Chapter XV: , Note 15), specific heat, and other properties of the metal

confirm the atomic weight In = 113.[40]

[40] The vapour density of indium chloride, InCl_{3} (Note 31),
determined by Nilson and Pettersson, confirms this atomic weight.
Indium is separated from zinc and cadmium, with which it occurs,
by taking advantage of the fact that its hydroxide is insoluble in
ammonia, that the solutions of its salts give indium when treated
with zinc (hence indium is dissolved after zinc by acids) and that
they give a precipitate with hydrogen sulphide even in acid
solutions. Metallic indium is grey, has a sp. gr. of 7·42, fuses
at 176°, and does not oxidise in the air; when ignited, it first
gives a black suboxide, In_{4}O_{3}, then volatilises and gives a
brown oxide, In_{2}O_{3}, whose salts, InX_{3}, are also formed by
the direct action of acids on the metal, hydrogen being evolved.
Caustic alkalis do not act on indium, from which it is evident
that it is less capable of forming alkaline compounds than
aluminium is; however, with potassium and sodium hydroxides,
solutions of indium salts give a colourless precipitate of the
hydroxide, which is soluble in an excess of the alkali, like the
hydroxides of aluminium and zinc. Its salts do not crystallise.
Nilson and Pettersson (1889), by the action of HCl upon In,
obtained volatile crystalline, InCl_{2}, and by treating this
compound with In, InCl also.

Inasmuch as we found among the analogues of magnesium in group II. a metal, mercury, heavier and more easily reduced than the rest, and giving two grades of oxidation, so we should expect to find a metal among the analogues of aluminium in group III. which would be heavy, easily reduced, and give two grades of oxidation, and would have an atomic weight greater than 200. Such is _thallium_. It forms compounds of a lower type, TlX, besides the higher unstable type TlX_{3}, just as mercury gives HgX_{2} and HgX. In the form of the thallic oxide, Tl_{2}O_{3}, the base is but feebly energetic, as would be expected by analogy with the oxides Al_{2}O_{3}, Ga_{2}O_{3}, and In_{2}O_{3}, whilst in thallous oxide, Tl_{2}O, the basic properties are sharply defined, as might be expected according to the properties of the type R_{2}O (Chapter XV.). _Thallium_ was discovered in 1861 by Crookes and by Lamy in certain pyrites. When pyrites are employed in the manufacture of sulphuric acid, they are burned, and give besides sulphurous anhydride the vapours of various substances which accompany the sulphur, and are volatile. Among these substances arsenic and selenium are found, and together with them, thallium. These substances accumulate in a more or less considerable quantity in the tubes through which the vapours formed in the combustion of the pyrites have to pass. When the methods of spectrum analysis were discovered (1860), a great number of substances were subjected to spectroscopic research, and it was observed that those sublimations which are obtained in the combustion of certain pyrites contained an element having a very sharply-defined and characteristic spectrum--namely, in the green portion of the spectra it gave a well-defined band (wave-length 535 millionth millimetres) which did not correspond with any then known element.[41]

[41] Thallium was afterwards found in certain micas and in the rare
mineral crookesite, containing lead, silver, thallium, and
selenium. Its isolation depends on the fact that in the presence
of acids thallium forms thallous compounds, TlX. Among these
compounds the chloride and sulphate are only slightly soluble, and
give with hydrogen sulphide a black precipitate of the sulphide
Tl_{2}S, which is soluble in an excess of acid, but insoluble in
ammonium sulphide.

Under the action of a galvanic current solutions of thallium salts deposit the metal in the form of a heavy powder. It is of a grey colour like tin, is soft like sodium, and has a metallic lustre. Its specific gravity is 11·8, it melts at 290°, and volatilises at a high temperature. When heated slightly above its melting point it forms an insoluble (in water) higher oxide, Tl_{2}O_{3}, as a dark-coloured powder, generally however accompanied by the lower oxide Tl_{2}O, which is also black but soluble in water and alcohol. This solution has a distinctly alkaline reaction. This _thallous oxide_, melts at 300°, and is easily obtained from the hydroxide TlHO by igniting it without access of air (in the presence of air the incandescent thallous oxide partly passes into thallic oxide). _Thallous hydroxide_, TlOH, crystallises with one molecule H_{2}O in yellow prisms which are very easily soluble in water. Metallic thallium may be used for its preparation, as the metal in the presence of water attracts oxygen from the air and forms the hydroxide. But metallic thallium does not decompose water, although it gives a hydroxide which is soluble in water.[41 bis] All the other data for the chemical and physical properties of thallium, of its two grades of oxidation and of their corresponding salts, are expressed by the position occupied by this metal in virtue of its atomic weight Tl = 204, between mercury Hg = 200, and lead Pb = 206.

[41 bis] The best method of preparing thallous hydroxide, TlOH, is by
the decomposition of the requisite quantity of baryta by thallous
sulphate, which is slightly soluble in water; barium sulphate is
then obtained in the precipitate and thallous hydroxide in
solution. This solubility of the hydroxide is exceedingly
characteristic, and forms one of the most important properties of
thallium. These lower (thallous) compounds are of the type TlX,
and recall the salts of the alkalis. The salts TlX are colourless,
do not give a precipitate with the alkalis or ammonia, but are
precipitated by ammonium carbonate, because thallous carbonate,
Tl_{2}CO_{3}, is sparingly soluble in water. Platinic chloride
gives the same kind of precipitate as it does with the salts of
potassium--that is, thallous platinochloride, PtTl_{2}Cl_{6}. All
these facts, together with the isomorphism of the salts TlX with
those of potassium, again point out what an important significance
the types of compounds have in the determination of the character
of a given series of substances. Although thallium has a greater
atomic weight and greater density than potassium, and although it
has a less atomic volume, nevertheless thallous oxide is analogous
to potassium oxide in many respects, for they both give compounds
of the same type, RX. We may further remark that thallous
fluoride, TlF, is easily soluble in water as well as thallous
silicofluoride, SiTl_{2}F_{6}, but that thallous cyanide, TlCN, is
sparingly soluble in water. This, together with the slight
solubility of thallous chloride, TlCl, and sulphate, Tl_{2}SO_{4},
indicates an analogy between TlX and the salts of silver, AgX.

As regards the higher oxide or the _thallic oxide_, Tl_{2}O_{3},
the thallium is trivalent in it--that is, it forms compounds of
the type TlX_{3}. The hydroxide, TlO(OH), is formed by the action
of hydrogen peroxide on thallous oxide, or by the action of
ammonia on a solution of thallic chloride, TlCl_{3}. It is
obtained as a brown precipitate, insoluble in water but easily
soluble in acids, with which it gives thallic salts, TlX_{3}.
Thallic chloride, which is obtained by cautiously heating the
metal in a stream of chlorine, forms an easily fusible white mass,
which is soluble in water and able to part with two-thirds of its
chlorine when heated. An aqueous solution of this salt yields
colourless crystals containing one equivalent of water. It is
evident from the above that all the thallic salts can easily be
reduced to thallous salts by reducing agents such as sulphurous
anhydride, zinc, &c. Besides these salts, thallic sulphate,
Tl_{2}(SO_{4})_{3},7H_{2}O, thallic nitrate
Tl(NO_{3})_{3},4H_{2}O, &c., are known. These salts are decomposed
by water, like the salts of many feeble basic metals--for example,
aluminium.

Gallium, indium, and thallium belong to the uneven series, and there should be elements of the even series in group III. corresponding with calcium, strontium, and barium in group II. These elements should in their oxides R_{2}O_{3} present basic characters of a more energetic kind than those shown by alumina, just as calcium, strontium, and barium give more energetic bases than magnesium, zinc, and cadmium. Such are _yttrium_ and _ytterbium_, which occur in a rare Swedish mineral called _gadolinite_, and are therefore termed the gadolinite metals. To these belong also the metal _lanthanum_, which accompanies the two other metals _cerium_ and _didymium_ in the mineral _cerite_, and it therefore belongs to the cerite metals. All these metals and certain others accompanying them, give basic oxides R_{2}O_{3}. At first their formula was supposed to be RO, but the application of the periodic system required their being counted as elements of groups III. and IV., which was also confirmed by the determination of the specific heats of these metals,[42] and better still by the fact that Nilson and Clève, in their researches on the gadolinite metals (1879), discovered that they contain a peculiar and very rare element, _scandium_, which by the magnitude of its atomic weight, Sc = 44, and in all its properties, exactly corresponds with the metal (previously foretold on the basis of the periodic system) _ekaboron_, whose properties were determined by taking the cerite and gadolinite metals as forming oxides R_{2}O_{3}.[43]

[42] The specific heat of cerium determined (1870) by me, and
afterwards confirmed by Hillebrand, corresponds with that atomic
weight of cerium according to which the composition of two oxides
should be Ce_{2}O_{3} and CeO_{2}. Hillebrand also obtained
metallic lanthanum and didymium by decomposing their salts by a
galvanic current, and he found their specific heats to be near
that of cerium and about 0·04, and it is therefore justifiable to
give them an atomic weight near that of cerium, as was done on the
basis of the periodic law. Up to 1870 yttrium oxide was also given
the formula RO. Having re-determined the equivalent of yttrium
oxide (with respect to water), and found it to be 74·6, I
considered it necessary to also ascribe to it the composition
Y_{2}O_{3}, because then it falls into its proper place in the
periodic system. If the equivalent of the oxide to water be 74·6,
it contains 58·6 of metal per 16 of oxygen, and consequently one
part by weight of hydrogen replaces 29·3 of yttrium, and if it be
regarded as bivalent (oxide RO), it would not, by its atomic
weight 58·6, find a place in the second group. But if it be taken
as trivalent--that is, if the formula of its oxide be R_{2}O_{3}
and salts RX_{3}--then Y = 88, and a position is open for it in
the third group in the sixth series after rubidium and strontium.
These alterations in the atomic weights of the cerite and
gadolinite metals were afterwards accepted by Clève and other
investigators, who now ascribe a formula R_{2}O_{3} to all the
newly discovered oxides of these metals. But still the position in
the periodic system of certain elements--for example of holmium,
thulium, samarium, and others--has not yet been determined for
want of a sufficient knowledge of their properties in a state of
purity.

[43] So, for example, in 1871, in the _Journal of the Russian
Physico-Chemical Society_ (p. 45) and in Liebig's _Annalen_, Supt.
Band viii. 198, I deduced, on the basis of the periodic law, an
atomic weight 44 for ekaboron, and Nilson in 1888 found that of
scandium, which is ekaboron, to be Sc = 44·03, The periodic law
showed that the specific gravity of the ekaboron oxide would be
about 8·5, that it would have decided but feeble basic properties
and that it would give colourless salts. And this proved to be the
case with scandium oxide. In describing scandium, Clève and Nilson
acknowledge that the particular interest attached to this element
is due to its complete identity with the expected element
ekaboron. And this accurate foretelling of properties could only
be arrived at by admitting that alteration of the atomic weights
of the cerite and gadolinite metals which was one of the first
results of the application of the periodic system of the elements
to the interpretation of chemical facts. In my first memoirs,
namely, in the _Bulletin of the St. Petersburg Academy of
Sciences_, vol. viii. (1870), and in Liebig's _Annalen_ (_l. c._
p. 168) and others, I particularly insisted on the necessity of
altering the then accepted atomic weights of cerium, lanthanum,
and didymium. Clève, Höglund, Hillebrand and Norton, and more
especially Brauner, and others accepted the proposed alteration,
and gave fresh proofs in favour of the proposed alterations of
these atomic weights. The study of the fluorides was particularly
important. Placing cerium in the fourth group, the composition of
its highest oxide would then be CeO_{2}, and its compounds CeX_{4}
and the lower oxide, Ce_{2}O_{3} or CeX_{3}. Brauner obtained the
fluoride CeF_{4},H_{2}O corresponding with the first, and a double
crystalline salt, 3KF,2CeF_{4},2H_{2}O, without any admixture of
compound of the lower grade CeX_{3}, which generally occur
together with the majority of salts corresponding with CeX_{4}. It
will be seen from these formulæ and from the tables of the
elements, that cerium and didymium do not belong to the third
group, which is now being described, but we mention them here for
convenience, as all the cerite and gadolinite metals have much in
common. These metals, which are rare in nature, resemble each
other in many respects, always accompany each other, are with
difficulty isolated from each other, and stand together in the
periodic system of the elements; they have acquired a peculiar
interest owing to their having been in 1870 the objects of the
study of Marignac, Delafontaine, Soret, Lecoq de Boisbaudran,
Brauner, Clève, Nilson, the professors of Upsala, and others.

The cerite and gadolinite metals occur in rare siliceous minerals
from Sweden, America, the Urals, and Baikal, such as cerite (in
Sweden), gadolinite, and orthite; and in still rarer minerals
formed by titanic, niobic, and tantalic acids, such as euxenite in
Norway and America, and samarskite in Norway, the Urals and
America, and in a few rare fluorides and phosphates. Among the
latter, monazite is found in somewhat considerable quantities in
Brazil and North Carolina; this contains the phosphate of cerium,
CePO_{4} (= Ce_{2}O_{3}P_{2}O_{3}), together with didymium,
thorium and lanthanum (according to W. Edron and Shapleigh's
analyses), and is now used for preparing that mixture of the
oxides of the rare metals (especially ThO_{2}, Ce_{2}O_{3},
La_{2}O_{3}, &c.), which is employed for incandescent burners
(Auer von Welsbach), as it has been found by experiment that these
oxides when raised to incandescence in a non-luminous gas flame,
give a far more brilliant flame with a smaller consumption of gas,
besides being suitable for such non-luminous gases as water gas.
The insufficiency of material to work upon, and the difficulty of
separating the oxides from each other, are the chief reasons why
the composition of the compounds of these rare metals is so
imperfectly known. Cerite is the most accessible of these
minerals. Besides silica it contains more than 50 p.c. of the
oxides of cerium, lanthanum (from 4 p.c.), and didymium. The
decomposition of its powder by sulphuric acid gives sulphates, all
of which are soluble in water. The other minerals mentioned above
are also decomposed in the same manner. The solution of sulphates
is precipitated with free oxalic acid, which forms salts insoluble
in water and dilute acids with all the cerite and gadolinite
oxides. The oxides themselves are obtained by igniting the
oxalates. When ignited in the air the cerium passes from its
ordinary oxide Ce_{2}O_{3} into the higher oxide CeO_{2}, which is
so feeble a base that its salts are decomposed by water, and it is
insoluble in dilute nitric acid. Therefore it is always possible
to remove all the cerium oxide by repeated ignitions and solutions
in sulphuric acid. The further separation of the metals is mainly
based on four methods employed by many investigators.

(_a_) A solution of the mixed salts is treated with an excess of
solid potassium sulphate. Double salts, such as
Ce_{2}(SO_{4})_{3},3K_{2}SO_{4}, are thus formed. The gadolinite
metals, namely yttrium, ytterbium, and erbium, then remain in
solution--that is, their double salts are soluble in a solution of
potassium sulphate, whilst the cerite metals--namely, cerium,
lanthanum, and didymium--are precipitated, that is, their double
salts are insoluble in a saturated solution of potassium sulphate.
This ordinary method of separation, however, appears from the
researches of Marignac to be so untrustworthy that a considerable
amount of didymium and the other metals remain in the soluble
portion, owing to the fact that, although individually insoluble,
they are dissolved when mixed together. Thus erbium and terbium
occur both in the solution and precipitate. Nevertheless,
beryllium, yttrium, erbium, and ytterbium belong to the soluble,
and scandium, cerium, lanthanum, didymium, and thorium to the
insoluble portion. The insoluble salt of scandium, for example
(_i.e._ insoluble in a solution of potassium sulphate), has a
composition Sc_{2}(SO_{4})_{3},3K_{2}SO_{4}.

(_b_) The oxides obtained by the ignition of the oxalates are
dissolved in nitric acid (the nitrates of the cerite metals easily
form double salts with those of the alkali metals, and as
some--for example, the ammonio-lanthanum salt--crystallise very
well, they should be studied and applied to the analytical
separation of these metals), the solution is then evaporated to
dryness, and the residue fused. All nitrates are destroyed by
heat; those of aluminium and iron, &c., very easily, those of the
cerite and gadolinite metals also easily (although not so easily
as the above) but in different degrees and sequence; so that by
carrying on the decomposition carefully from the beginning it is
possible to destroy the nitrate of only one metal without touching
the others, or leaving them as insoluble basic salts. This method,
like the preceding and the two following, must be repeated as many
as seventy times to attain a really constant product of fixed
properties, that is, one in which the decomposed and undecomposed
portions contain one and the same oxide. This method, due to
Berlin and worked out by Bunsen, has given in the hands of
Marignac and Nilson the best results, especially for the
separation of the gadolinite metals, ytterbium and scandium.

(_c_) A solution of the salts is partially precipitated by
ammonia; that is, the solution is mixed with a small quantity of
ammonia insufficient for the precipitation of the entire quantity
of the bases (fractional precipitation). Thus, the didymium
hydroxide is first precipitated from a mixture of the salts of
didymium and lanthanum. A partial separation may be effected by
repeating the solution of the precipitate and fractional
precipitation, but a perfectly pure product is scarcely
attainable.

(_d_) The formates having different degrees of solubility
(lanthanum formate 420 parts of water per one of salt, didymium
formate 221, cerium formate 360, yttrium and erbium formates
easily soluble) give a possible means of separating certain of the
gadolinite metals from each other by a method of fractional
solution and precipitation, as Bunsen, Bahr, Clève, and others
have pointed out.

(_e_) Crookes (1893) took advantage of the fractional
precipitation of alcoholic solutions of the chlorides by amylene,
and by this means separated, for example, erbium, terbium, and
others.

(_f_) Lastly, oxide of thorium ThO_{2} (Chapter VIII., Note 59) is
separated by means of its solubility in a solution of sodium
carbonate.

A good method of separating these metals is not known, for they
are so like each other. There are also only a few _methods of
distinguishing_ them from each other, and we can only add the
following four to the above.

^a The faculty of oxidising into a higher oxide. This is very
characteristic for cerium, which gives the oxides Ce_{2}O_{3} and
CeO_{2} or Ce_{2}O_{4}. Didymium also gives one colourless oxide,
Di_{2}O_{3}, which is capable of forming salts (of a lilac
colour), and another, according to Brauner, Di_{2}O_{5} which is
dark brown and does not form salts, so far as is known, and (like
ceric oxide) acts as an oxidising agent, like the higher oxides of
tellurium, manganese, lead, and others. Lanthanum, yttrium, and
many others are not capable of such oxidation. The presence of the
higher oxides may be recognised by ignition in a stream of
hydrogen, by which means the higher oxides are reduced to the
lower, which then remain unaltered.

^b The majority of the salts of the gadolinite and cerite metals
are colourless, but those of didymium and erbium are
rose-coloured, the salts of the higher oxide of cerium, CeX_{4},
yellow, of the higher oxide of terbium, yellow, &c. Thus, the
first metals obtained from gadolinite were yttrium, giving
colourless, and erbium, giving rose-coloured, salts. Afterwards it
was found that the salts of erbium of former investigators
contained numerous colourless salts of scandium, ytterbium, &c.,
so that a coloration sometimes indicates the presence of a small
impurity, as was long known to be the case in minerals, and
therefore this point of distinction cannot be considered
trustworthy.

^c In a solid state and in solutions, the salts of didymium,
samarium, holmium, &c., give characteristic absorption spectra, as
we pointed out in Chapter XIII., and this naturally is connected
with the colour of these salts. The most important point is, that
those metals which do not give an absorption spectrum--for
example, lanthanum, yttrium, scandium, and ytterbium--may be
obtained free from didymium, samarium, and the other metals giving
absorption spectra, because the presence of the latter may be
easily recognised by means of the spectroscope, whilst the
presence of the former in the latter cannot be distinguished, and
therefore the purification of the former can be carried further
than that of the latter. We may further remark that the
sensitiveness of the spectrum reaction for didymium is so great
that it is possible with a layer of solution half a metre thick to
recognise the presence of 1 part of didymium oxide (as salt) in
40,000 parts of water. Cossa determined the presence of didymium
(together with cerium and lanthanum) in apatites, limestones,
bones, and the ashes of plants by this method. The main group of
dark lines of didymium correspond with wave-lengths of from 580 to
570 millionths mm.; and the secondary to about 520, 730, 480, &c.
The chief absorption bands of samarium are 472-486, 417, 500, and
559. Besides which, Crookes applied the investigation of the
spectra of the phosphorescent light which is emitted by certain
earths in an almost perfect vacuum, when an electric discharge is
passed through it, to the discovery and characterisation of these
rare metals. But it would seem that the smallest admixture of
other oxides (for example, bismuth, uranium) so powerfully
influences these spectra that the fundamental distinctions of the
oxides cannot be determined by this method. Besides which, the
spectra obtained by the passage of sparks through solutions or
powders of the salts are determined and applied to distinguishing
the elements, but as spectra vary with the temperature and
elasticity (concentration) this method cannot be considered as
trustworthy.

^d The most important point of distinction of individual metallic
oxides is given by the direct _determination of their equivalent
with respect to water_--that is, the amount of the oxide by weight
which combines (like water) with 80 parts by weight of sulphuric
anhydride, SO_{3}, for the formation of a normal salt. For this
purpose the oxide is weighed and dissolved in nitric acid,
sulphuric acid is then added, and the whole is evaporated to
dryness over a water-bath and then heated over a naked flame
sufficiently strongly to drive off the excess of sulphuric acid,
but so as not to decompose the salt (the product would in that
case not be perfectly soluble in water); then, knowing the weight
of the oxide and of the anhydrous sulphate, we can find the
equivalent of the oxide. The following are the most trustworthy
figures in this connection: scandium oxide 45·35 (Nilson), yttrium
oxide 75·7 (Clève; according to my determination, 1871--74·6),
cerous oxide--that is, the lower form of oxidation of cerium,
according to various investigators (Bunsen, Brauner, and others)
from 108 to 111, the higher oxide of cerium from 85 to 87,
lanthanum oxide, according to Brauner, 108, didymium oxide (in
salts of the ordinary lower form of oxidation) about 112
(Marignac, Brauner, Clève), samarium oxide about 116 (Clève),
ytterbium oxide 131·3 (Nilson). It may not be superfluous here to
draw attention to the fact that the equivalent of the oxides of
all the gadolinite and cerite metals for water distribute
themselves into four groups with a somewhat constant difference of
nearly 30. In the first group is scandium oxide with equivalent
45, in the second, yttrium oxide 76, in the third, lanthanum,
cerium, didymium, and samarium oxides with equivalent about 110,
and, in the fourth, erbium, ytterbium, and thorium oxides with
equivalent about 131. The common difference of period is nearly
45. And if we ascribe the type R_{2}O_{3} to all the oxides--that
is, if we triple the weight of the equivalent of the oxide--we
shall obtain a difference of the groups nearly equal to 90, which,
for two atoms of the metal, forms the ordinary periodic difference
of 45. If one and the same type of oxide R_{2}O_{3} be ascribed to
all these elements (as now generally accepted, in many cases there
being insufficiently trustworthy data), then the atomic weights
should be Sc = 44, Y = 89, La = 138, Ce = 140, Di = 144,
(neodymium 140, praseodymium 144), Sm = 150, Yb = 173, also
terbium 147, holmium 162, alphayttrium 157, erbium 166, thulium
170, decipium 171. It should be observed that there may be
instances of basic salts. If, for example, an element with an
atomic weight 90 gave an oxide RO_{2}, but salts ROX_{2}, then by
counting its oxide as R_{2}O_{3} its atomic weight would be 159.

All the points distinguishing many gadolinite and cerite elements
have not been sufficiently well established in certain cases (for
example, with decipium, thulium, holmium, and others). At present
the most certain are yttrium, scandium, cerium, and lanthanum. In
the case of didymium, for example, there is still much that is
doubtful. Didymium, discovered in 1842 by Mosander after
lanthanum, differs from the latter in its absorption spectrum and
the lilac-rose colour of its salts. Delafontaine (1878) separated
samarium from it. Welsbach showed that it contains two particular
elements, neodymium (salts bluish-red) and praseodymium (salts
apple-green), and Becquerel (1887) by investigating the spectra of
crystals, recognised the presence of six individual elements.
Probably, therefore, many of the now recognised elements contain a
mixture of various others, and as yet there is not enough
confirmation of their individuality. As regards yttrium, scandium,
cerium, and lanthanum, which have been established without doubt,
I think that, owing to their great rarity in nature and chemical
art, it would be superfluous to describe them further in so
elementary a work as the present. We may add that Winkler (1891)
obtained a hydrogen compound of lanthanum, whose composition
(according to Brauner) is La_{2}H_{3}, as would be expected from
the composition of Na_{2}H, Mg_{2}H_{2}, &c. C. Winkler (1891), on
reducing CeO_{2} with magnesium, also remarked a rapid absorption
of hydrogen, and showed that a _hydride of cerium_, CeH_{2},
corresponding to CaH, and the other similar hydrides of metals of
the alkaline earths, is formed (Chapter XIV., Note 63).

The brevity of this work and the great rarity of the above-mentioned elements will give me the right to exclude their description, all the more as the principles of the periodic system enable many of their properties to be foreseen, and as their practical uses (cerium oxalate is used in medicine, and didymium oxide in the manufacture of glass, a mixture of the oxides of lanthanum and similar metals is employed for giving a bright light, as this mixture emits a brilliant white light when brought to incandescence) are very limited, by reason of their great rarity in nature, and the difficulty of separating them from one another.