cerium. As the above-mentioned elements are rather rare in nature, have
but little practical application, and do not present any new forms of
combination, it is unadvisable to dwell on them in this treatise.
_Titanium_ is found in nature in the form of its anhydride or oxide,
TiO_{2}, mixed with silicon in many minerals, but the oxide is also found
separately in the form of semi-metallic _rutile_ (sp. gr. 4·2). Another
titanic mineral is found as a mixture in other ores, known as _titanic
iron ore_ (in the Thuensky mountains of the southern Ural; it is known as
_thuenite_), FeTiO_{3}. This is a salt of ferrous oxide and titanic
anhydride. It crystallises in the rhombohedric system, has a metallic
lustre, grey colour, sp. gr. 4·5. The third mineral in which titanium is
found in considerable quantities in nature is _sphene_ or _titanite_,
CaTiSiO_{5} = CaO,SiO_{2},TiO_{2}, sp. gr. 3·5, colour yellow, green, or
the like, crystallises in tablets. The fourth, but rare, titanic mineral
is _peroffskite_, calcium titanate, CaTiO_{3}; it forms blackish-grey or
brown cubic crystals, sp. gr. 4·02, and occurs in the Ural and other
localities. It may be prepared artificially by fusing sphene in an
atmosphere of water vapour and carbonic anhydride. At the end of the last
century Klaproth showed the distinction between titanic compounds and all
others then known.[57]
[57] The compounds of titanium are generally obtained from rutile; the
finely-ground ore is fused with a considerable amount of acid
potassium sulphate, until the titanic anhydride, as a feeble base,
passes into solution. After cooling, the resultant mass is ground
up, dissolved in cold water, and treated with ammonium
hydrosulphide; a black precipitate then separates out from the
solution. This precipitate contains TiO_{2} (as hydrate) and
various metallic sulphides--for example, iron sulphide. It is
first washed with water and then with a solution of sulphurous
anhydride until it becomes colourless. This is due to the iron
sulphide contained in the precipitate, and rendering it black,
being converted into dithionate by the action of the sulphurous
acid. The titanic acid left behind is nearly pure. The
considerable volatility of titanium chloride may also be taken
advantage of in preparing the compounds of titanium from rutile.
It is formed by heating a mixture of rutile and charcoal in dry
chlorine; the distillate then contains _titanium chloride_,
TiCl_{4}. It may be easily purified, owing to its having a
constant boiling point of 136°. Its specific gravity is 1·76; it
is a colourless liquid, which fumes in the air, and is perfectly
soluble in water if it be not heated. When hot water acts on
titanic chloride, a large proportion of titanic acid separates out
from the solution and passes into metatitanic acid. A similar
decomposition of acid solutions of titanic acid is accomplished
whenever they are heated, and especially in the presence of
sulphuric acid, just as with metastannic acid, which titanic acid
resembles in many respects. On igniting the titanic acid a
colourless powder of the anhydride, TiO_{2}, is obtained. In this
form it is no longer soluble in acids or alkalis, and only fuses
in the oxyhydrogen flame; but, like silica, it dissolves when
fused with alkalis and their carbonates; as already mentioned, it
dissolves when fused with a considerable excess of acid potassium
sulphate--that is, it then reacts as a feeble base. This shows the
basic character of titanic anhydride; it has at once, although
feebly developed, both basic and acid properties. The fused mass,
obtained from titanic anhydride and alkali when treated with
water, parts with its alkali, and a residue is obtained of a
sparingly-soluble poly-titanate, K_{2}TiO_{3}_n_TiO_{2}. The
hydrate, which is precipitated by ammonia from the solutions
obtained by the fusion of TiO_{2} with acid potassium sulphate,
when dried forms an amorphous mass of the composition Ti(OH)_{4}.
But it loses water over sulphuric acid, gradually passing into a
hydrate of the composition TiO(OH)_{2}, and when heated it parts
with a still larger proportion of water; at 100° the hydrate
Ti_{2}O_{3}(OH)_{2} is obtained, and at 300° the anhydride itself.
The higher hydrate, Ti(OH)_{4}, is soluble in dilute acid, and the
solution may be diluted with water; but on boiling the sulphuric
acid solution (though not the solution in hydrochloric acid), all
the titanic acid separates in a modified form, which is, however,
not only insoluble in dilute acids, but even in strong sulphuric
acid. This hydrate has the composition Ti_{2}O_{3}(OH)_{2}, but
shows different properties from those of the hydrate of the same
composition described above, and therefore this modified hydrate
is called _metatitanic acid_. It is most important to note the
property of the ordinary gelatinous hydrate (that precipitated
from acid solutions by ammonia) of dissolving in acids, the more
so since silica does not show this property. In this property a
transition apparently appears between the cases of common solution
(based on a capacity for unstable combination) and the case of the
formation of a hydrosol (the solubility of germanium oxide,
GeO_{2}, perhaps presents another such instance). If titanium
chloride be added drop by drop to a dilute solution of alcohol and
hydrogen peroxide, and then ammonia be added to the resultant
solution, a yellow precipitate of _titanium trioxide_,
TiO_{3}H_{2}O, separates out, as Piccini, Weller, and Classen
showed. This substance apparently belongs to the category of true
peroxides.
Titanium chloride absorbs ammonia and forms a compound,
TiCl_{4},4NH_{3}, as a red-brown powder which attracts moisture
from the air and when ignited forms _titanium nitride_,
Ti_{3}N_{4}. Phosphuretted hydrogen, hydrocyanic acid, and many
similar compounds are also absorbed by titanium chloride, with the
evolution of a considerable amount of heat. Thus, for example, a
yellow crystalline powder of the composition TiCl_{4},2HCN is
obtained by passing dry hydrocyanic acid vapour into cold titanium
chloride. Titanium chloride combines in a similar manner with
cyanogen chloride, phosphorus pentachloride, and phosphorus
oxychloride, forming molecular compounds, for example
TiCl_{4},POCl_{3}. This faculty for further combination probably
stands in connection, on the one hand, with the capacity of
titanium oxide to give polytitanates, TiO(MO)_{2},_n_TiO_{2}; on
the other hand, it corresponds with the kindred faculty of stannic
chloride for the formation of poly-compounds (Note 41), and lastly
it is probably related to the remarkable behaviour of titanium
towards nitrogen. Metallic titanium, obtained as a grey powder by
reducing potassium titanofluoride, K_{2}TiF_{6}, (sp. gr. 3·55 K.
Hofman 1893), with iron in a charcoal crucible, combines directly
with nitrogen at a red heat. If titanic anhydride be ignited in a
stream of ammonia, all the oxygen of the titanic oxide is
disengaged, and the compound TiN_{2} is formed as a dark violet
substance having a copper-red lustre. A compound Ti_{5}N_{6} is
also known; it is obtained by igniting the compound Ti_{3}N_{4} in
a stream of hydrogen, and is of a golden-yellow colour with a
metallic lustre. To this order of compounds also belongs the
well-known and chemically historical compound known as _titanium
nitrocyanide_; its composition is Ti_{5}CN_{4}. This substance
appears as infusible, sometimes well-formed, cubical crystals of
sp. gr, 4·3, and having a red copper colour and metallic lustre;
it is found in blast furnace slag. It is insoluble in acids but is
acted on by chlorine at a red heat, forming titanium chloride. It
was at first regarded as metallic titanium; it is formed in the
blast furnace at the expense of those cyanogen compounds
(potassium cyanide and others) which are always present, and at
the expense of the titanium compounds which accompany the ores of
iron. Wöhler, who investigated this compound, obtained it
artificially by heating a mixture of titanic oxide with a small
quantity of charcoal, in a stream of nitrogen, and thus proved the
direct power for combination between nitrogen and titanium. When
fused with caustic potash, all the nitrogen compounds of titanium
evolve ammonia and form potassium titanate. Like metals they are
able to reduce many oxides--for example, oxides of copper--at a
red heat. Among the alloys of titanium, the crystalline compound
Al_{4}Ti is remarkable. It is obtained by directly dissolving
titanium in fused aluminium; its specific gravity is 3·11. The
crystals are very stable, and are only soluble in aqua regia and
alkalis.
The comparatively rare element _zirconium_, Zr = 90, is very similar to
titanium, but has a more basic character. It is rarer in nature than
titanium, and is found principally in a mineral called _zircon_,
ZrSiO_{4} = ZrO_{2}.SiO_{2}, crystallising in square prisms, sp. gr. 4·5.
It has considerable hardness and a characteristic brownish-yellow colour,
and is occasionally found in the form of transparent crystals, as a
precious stone called hyacinth.[58] Metallic zirconium was obtained, by
Berzelius and Troost, by the action of aluminium on potassium
zirconofluoride in the same way that silicon is prepared; it forms a
crystalline powder, similar in appearance to graphite and antimony, but
having a very considerable hardness, not much lustre, sp. gr. 4·15. In
many respects it resembles silicon; it does not fuse when heated, and
even oxidises with difficulty, but liberates hydrogen when fused with
potash. When fused with silica it liberates silicon. With carbon in the
electrical furnace it forms ZrC_{2}, with hydrogen it gives ZrH_{2} (like
CaH_{2}, Winkler, Vol. I., p. 621); hydrochloric and nitric acids act
feebly on it, but aqua regia easily dissolves it. It is distinguished
from silicon by the fact that hydrofluoric acid acts on it with great
facility, even in the cold and when diluted, whilst this acid does not
act on silicon at all.
[58] The formula ZrO was first given to the oxide of zirconium as a
base, in this case Zr = 45 whilst the present atomic weight is Zr
= 90--that is, the formula of the oxide is now recognised as being
ZrO_{2}. The reasons for ascribing this formula to the compounds
of zirconium are as follows. In the first place, the investigation
of the crystalline forms of the zirconofluorides--for example,
K_{2}ZrF_{6}, MgZrF_{6},5H_{2}O--which proved to be analogous in
composition and crystalline form with the corresponding compounds
of titanium, tin, and silicon. In the second place, the specific
heat of Zr is 0·067, which corresponds with the combining weight
90. The third and most important reason for doubling the combining
weight of zirconium was given by Deville's determination of the
vapour density of _zirconium chloride_, ZrCl_{4}. This substance
is obtained by igniting zirconium oxide mixed with charcoal in a
stream of dry chlorine, and is a colourless, saline substance
which is easily volatile at 440°. Its density referred to air was
found to be 8·15, that is 117 in relation to hydrogen, as it
should be according to the molecular formula of this substance
above-cited. It exhibits, however, in many respects, a saline
character and that of an acid chloranhydride, for zirconium oxide
itself presents very feebly developed acid properties but clearly
marked basic properties. Thus zirconium chloride dissolves in
water, and on evaporation the solution only partially disengages
hydrochloric acid--resembling magnesium chloride, for example.
Zirconium was discovered and characterised as an individual
element by Klaproth.
Pure compounds of zirconium are generally prepared from zircon,
which is finely ground, but as it is very hard it is first heated
and thrown into cold water, by which means it is disintegrated.
Zircon is decomposed or dissolved when fused with acid potassium
sulphate, or still more easily when fused with acid potassium
fluoride (a double soluble salt, K_{2}ZrF_{6}, is then formed);
however, zirconium compounds are generally prepared from powdered
zircon by fusing it with sodium carbonate and then boiling in
water. An insoluble white residue is obtained consisting of a
compound of the oxides of sodium and zirconium, which is then
treated with hydrochloric acid and the solution evaporated to
dryness. The silica is thus converted into an insoluble form, and
zirconium chloride obtained in solution. Ammonia precipitates
_zirconium hydroxide_ from this solution, as a white gelatinous
precipitate, ZrO(OH)_{2}. When ignited this hydroxide loses water
and in so doing undergoes a spontaneous recalescence and leaves a
white infusible and exceedingly hard mass of _zirconium oxide_,
ZrO_{2}, having a specific gravity of 5·4 (in the electrical
furnace ZrO_{2} fuses and volatilises like SiO_{2}, Moissan).
Owing to its infusibility, zirconium oxide is used as a substitute
for lime and magnesia in the Drummond light. This oxide, in
contradistinction to titanium oxide, is soluble, even after
prolonged ignition, in hot strong sulphuric acid. The hydroxide is
easily soluble in acids. The composition of the salts is ZrX_{4},
or ZrOX_{2}, or ZrOX_{2},ZrO_{2}, just as with those of its
analogues. But although zirconium oxide forms salts in the same
way with acids, it also gives salts with bases. Thus it liberates
carbonic anhydride when fused with sodium carbonate, forming the
salts Zr(NaO)_{4}, ZrO(NaO)_{2}, &c. Water, however, destroys
these salts and extracts the soda.
The very similar element _thorium_ (Th = 232) was distinguished by
Berzelius from zirconium. It is very rarely met with, in _thorite_ and
_orangeite_, ThSiO_{4},2H_{2}O. The latter is isomorphous with zircon
(sp. gr. 4·8).[59]
[59] Thorium has also been found in the form of oxide in certain
pyrochlores, euxenites, monazites, and other rare minerals
containing salts of niobium and phosphates. The compounds of
thorium are prepared by decomposing thorite or orangeite with
strong sulphuric acid at its boiling point; this renders the
silica insoluble, and the thorium oxide passes into solution when
the residue is treated with cold water, after having been
previously boiled with water (boiling water does not dissolve the
oxide of thorium). Lead and other impurities are separated by
passing sulphuretted hydrogen through the solution, and the
thorium hydroxide is then precipitated by ammonia. If this
hydroxide be dissolved in the smallest possible amount of
hydrochloric acid, and oxalic acid be then added, thorium oxalate
is obtained as a white precipitate, which is insoluble in an
excess of oxalic acid; this reaction is taken advantage of for
separating this metal from many others. It, however, resembles the
cerite metals (Chapter XVII., Note 43) in this and many other
respects. The thorium hydroxide is gelatinous; on ignition it
leaves an infusible oxide, ThO_{2}, which, when fused with borax,
gives crystals of the same form as stannic oxide or titanic
anhydride; sp. gr. 9·86. But the basic properties are much more
developed in thorium oxide than in the preceding oxides, and it
does not even disengage carbonic acid when fused with sodium
carbonate--that is, it is a much more energetic base than
zirconium oxide. The hydrate, ThO_{2}, however, is soluble in a
solution of Na_{2}CO_{3} (Chapter XVII., Note 43). Thorium
chloride, ThCl_{4} is obtained as a distinctly crystalline
sublimate when thorium oxide, mixed with charcoal, is ignited in a
stream of dry chlorine. When heated with potassium, thorium
chloride gives a metallic powder of thorium having a sp. gr. 11·1.
It burns in air, and is but slightly soluble in dilute acids. The
atomic weight of thorium was established by Chydenius and
Delafontaine on the basis of the ismorphism of the double
fluorides.