrate, Bi_{2}O_{5},H_{2}O, or BiHO_{3}, known as bismuthic
acid, and the pyrohydrate, Bi_{2}H_{4}O_{7}. _Bismuth pentoxide_
is obtained by the prolonged passage of chlorine through a boiling
solution of potassium hydroxide (sp. gr. 1·38), containing bismuth
oxide in suspension; the precipitate is washed with water, with
boiling nitric acid (but not for long, as otherwise the bismuthic
acid is decomposed), then again with water, and finally the
resultant bright red powder of the hydrate BiHO_{3} is dried at
125°. The prolonged action of nitric acid on bismuthic anhydride,
Bi_{2}O_{5}, results in the formation of the compound
Bi_{2}O_{4},H_{2}O, which decomposes in moist air, forming
Bi_{2}O_{3}. The density of bismuthic anhydride is 5·10, of the
tetroxide, Bi_{2}O_{4}, 3·60, and of bismuthic acid, BiHO_{3},
5·75. _Pyrobismuthic acid_, Bi_{2}H_{4}O_{7}, forms a brown
powder, which loses a portion of its water at 150°, and decomposes
on further heating, with the evolution of oxygen and water. It is
obtained by the action of potassium cyanide on a solution of
bismuth nitrate. The meta-salts of bismuthic acid are known, for
example KBiO_{3}. They generally occur, however, in combinations
with metabismuthic acid itself. Thus André (1891) took a solution
of the double salt of BiBr_{3} and KBr, treated it with bromine
after adding ammonia, and obtained a red-brown precipitate, which
after being washed (for several weeks) had the composition
KBiO_{3},HBiO_{3} When washed with dilute nitric acid this salt
gave bismuthic acid.

In nature, bismuth occurs in only a few localities and in small quantities, most frequently in a native state, and more rarely as oxide and as a compound of bismuth sulphide with the sulphides of other metals, and sometimes in gold ores. It is extracted from its native ores by simple fusion in the furnace shown in fig. 85. This furnace contains an inclined iron retort, into the upper extremity of which the ore is charged, and the molten _metal_ flows from the lower extremity. It is refined by re-melting, and the pure metal may be obtained by dissolving in nitric acid, decomposing the resultant salt with water, and reducing the precipitate by heating it with charcoal. Bismuth is a metal which crystallises very well from a molten state. Its specific gravity is 9·8; it melts at 269°, and if it be melted in a crucible, allowed to cool slowly, and the crust broken and the remaining molten liquid poured out, perfect rhombohedral crystals of bismuth are obtained on the sides of the crucible.[44 bis] It is brittle, has a grey-coloured fracture with a reddish lustre, is not hard, and is but very slightly ductile and malleable; it volatilises at a white heat and easily oxidises. It recalls antimony and lead in many of its properties. When oxidised in air, or when the nitrate is ignited, bismuth forms the _oxide_, Bi_{2}O_{3}, as a white powder which fuses when heated and resembles massicot. The addition of an excess of caustic potash to a solution of a bismuthous salt gives a white precipitate of the hydroxide, BiO(OH), which loses its water and gives the anhydrous oxide when boiled with a solution of caustic potash. Both the hydroxide and oxide easily dissolve in acids and form bismuthous salts.

[44 bis] Hérard (1889) obtained a peculiar variety of bismuth by
heating pure crystalline bismuth to a bright red heat in a stream
of nitrogen. A greenish vapour was deposited in the cooler
portions of the apparatus in the form of a grey powder, which
under the microscope had the appearance of minute globules. An
atmosphere of nitrogen is necessary for this transformation, other
gases such as hydrogen and carbonic oxide do not favour the
transition. The resultant amorphous bismuth fuses at 410° (the
crystalline variety at 269°), sp. gr. 9·483. (Does it not contain
a nitride?)

_Bismuthous oxide_, Bi_{2}O_{3}, is a feeble and unenergetic base. The normal hydroxide of the oxide Bi_{2}O_{3} is Bi(OH)_{3}; it parts with water and forms a metahydroxide (bismuthyl hydroxide), BiO(OH). Both of these hydroxides have their corresponding saline compounds of the composition BiX_{3} and BiOX. And the form BiOX is nothing else but the type of the basic salt, because 3ROX = RX + R_{2}O_{3}. It is evident that in the type BiX_{3} the bismuth replaces three atoms of hydrogen. And indeed with phosphoric acid solutions of the bismuthous salts give a precipitate of the composition BiPO_{4}. On the other hand, in the form of compounds BiOX or Bi(OH)_{2}X, the univalent group (BiO) or (BiH_{2}O_{2}) is combined with X. Many bismuth salts are formed according to the type BiOX. For instance the carbonate, (BiO)_{2}CO_{3}, which corresponds with the other carbonates M_{2}CO_{3}. It is obtained as a white precipitate when a solution of sodium carbonate is added to a solution of a bismuth salt.[45] The compound radicle BiO is not a special natural grouping, as it was formerly represented to be; it is simply a mode of expression for showing the relation between the compound in question and the compounds of other oxides.

[45] Basic bismuth carbonate is employed for whitening the skin
(veloutine, &c.)

Three _salts of nitric acid_ are known containing bismuthous oxide. If metallic bismuth or its oxide be dissolved in nitric acid, it forms a colourless transparent solution containing a salt which separates in large transparent crystals containing Bi(NO_{3})_{3},5H_{2}O. When heated at 80° these crystals melt in their water of crystallisation, and in so doing lose a portion of their nitric acid together with water, forming a salt whose empirical formula is Bi_{2}N_{2}H_{2}O_{9}. If the preceding salt belongs to the type BiX_{3}, this one should belong to the form BiOX, because it = BiO(NO_{3}) + Bi(H_{2}O_{2})(NO_{3}). This salt may be heated to 150° without change. When the first colourless crystalline salt dissolves in water _it is decomposed_. There is no decomposition if an excess of acid be added to the water--that is to say, the salt is able to exist in an acid solution without decomposing, without separation of the so-called basic salt--but by itself it cannot be kept in solution; water decomposes this salt, acting on it like an alkali. In other words the basic properties of bismuthic oxide are so feeble that even water acts by taking up a portion of the acid from it. Here we see one of the most striking facts, long since observed, confirming that action of water on salts about which we have spoken in Chapter X. and elsewhere. This action on water may be expressed thus:--BiX_{3} + 2H_{2}O = Bi(OH)_{2}X + 2XH. A salt of the type Bi(OH)_{2}X is obtained in the precipitate. But if the quantity of acid, HX, be increased, the salt BiX_{3} is again formed and passes into solution. The quantity of the salt BiOX which passes into solution on the addition of a given quantity of acid depends indisputably on the amount (mass) of water (Muir). The solution, which is perfectly transparent with a small amount of water, becomes cloudy and deposits the salt of the type BiOX, when diluted. The white flaky precipitate of Bi(OH)_{2}NO_{3} formed from the normal salt Bi(NO_{3})_{3} by mixing it with five parts of water, and in general with a small amount of water, is used in medicine under the name of magistery of bismuth.[46]

[46] With an excess of water a further quantity of acid is separated
and a still more basic salt formed. The ultimate product, on which
an excess of water has apparently no action whatever, is a
substance having the composition BiO(NO_{3}).BiO(OH). In the
latter salt we see the limit of change, and this limit appears to
show that the type of the saline compounds of bismuthic oxide is
of the form Bi_{2}X_{6}, and not BiX_{3}; but it is very probable,
on the basis of the examples which we considered in the case of
lead, that this type should be still further polymerised in order
to give a correct idea of the type of the bismuthous compounds. If
we refer all the bismuthous compounds to this type, Bi_{2}X_{6},
we shall obtain the following expression for the composition of
the nitrates: normal salt, Bi_{2}(NO_{3})_{6}, first basic salt,
Bi_{2}O(OH)_{2}(NO_{3})_{2}, magistery of bismuth,
Bi_{2}(OH)_{4}(NO_{3})_{2}, and the limiting form
Bi_{2}O_{2}(OH)(NO_{3}).

The general character of bismuthous oxide in its compounds is well
exemplified in the nitrate; bismuthous chloride, BiCl_{3}, which
is obtained by heating bismuth in chlorine, or by dissolving it in
aqua regia, and then distilling without access of air, is also
decomposed by water in exactly the same manner, and forms basic
salts--for instance, first, BiOCl, like the above salt of nitric
acid. Bismuth chloride boils at 447° and probably its formula is
BiCl_{3}. Polymerisation may take place in some compounds and not
in others. A volatile compound of the composition
Bi(C_{2}H_{5})_{3} is also known as a liquid which is insoluble in
water and decomposes with explosion when heated at 130°. Double
salts containing chloride of bismuth are: 2(KCl)BiCl_{3}2H_{2}O
(from a solution of Bi_{2}O_{3} and KCl in hydrochloric acid) and
KClBiCl_{3}H_{2}O. Bigham (1892) also obtained KBr(SO_{4})_{2} in
tabular crystals by treating the above-named double salt with
strong sulphuric acid. The composition of this salt recalls that
of alum.

Metallic bismuth is used in the preparation of fusible alloys. The addition of bismuth to many metals renders them very hard, and at the same time generally lowers their melting point to a considerable extent. Thus Wood's metal, which contains one part of cadmium, one part of tin, two parts of lead, and four parts of bismuth, fuses at about 60°, and in general many alloys composed of bismuth, tin, lead, and antimony melt below or about the boiling point of water.[47]

[47] As the metals contained in alloys like the above (bismuth, lead,
tin, cadmium) are difficultly volatile and their alloys are
fusible, they may be employed in the place of mercury in many
physical experiments conducted at or above 70°, and they offer the
advantage that they do not give any vapour having an appreciable
tension (mercury at 100°, 0·75 mm.) Bismuth expands in passing
into a molten state, but it has a temperature of maximum density.
According to Luedeking the mean coefficient of expansion of liquid
bismuth is 0·0000442 (between 270° and 303°), and of solid bismuth
0·0000411.

Just as in group II., side by side with the elements zinc, cadmium, and mercury in the uneven series, we found calcium, strontium, and barium in the even series; and as in group IV., parallel to silicon, germanium, tin, and lead, we noticed thallium, zirconium, cerium, and thorium; so also in group V. we find, beside those elements of the uneven series just considered by us, a series of analogues in the even series, which, with a certain degree of similarity (mainly quantitative, or relative to the atomic weights), also present a series of particular (qualitative) independent points of distinction. In the even series are known _vanadium_, which stands between titanium and chromium, _niobium_, between zirconium and molybdenum, and _tantalum_, situated near tungsten (an element of group VI. like chromium and molybdenum). Just as bismuth is similar in many respects to its neighbour lead, so also do these neighbouring elements resemble each other, even in their external appearance, not to mention the quality of their compounds, naturally taking into account the differences of type corresponding with the different groups. The occurrence in group V. determines the type of the oxides, R_{2}O_{3} and R_{2}O_{5}, and the development of an acid character in the higher oxides. The occurrence in the even series determines the absence of volatile compounds, RH_{3}, for these metals, and a more basic character of the oxides of a given composition than in the uneven series, &c.[48] Vanadium, niobium, and tantalum belong to the category of rare metals, and are exceedingly difficult to obtain pure, more especially owing to their similarity to, and occurrence with, chromium, tungsten and other metals, and also in combination among themselves; therefore it is natural that they have been far from completely studied, although since 1860 chemists have devoted not a little time to their investigation. The researches carried out by Marignac, at Geneva, on niobium, and by Sir Henry Roscoe, at Manchester, on vanadium deserve special attention. The undoubted external resemblance of the compounds of chromium and vanadium, as well as the want of completeness in the knowledge of the compounds of vanadium, long caused its oxides to be considered analogous in atomic composition to those formed by chromium. The higher oxide of vanadium was therefore supposed to have the formula VO_{3}. But the fact of the matter is, that the chemical analogy of the elements does not hold in one direction only; vanadium is at one and the same time the analogue of chromium, and consequently of the elements like sulphur of group VI, and also the analogue of phosphorus, arsenic, and antimony; just as bismuth stands in respect to lead and antimony. Investigation has shown that the compounds of vanadium are always accompanied by those of phosphorus as well as of iron, and that it is even more difficult to separate it from the compounds of phosphorus than from those of iron and tungsten. We should have to extend our description considerably if we wished to give the complete history, even of vanadium alone, not to mention niobium and tantalum, all the more as questions would not unfrequently arise concerning the compounds of these elements which have not yet been fully elucidated. We shall therefore limit ourselves to pointing out the most important features in the history of these elements, the more so since the minerals themselves in which they occur are exceedingly rare and only accessible to a few investigators.

[48] Although, guided by Brauner, who showed that didymium gives a
higher oxide, Di_{2}O_{5}, I place this element in the fifth
group, still I am not certain as to its position, because I
consider that the questions relating to this metal are still far
from being definitely answered.

An important point in the history of the members of this group is the circumstance that they form volatile compounds with chlorine, similar to the compounds of the elements of the phosphorus group, namely, of the type RX_{5}. The vapour densities of the compounds of this kind were determined, and served as the most important basis for the explanation of the atomic composition of these molecules. In this we see the power of general and fundamental laws, like the law of Avogadro-Gerhardt. An oxychloride, VOCl_{3}, is known for vanadium, which is the perfect analogue of phosphorus oxychloride. It was formerly considered to be vanadium chloride, for just as in the case of uranium (Chapter XXI.), its lower oxide, VO, was considered to be the metal, because it is exceedingly difficultly reduced--even potassium does not remove all the oxygen, besides which it has a metallic appearance, and decomposes acids like a metal; in a word, it simulates a metal in every respect. _Vanadium oxychloride_ is obtained by heating the trioxide, V_{2}O_{3}, mixed with charcoal, in a current of hydrogen; the lower oxide of vanadium is then formed, and this, when heated in a current of dry chlorine, gives the oxychloride VOCl_{3} as a reddish liquid which does not act on sodium and may be purified by distillation over this metal. It fumes in the air, giving reddish vapours; it reacts on water, forming hydrochloric and vanadic acids; hence, on the one hand it is very similar to phosphorus oxychloride, and on the other hand to chromium oxychloride, CrO_{2}Cl_{2} (Chapter XXI.). It is of a yellow colour, its specific gravity is 1·83, it boils at 120°, and its vapour density is 86 with respect to hydrogen; therefore the above formula expresses its molecular weight.[49]

[49] When the vapours of vanadium oxychloride are heated with zinc in a
closed tube at 400°, they lose a portion of their chlorine and
form a green crystalline mass of sp. gr. 2·88, which is
deliquescent in air and has the composition VOCl_{2}. Only its
vapour density is unknown, and it would be extremely important to
determine whether its molecular composition is that given above,
or whether it corresponds with the formula V_{2}O_{2}Cl_{4}.
Another less volatile oxychloride, VOCl, is formed with it as a
brown insoluble substance, which is, however, soluble in nitric
acid like the preceding. Roscoe obtained a still less chlorinated
substance, namely, (VO)_{2}Cl; but it may only consist of a
mixture of VO and VOCl. At all events, we here find a graduated
series such as is met with in the compounds of very few other
elements.

_Vanadic anhydride_, V_{2}O_{5}, is obtained either in small quantities from certain clays where it accompanies the oxides of iron (hence some sorts of iron contain vanadium) and phosphoric acid, or from the rare minerals: _volborthite_, CuHVO_{4}, or basic vanadate of copper; _vanadinite_, PbCl_{2}3Pb_{3}(VO_{4})_{2}; lead vanadate, Pb_{3}(VO_{4})_{2}, &c. The latter salts are carefully ignited for some time with one-third of their weight of nitre; the fused mass thus formed is powdered and boiled in water: the yellow solution obtained contains potassium vanadate. The solution is neutralised with acid, and barium chloride added; a meta-salt, Ba(VO_{3})_{2}, is then precipitated as an almost insoluble white powder, which gives a solution of vanadic acid when boiled with sulphuric acid. (The precipitate is at first yellow, as long as it remains amorphous, but it afterwards becomes crystalline and white.) The solution thus obtained is neutralised with ammonia, which thus forms ammonium (meta) vanadate, NH_{4}VO_{3}, which, when evaporated, gives colourless crystals, insoluble in water containing sal-ammoniac; hence this salt is precipitated by adding solid sal-ammoniac to the solution. Ammonium vanadate, when ignited, leaves vanadic acid behind. In this it differs from the corresponding chromium salt, which is deoxidised into chromium oxide when ignited. In general, vanadic acid has but a small oxidising action. It is reduced with difficulty, like phosphoric or sulphuric acid, and in this differs from arsenic and chromic acids. Vanadic acid, like chromic acid, separates from its solution as the anhydride V_{2}O_{5}, and not in a hydrous state. Vanadic anhydride, V_{2}O_{5}, forms a reddish-brown mass, which easily fuses and re-solidifies into transparent crystals having a violet lustre (another point of resemblance to chromic acid); it dissolves in water, forming a yellow solution with a slightly acid reaction.[50]

[50] Strong acids and alkalis dissolve vanadic anhydride in
considerable quantities, forming yellow solutions. When it is
ignited, especially in a current of hydrogen, it evolves oxygen
and forms the lower oxides; V_{2}O_{4} (acid solutions of a green
colour, like the salts of chromic oxide), V_{2}O_{3}, and the
lowest oxide, VO. The latter is the metallic powder which is
obtained when the vanadium oxychloride is heated in an excess of
hydrogen, and was formerly mistaken for metallic vanadium. When a
solution of vanadic acid is treated with metallic zinc it forms a
blue solution, which seems to contain this oxide. It acts as a
reducing agent (and forms a close analogue to chromous oxide,
CrO). Metallic _vanadium_ can only be obtained from vanadium
chloride which is quite free from oxygen. Moissan (1893) obtained
it by reducing the oxide with carbon in the electric furnace, and
considered it to be most infusible of the metals in the series Pt,
Cr, Mo, U, W, and V (he also obtained a compound of vanadium and
carbon). The specific gravity of this metal is 5·5. It is of a
grey-white colour, is not decomposed by water, and is not oxidised
in air, but burns when strongly heated, and can be fused in a
current of hydrogen (forming perhaps a compound with hydrogen). It
is insoluble in hydrochloric acid, but easily dissolves in nitric
acid, and when fused with caustic soda it forms sodium vanadate.

As regards the salts of vanadic acid, three different classes are
known; the first correspond with metavanadic acid, VMO_{3} =
M_{2}OV_{2}O_{5}, the second correspond with the dichromates--that
is, have the composition V_{4}M_{2}O_{11}, which is equal to
M_{2}O + 2V_{2}O_{5}--and the third correspond with orthovanadic
acid, VM_{3}O_{4} or 3M_{2}O + V_{2}O_{5}. The latter are formed
when vanadic anhydride is fused with an excess of an alkaline
carbonate.

Vanadic acid gives the so-called 'complex' acids (which are
considered more fully in Chapter XXI. in speaking of Mo and
W)--_i.e._ acids formed of two acids assimilated into one. Thus
Friedheim (1890) obtained phosphor-vanadic acid, and
Schmitz-Dumont (1890) a similar arseno-vanadic acid. The former is
obtained by heating V_{2}O_{5} with sirupy phosphoric acid. The
resultant golden-yellow tabular crystals have the composition
H_{2}OV_{2}O_{5}P_{2}O_{5}9H_{2}O, and there are corresponding
salts--for example, (NH_{4})_{2}V_{2}O_{5}P_{2}O_{5} with 3 and
7H_{2}O, &c. These salts cannot be separated by crystallisation,
so that there are 'complexes' of these acids in a whole series of
salts (and also in nature). It may be supposed (Friedheim) that
V_{2}O_{5} here, as it were, plays the part of a base, or that
those acids may be looked upon as double salts. Among the true
double salts of vanadium (Nb and Ta) very many are known among the
fluorides, such as VF_{3}2NH_{4}F, VOF_{2}2NH_{4}F,
VO_{2}F,3NH_{4}F, &c. (Pettersson, Piccini, and Georgi, 1890-92).

Vanadium was discovered at the beginning of this century by
Del-Rio, and afterwards investigated by Sefström, but it was only
in 1868 that Roscoe established the above formulæ of the vanadic
compounds.

_Niobium and tantalum_[51] occur as acids in rare minerals, and are mainly extracted from _tantalite_ and _columbite_, which are found in Bavaria, Finland, North America, and in the Urals. These minerals are composed of the ferrous salts of niobic and tantalic acids; they contain about 15 per cent. of ferrous oxide in isomorphous mixture with manganous oxide, in combination with various proportions of tantalic and niobic anhydrides. These minerals are first fused with a considerable amount of potassium bisulphate, and the fused mass is boiled in water, which dissolves the ferrous and potassium salts and leaves an insoluble residue of impure niobic and tantalic acids. This raw product is then treated with ammonium sulphide, in order to extract the tin and tungsten, which pass into solution. The residue containing the acids (according to Marignac) is then treated with hydrofluoric acid, in which it entirely dissolves, and potassium fluoride is added to the resultant hot solution; on cooling, a sparingly soluble double fluoride of potassium and tantalum separates out in fine crystals, while the much more soluble niobium salt remains in solution. The difference in the solubility of these double salts in water acidified with hydrofluoric acid (in pure water the solution becomes cloudy after a certain time) is so great that the tantalum compound requires 150 parts of water for its solution, and the niobium compound only 13 parts. The Greenland columbite (specific gravity 5·36) only contains niobic acid, and that from Bodenmais, Bavaria (specific gravity 6·06) almost equal quantities of tantalic and niobic acids. Having isolated tantalic and niobic salts, Marignac found that the relation between the potassium and fluorine in them is very variable--that is, that there exist various double salts of fluoride of potassium, and of the fluorides of the metals of this group, but that with an excess of hydrofluoric acid both the tantalum and niobium compounds contain seven atoms of fluorine to two of potassium, whence it must be concluded that the simplest formula for these double salts will be K_{2}RF_{7} = RF_{5},2KF; that is, that the type of the higher compounds of niobium and tantalum is RX_{5}, and hence is similar to phosphoric acid. A chloride, TaCl_{5}, may be obtained from pure tantalic acid by heating it with charcoal in a current of chlorine. This is a yellow crystalline substance, which melts at 211°, and boils at 241°; its vapour density with respect to hydrogen is 180, as would follow from the formula TaCl_{5}. It is completely decomposed by water into tantalic and hydrochloric acids. _Niobium pentachloride_ may be prepared in the same manner; it fuses at 194°, and boils at 240°. When treated with water this substance gives a solution containing niobic acid, which only separates out on boiling the solution. Delafontaine and Deville found its vapour density to be 9·3 (air = 1), as is shown by its formula NbCl_{5}.[52]

[51] The researches made by Roscoe were preceded by those of Marignac
in 1865, on the _compounds_ of _niobium_ and _tantalum_, to which
were also ascribed different formulæ from those now recognised.
Tantalum was discovered simultaneously with vanadium by Hatchett
and Ekeberg, and was afterwards studied by Rose, who in 1844
discovered niobium in it. Notwithstanding the numerous researches
of Hermann (in Moscow), Kobell, Rose, and Marignac, still there is
not yet any certainty as to the purity of, and the properties
ascribed to, the compounds of these elements. They are difficult
to separate from each other, and especially from the cerite metals
and titanium, &c., which accompany them. Before the investigations
of Rose the highest oxide of tantalum was supposed to belong to
the type TaX_{6}--that is, its composition was taken as TaO_{3},
and to the lower oxide was ascribed a formula TaO_{2}. Rose gave
the formula TaO_{2} to the higher oxide, and discovered a new
element called niobium in the substance previously supposed to be
the lower oxide. He even admitted the existence of a third element
occurring together with tantalum and niobium, which he named
pelopium, but he afterwards found that pelopic acid was only
another oxide of niobium, and he considered it probable that the
higher oxide of this element is NbO_{2}, and the lower
Nb_{2}O_{3}. Hermann found that niobic acid which was considered
pure contained a considerable quantity of tantalic acid, and
besides this he admitted the existence of another special metallic
acid, which he called ilmenic acid, after the locality (the Ilmen
mountains of the Urals) of the mineral from which he obtained it.
V. Kobell recognised still another acid, which he called dianic
acid, and these diverse statements were only brought into
agreement in the sixties by Marignac. He first of all indicated an
accurate method for the separation of tantalic and niobic
compounds, which are always obtained in admixture.

[52] If niobic acid be mixed with a small quantity of charcoal and
ignited in a stream of chlorine, a difficultly-fusible and
difficultly-volatile oxychloride, NbOCl_{3} separates. The vapour
density of this compound with respect to air is 7·5, and this
vapour density perfectly confirms the accuracy of the formulæ
given by Marignac, and indicates the quantitative analogy between
the compounds of niobium and tantalum, and those of phosphorus and
arsenic, and consequently also of vanadium. In their qualitative
relations (as is evident also from the correspondence of the
atomic weights), the compounds of tantalum and niobium exhibit a
great analogy with the compounds of molybdenum and tungsten. Thus
zinc, when acting on acid solutions of tantalic and niobic
compounds, gives a blue coloration, exactly as it does with those
of tungsten and molybdenum (also titanium). These acids form the
same large number of salts as those of tungsten and molybdenum.
The anhydrides of the acids are also insoluble in water, but as
colloids are sometimes held in solution, just like those of
titanic and molybdic acids. Furthermore, niobium is in every
respect the nearest analogue of molybdenum, and tantalum of
tungsten. _Niobium_ is obtained by reducing the double fluoride of
niobium and sodium, with sodium. It is difficult to obtain in a
pure state. It is a metal on which hydrochloric acid acts with
some energy, as also does hydrofluoric acid mixed with nitric
acid, and also a boiling solution of caustic potash. _Tantalum_,
which is obtained in exactly the same way, is a much heavier
metal. It is infusible, and is only acted on by a mixture of
hydrofluoric and nitric acids. Rose in 1868 showed that in the
reduction of the double fluoride, NbF_{5},2KF, by sodium, a
greyish powder is obtained after treating with water. The specific
gravity of this powder is 6·8, and he considers it to be niobium
hydride, NbH. Neither did he obtain metallic niobium when he
reduced with magnesium and aluminium, but an alloy, Al_{3}Nb,
having a sp. gr. of 4·5.

Niobium, so far as is known, unites in three proportions with
oxygen. NbO, which is formed when NbOF_{3},2KF is reduced by
sodium; NbO_{2}, which is formed by igniting niobic acid in a
stream of hydrogen, and niobic anhydride, Nb_{2}O_{5}, a white
infusible substance, which is insoluble in acids, and has a
specific gravity of 4·5. Tantalic anhydride closely resembles
niobic anhydride, and has a specific gravity of 7·2. _The
tantalates and niobates_ present the type of ortho-salts--for
example, Na_{2}HNbO_{4},6H_{2}O, and also of pyro-salts, such as
K_{3}HNb_{2}O_{7},6H_{2}O, and of meta-salts--for example,
KNbO_{3},2H_{2}O. And, besides these, they give salts of a more
complex type, containing a larger amount of the elements of the
anhydride; thus, for instance, when niobic anhydride is fused with
caustic potash it forms a salt which is soluble in water, and
crystallises in monoclinic prisms, having the composition
K_{8}Nb_{6}O_{19},16H_{2}O. There is a perfectly similar
isomorphous salt of tantalic acid. Tantalite is a salt of the type
of metatantalic acid, Fe(TaO_{3})_{2}. The composition of
Yttrotantalite appears to correspond with orthotantalic acid.