Chapter XVIII: Silicon and the Other Elements of the Fourth Group (2)

The hydrosol of silica prepared by mixing an excess of hydrochloric acid with a solution of sodium silicate, may be freed from the admixtures both of hydrochloric acid and salt, sodium chloride, _by means of dialysis_,[17] as Graham showed (in 1861) in enquiring into the nature of colloids (Chapter I.), and making many other important chemical investigations. The solution, containing the acid, salt, and silica, all dissolved in water, is poured into a dialyser--that is, a vessel with a porous diaphragm surrounded by water. Certain substances pass more easily through the diaphragm than others. This may be represented thus: the passage through the diaphragm proceeds in both directions, and if the solutions on each side of the diaphragm be equally strong, there will be equal numbers of molecules of the soluble substance passing into either side in a given time, some passing quickly and others slowly. The metallic chlorides and hydrochloric acid belong to the series of crystalloids which easily pass through a diaphragm, and therefore the hydrochloric acid and sodium chloride contained in the above-mentioned dialyser pass from the solution through the diaphragm into the water of the external vessel with considerable rapidity. The aqueous solution of colloidal silica also penetrates through the diaphragm, but very much more slowly. But if the amount of the substance dissolved is not equal on either side of the diaphragm, the whole system strives to attain a state of equilibrium; that is, the given substance penetrates through the diaphragm from the side where it is in excess to the part where there is a smaller quantity of it. All substances which are soluble in water have the faculty of penetrating through a membrane swollen in water, but the velocity of penetration is not equal, and in this respect the dialyser separates substances like a sieve. The silica passes less rapidly through the diaphragm than the sodium chloride and hydrochloric acid, so that by repeatedly changing the external water it is easy to effect the extraction of the chlorine compounds from the dialyser, which will finally only contain a solution of silica. This extraction (of HCl and NaCl) may be so complete that the liquid taken from the dialyser will not give any precipitate with a solution of silver nitrate. Graham obtained in this way soluble silica having a distinctly acid reaction, which, however, disappeared on the addition of a very minute quantity of alkali; for ten parts of silica in the solution it was sufficient to take one part of alkali in order to give the liquid an alkaline reaction, so slightly energetic are the acid properties of silicic acid. The solution of silica obtained by this method becomes gelatinous on standing, on being heated, or on evaporation under the receiver of an air-pump, &c. The hydrosol is transformed into the hydrogel, the soluble hydrate into the gelatinous.

[17] _See_ Chapter I., Note 18. A solution of water-glass mixed with an
excess of hydrochloric acid is poured into the dialyser, and the
outer vessel is filled with water, which is continually renewed.
The water carries off the sodium chloride and hydrochloric acid,
and the hydrosol remains in the dialyser.

Thus in addition to the gelatinous form of the silicic acid, there exists also a variety of this substance, soluble in water, as is the case with alumina. Such variation in properties and exactly the same relations with regard to water characterise an immense series of other substances having a great significance in nature. The number of such substances is especially great among organic compounds, and particularly in those classes of them which compose the principal material of the bodies of animals and plants. It is sufficient to mention, for instance, the gelatin which is familiar to all as carpenter's and other glues, and in the form of size and jelly. The same substance is also known in the solution which is used to join objects together. In a peculiar insoluble condition it enters into the composition of hides and bones. These various forms of gelatin differ in the same way as the different varieties of silica. The property of forming a jelly is exactly the same as in silica, and the adhesiveness of the solutions of both substances is identical; soluble silica adheres like a solution of gelatin. The same properties are again shown by starch, rosin, and albumin, and by a series of similar substances. The diaphragms used in dialysis are also insoluble, gelatinous, forms of colloids. The bodies of animals and plants consist largely of similar matter, insoluble in water, corresponding with the gelatinous or insoluble silicon hydrate, or with glue. The albumin which coagulates when eggs are boiled is a typical form of the gelatinous condition of such substances in the body. These slight indications are sufficient in order to show how great is the significance of those transformations which are so well marked in silica. The facts discovered by _Graham_ in 1861-1864 comprise the most essential acquisitions in the general association of these phenomena of nature in the history of organic forms. The facility of transit from hydrogel to hydrosol is the first condition of the possibility of the development of organisms. The blood contains hydrosols, and the hydrogels of the same substances are contained in the muscles and tissues, and especially on the surface, of the body. All tissues are formed from the blood, and in that case the hydrosols are converted into hydrogels.[18] The absence of crystallisation, the property, apparently under the influence of feeble agencies, of passing from the soluble condition to the insoluble, to the gelatinous condition of the hydrogel, constitute the fundamental properties of all colloids.[19]

[18] A similar process occurs in plants--for example, when they secrete
a store of material for the following year in their bulbs, roots,
&c. (for instance, the potato in its tubers), the solutions from
the leaves and stems penetrate into the roots and other parts in
the form of hydrosols, where they are converted into
hydrogels--that is, into an insoluble form, which is acted on with
difficulty and is easily kept unaltered until the period of
growth--for example, until the following spring--when they are
reconverted into hydrosols, and the insoluble substance re-enters
into the sap, and serves as a source of the hydrogels in the
leaves and other portions of plants.

[19] As regards their chemical composition the colloids are very
complex--that is, they have a high molecular weight and a large
molecular volume--in consequence of which they do not penetrate
through membranes, and are easily subject to variation in their
physical and chemical properties (owing to their complex structure
and polymerism?) They have but little chemical energy, and are
generally feeble acids, if belonging to the order of oxides or
hydrates, such as the hydrates of molybdic and tungstic acids
(Chapter XXI.). But now the number of substances capable, like
colloids, of passing into aqueous solutions and of easily
separating out from them, as well as of appearing in an insoluble
form, must be supplemented by various other substances, among
which soluble gold and silver (Chapter XXIV.) are of particular
interest. So that now it may be said that the capacity of forming
colloid solutions is not limited to a definite class of compounds,
but is, if not a general, at all events, an exceedingly widely
distributed phenomenon.

Silica, as regards its _salt forming properties_, stands in the series of oxides on the boundary line on the side of the acids in just such a place as alumina occupies on the side of the bases--that is, aluminium hydroxide is the representative of the feeblest bases and silicic acid is the least energetic of acids (at least in the presence of water--that is, in aqueous solutions); in alumina, however, the basic properties are distinctly expressed, while in silica the acid properties preponderate. Like all feeble acid oxides it is capable of forming, with other acids, saline compounds which are but slightly stable and are very easily decomposed in the presence of water. The chief peculiarity of the silicates consists in the number of their types. The salts formed with nitric or sulphuric acid exist in one, two, and three tolerably stable forms, but for acids like silicic acid the number of forms is very great, almost unlimited. The natural silicates in particular furnish proof of this fact; they contain various bases in combination with silica, and for one and the same base there often exist various degrees of combination. As feeble bases are capable of forming basic salts in addition to normal salts--that is, a compound of a normal salt with a feeble base (either the hydroxide or the oxide)--so the feeble acid oxides (although not all) form, in addition to normal salts, highly acid salts--that is, normal salts _plus_ acid (hydrate or anhydride). Such acids are boric, phosphoric, molybdic, chromic, and especially silicic, acid.

In order to explain these relations it is necessary first to recollect the existence of the various hydrates of silica, or silicic acids,[20] and then to turn our attention to the similarity between silicon compounds and metallic alloys. Silica is an oxide having the appearance of, and in many respects the same properties as, those oxides which combine with it, and if two metals are capable of forming homogeneous alloys in which there exist definite or indefinite compounds, it is permissible to assume a similar power of forming alloys in the case of analogous oxides. Such alloys are found in indefinite, amorphous masses in the form of glass, lava, slags, and a number of similar siliceous compounds which do not contain any definite types of combination, but nevertheless are homogeneous throughout their mass. By slow cooling, or under other circumstances, definite crystalline compounds may--and sometimes do--separate from this homogeneous mass, as also sometimes definite crystalline alloys separate from metallic alloys.

[20] This is in accordance with the generally-accepted representation
of the relations between salts and the hydrates of acids, but it
is of little help in the study of siliceous compounds. Generally
speaking, it becomes necessary to explain the property of
(SiO_{2})_{_n_} to combine with (RO)_{_m_}, where _n_ may be
greater than _m_, and where R may be H_{2}, Ca, &c. Here we are
aided by those facts which have been attained by the investigation
of carbon compounds, especially with respect to glycol. Glycol is
a compound having the composition C_{2}H_{6}O_{2}, only differing
from alcohol, C_{2}H_{6}O, by an extra atom of oxygen. This
hydrate contains two hydroxyl groups, which may be successively
replaced by chlorine, &c. Hence the composition of glycol should
be represented as C_{2}H_{4}(OH)_{2}. It has been found that
glycol forms so-called polyglycols. Their origin will be
understood from the fact that glycol as a hydrate has a
corresponding anhydride of the composition C_{2}H_{4}O, known as
ethylene oxide. This substance is ethane, C_{2}H_{6}, in which two
hydrogens are replaced by one atom of oxygen. Ethylene oxide is
not the only anhydride of glycol, although it is the simplest one,
because C_{2}H_{4}O = C_{2}H_{4}(OH)_{2} - H_{2}O. Various other
anhydrides of glycol are possible, and have actually been
obtained, of the composition _n_C_{2}H_{4}(OH)_{2} - (_n_ -
1)H_{2}O = (C_{2}H_{4})_{_n_}O_{_n_ - 1}(OH)_{2}. These imperfect
anhydrides of glycol, or _polyglycols_, still contain hydroxyls
like glycol itself, and therefore are of an alcoholic character in
the same sense as glycol itself. They are obtained by various
methods, and, amongst others, by the direct combination of
ethylene oxide with glycol, because C_{2}H_{4}(OH)_{2} + (_n_ -
1)C_{2}H_{4}O = (C_{2}H_{4})_{_n_}O_{_n_ - 1}(OH)_{2}. The most
important circumstance, from a theoretical point of view, is that
these polyglycols may be distilled without undergoing
decomposition, and that the general formula given above expresses
their actual molecular composition. Hence we have here a direct
combination of the anhydride with the hydrate, and, moreover, a
repeated one. The formula A_{_n_}H_{2}O may be used to express the
composition of glycol and polyglycols with respect to ethylene
oxide in the most simple manner, if A stand for ethylene oxide.
When _n_ = 1 we have glycol, when _n_ is greater than 1 a
polyglycol. Such also is the relationship of the salts of hydrate
of silica, if A stand for silica, and if we imagine that H_{2}O
may also be taken _m_ times. Such a representation of the
_polysilicic acids_ corresponds with the representation of the
polymerism of silica. Laurent supposed the existence of several
polymeric forms, Si_{2}O_{4}, Si_{3}O_{6}, &c., besides silica,
SiO_{2}.

The formation of crystalline rocks in nature is partly of such a nature. By aqueous or igneous agency, but in any case in a liquid condition, those oxides which form the earth's crust and her crystalline minerals came into mutual contact. First of all they formed a shapeless mass, of which lava, glass, slags and solutions are examples, but little by little, or else suddenly, some definite compounds of certain oxides existing in this alloy or in the shapeless mass were formed. This is entirely similar to two metals forming a homogeneous alloy,[21] and under known circumstances (for instance, on cooling the alloy, or in the case of aqueous solution when the two metals are simultaneously liberated from the solution), definite crystalline compounds are separated. In any case there is no doubt that there is less distinction between silica and bases, than between bases and such anhydrides as, for instance, sulphuric or nitric, or even carbonic, as is seen on comparing the physical and chemical properties of silica and various kinds of oxides. Alumina, especially, is exceedingly near akin to silica; not only in the hydrated state, but also in the anhydrous condition, there exists a certain similarity between the crystalline forms of alumina and silica, in the uncombined state. Both are very hard, transparent, inactive, non-volatile, infusible, and crystallise in the hexagonal system--in a word, they are remarkably similar, and for this reason they are capable, like two kindred metals, of entering into many different degrees of combination. Isomorphous mixtures--that is, differing by the substitution of oxides akin both in their physical and chemical characters--are very frequently met with among minerals, and the study of the latter gave the principal impetus to the study of isomorphism. Thus, in a whole series of minerals, lime and magnesia are found in variable and interchangeable proportions. Exactly the same may be said of potassium and sodium, of alumina and ferric oxide, of manganous, ferrous, magnesium oxides, &c. Such isomorphism does not, however, extend without change of form and properties beyond certain rather narrow limits.[22] What I mean by this is that lime is not always replaced totally, but often only in small quantities, by magnesia, or by the manganous and ferrous oxides, without changing the crystalline form. The same may be observed with regard to potassium and lithium, which may be in part, but not completely, replaced by sodium. On the total substitution of one metal for another, often (although not invariably) the entire nature of the substance is changed; for instance, _enstatite_ (or bronzite) is a magnesium bisilicate with a small isomorphous substitution of calcium for magnesium; its composition is expressed by the formula MgSiO_{3}, it belongs to the rhombic system. On the entire substitution of calcium, _wollastonite_, CaSiO_{3}, of the monoclinic system, is obtained; when manganese is substituted, _rhodonite_, of the triclinic system, is produced; but in all of them the angles of the prism are 86° to 88°.[23]

[21] For us the latter have not a saline character, only because they
are not regarded from this point of view, but an alloy of sodium
and zinc is, in a broad sense, a salt in many of its reactions,
for it is subject to the same double decompositions as sodium
phosphide or sulphide, which clearly have saline properties. The
latter (sodium phosphide), when heated with ethyl iodide, forms
ethyl phosphide, and the former--_i.e._ the alloy of zinc and
sodium--gives zinc ethyl; that is, the element (P, S, Zn) which
was united with the sodium passes into combination with the ethyl:
RNa + EtI = REt + NaI. By combining sodium successively with
chlorine, sulphur, phosphorus, arsenic, antimony, tin, and zinc,
we obtain substances having less and less the ordinary appearance
of salts, but if the alloy of sodium and zinc cannot be termed a
salt, then perhaps this name cannot be given to sodium sulphide,
and the compounds of sodium with phosphorus. The following
circumstance may also be observed: with chlorine, sodium gives
only one compound (with oxygen, at the most three), with sulphur
five, with phosphorus probably still more, with antimony naturally
still more, and the more analogous an element is to sodium, the
more varied are the proportions in which it is able to combine
with it, the less are the alterations in the properties which take
place by this combination, and the nearer does the compound formed
approach to the class of compounds known as indefinite chemical
compounds. In this sense a siliceous alloy, containing silica and
other acids, is a salt. The oxide to a certain extent plays the
same part as the sodium, whilst the silica plays the part of the
acid element which was taken up successively by zinc, phosphorus,
sulphur, &c., in the above examples. Such a comparison of the
silica compounds with alloys presents the great advantage of
including under one category the definite and indefinite silica
compounds which are so analogous in composition--that is, brings
under one head such crystalline substances as certain minerals,
and such amorphous substances as are frequently met with in
nature, and are artificially prepared, as glass, slags, enamels,
&c.

If the compounds of silica are substances like the metallic
alloys, then (1) the chemical union between the oxides of which
they are composed must be a feeble one, as it is in all compounds
formed between analogous substances. In reality such feeble
agencies as water and carbonic acid are able, although slowly, to
act on and destroy the majority of the complex silica compounds in
rocks, as we saw in the preceding chapter; (2) their formation,
like that of alloys, should not be accompanied by a considerable
alteration of volume; and this is actually the case. For example,
felspar has a specific gravity of about 2·6, and therefore, taking
its composition to be K_{2}O,Al_{2}O_{3},6SiO_{2}, we find its
volume, corresponding with this formula, to be 556·8. 2·6 = 214,
the volume of K_{2}O = 35, of Al_{2}O_{3} = 26, and of SiO_{2} =
22·6. Hence the sum of the volumes of the component oxides, 35 +
26 + 6 × 22·6 = 196, which is very nearly equal to that of the
felspar; that is, its formation is attended by a slight expansion,
and not by contraction, as is the case in the majority of other
cases when combinations determined by strong affinities are
accomplished. In the case in question the same phenomenon is
observed as in solutions and alloys--that is, as in cases of
feeble affinities. So also the specific gravity of glass is
directly dependent on the amount of those oxides which enter into
its composition. If in the preceding example we take the sp. gr.
of silica to be, not 2·65, but 2·2, its volume = 27·3, and the sum
of the volumes will be = 224--that is, greater than that of
orthoclase.

[22] It is, however, easy to imagine, and experience confirms the
supposition, that in a complex siliceous compound containing for
instance sodium and calcium, the whole of the sodium may be
replaced by potassium, and _at the same time_ the whole of the
calcium by magnesium, because then the substitution of potassium
for the sodium will produce a change in the nature of the
substance contrary to that which will occur from the calcium being
replaced by magnesium. That increase in weight, decrease in
density, increase of chemical energy, which accompanies the
exchange of sodium for potassium will, so to speak, be compensated
by the exchange of calcium for magnesium, because both in weight
and in properties the sum of Na + Ca is very near to the sum of K
+ Mg. _Pyroxene_ or _augite_ can be taken as an example; its
composition may be expressed by the formula CaMgSi_{2}O_{6}; that
is, it corresponds with the acid H_{2}SiO_{3}; it is a bisilicate.
In many respects it closely resembles another mineral called
'_spodumene_' (they are both monoclinic). This latter has the
composition Li_{6}Al_{8}Si_{15}O_{45}. On reducing both formulæ to
an equal contents of silica the following distinction will be
observed between them: spodumene
(Li_{2}O)_{6}(Al_{2}O_{3})_{8}30SiO_{2}; augite
(CaO)_{15}(MgO)_{15}30SiO_{2}. That is, the difference between
them consists in the sum of the magnesia and lime (MgO)_{15} +
(CaO)_{15} replacing the sum of the lithium oxide and alumina
(Li_{2}O)_{6} + (Al_{2}O_{3})_{8}; and in the chemical relation
these sums are near to one another, because magnesium and calcium,
both in forms of oxidation and in energy (as bases), in all
respects occupy a position intermediate between lithium and
aluminium, and therefore the sum of the first may be replaced by
the sum of the second.

If we take the composition of spodumene, as it is often
represented to be, Li_{2}O,Al_{2}O_{3},4SiO_{2}, the corresponding
formula of augite will be (CaO)_{2},(MgO)_{2},4SiO_{2}, and also
the amount of oxygen in the sum of Li_{2}OAl_{2}O_{3} will be the
same as in (CaO)_{2}(MgO)_{2}. I may remark, for the sake of
clearness, that lithium belongs to the first, aluminium to the
third group, and calcium and magnesium to the intermediate second
group; lithium, like calcium, belongs to the even series, and
magnesium and aluminium to the uneven.

The representation of the substitutions of analogous compounds
here introduced was first deduced by me in 1856. It finds much
confirmation in facts which have been subsequently discovered--for
example, with respect to tourmalin. Wülfing (1888), on the basis
of a number of analyses (especially of those by Röggs), states
that all varieties contain an isomorphous mixture of alkali and
magnesia tourmalin; into the composition of the former there
enters 12SiO_{2},3B_{2}O_{3},8Al_{2}O_{3},2Na_{2}O,4H_{2}O, and of
the latter 12SiO_{2},3B_{2}O_{3},5Al_{2}O_{3},12MgO,3H_{2}O. Hence
it is seen that the former contains in addition the sum of
3Al_{2}O_{3},2Na_{2}O,H_{2}O, whilst in the latter this sum of
oxides is replaced by 12MgO, in which there is as much oxygen as
in the sum of the more clearly-defined base 2Na_{2}O and less
basic 3Al_{2}O_{3}H_{2}O--that is, the relation is just the same
here as between augite and spodumene.

[23] With respect to the silica compounds of the various oxides, it
must be observed that only the _alkali salts_ are known in a
soluble form; all the others only exist in an insoluble form, so
that a solution of the alkali compounds of silica, or soluble
glass, gives a precipitate with a solution of the salts of the
majority of other metals, and this precipitate will contain the
silica compounds of the other bases. The maximum amount of the
gelatinous hydrate of silica, which dissolves in caustic potash,
corresponds with the formation of a compound, 2K_{2}O,9SiO_{2}.
But this compound is partially decomposed, with the precipitation
of hydrate of silica, on cooling the solution. Solutions
containing a smaller amount of silica may be kept for an
indefinite time without decomposing, and silica does not separate
out from the solution; but such compounds crystallise from the
solutions with difficulty. However, a crystalline bisilicate (with
water) has been obtained for sodium having the composition
Na_{2}O,SiO_{2}--_i.e._ corresponding to sodium carbonate. The
whole of the carbonic acid is evolved, and a similar soluble
sodium metasilicate is obtained on fusing 3·5 parts of sodium
carbonate with 2 parts of silica. If less silica be taken a
portion of the sodium carbonate remains undecomposed; however, a
substance may then be obtained of the composition Si(ONa)_{4},
corresponding with orthosilicic acid. It contains the maximum
amount of sodium oxide capable of combining with silica under
fusion. It is a sodium orthosilicate, (Na_{2}O)_{2},SiO_{2}.

Calcium carbonate, and the carbonates of the alkaline earths in
general, also evolve all their carbonic acid when heated with
silica, and in some instances even form somewhat fusible
compounds. Lime forms a fusible slag of _calcium silicate_, of the
composition CaO,SiO_{2} and 2CaO,3SiO_{2}. With a larger
proportion of silica the slags are infusible in a furnace. The
magnesium _slags_ are less fusible than those with lime, and are
often formed in smelting metals. Many compounds of the metals of
the alkaline earths with silica are also met with in nature. For
instance, among the magnesium compounds there is _olivine_,
(MgO)_{2},SiO_{2}, sp. gr. 3·4, which occurs in meteorites, and
sometimes forms a precious stone (peridote), and occurs in slags
and basalts. It is decomposed by acids, is infusible before the
blow-pipe, and crystallises in the rhombic system. _Serpentine_
has the composition 3MgO,2SiO_{2},2H_{2}O; it sometimes forms
whole mountains, and is distinguished for its great cohesiveness,
and is therefore used in the arts. It is generally tinted green;
its specific gravity is 2·5; it is exceedingly infusible, even
before the blowpipe. It is acted on by acids. Among the magnesium
compounds of silica, _talc_ is very widely used. It is frequently
met with in rocks which are widely distributed in nature, and
sometimes in compact masses; it can be used for writing like a
slate pencil or chalk, and being greasy to the touch, is also
known as _steatite_. It crystallises in the rhombic system, and
resembles mica in many respects; like it, it is divisible into
laminæ, greasy to the touch, and having a sp. gr. 2·7. These
laminæ are very soft, lustrous, and transparent, and are infusible
and insoluble in acids. The composition of talc approaches nearly
to 6MgO,5SiO_{2},2H_{2}O.

Among the crystalline silicates the following minerals are
known:--_Wollastonite_ (tabular-spar), crystallises in the
monoclinic system; sp. gr. 2·8; it is semi-transparent,
difficultly fusible, decomposed by acids, and has the composition
of a metasilicate, CaOSiO_{2}. But isomorphous mixtures of calcium
and magnesium silicates occur with particular frequency in nature.
The _augites_ (sp. gr. 3·3), diallages, hypersthenes, hornblendes
(sp. gr. 3·1), amphiboles, common asbestos, and many similar
minerals, sometimes forming the essential parts of entire rock
formations, contain various relative proportions of the
bisilicates of calcium and magnesium partially mixed with other
metallic silicates, and generally anhydrous, or only containing a
small amount of water. In the pyroxenes, as a rule, lime
predominates, and in the amphiboles (also of the monoclinic
system) magnesia predominates. Details upon this subject must be
looked for in works upon mineralogy.

The most remarkable complex siliceous compounds are the _felspars_, which enter into nearly all the primary rocks like porphyry, granite, gneiss, &c. These felspars always contain, in addition to silica and alumina, oxides presenting more marked basic properties, such as potash, soda, and lime. Thus the _orthoclase_ (adularia), or ordinary felspar (monoclinic) of the granites, contains K_{2}O,Al_{2}O_{3},6SiO_{2}; _albite_ contains the same substances, only with Na_{2}O instead of K_{2}O (it already appertains to the triclinic system); _anorthite_ contains lime, and its composition is CaO,Al_{2}O_{3},2SiO_{2}. On expressing the two last as containing equal quantities of oxygen, we have:--

Albite Na_{2} Al_{2} Si_{6} O_{16}
Anorthite Ca_{2} Al_{4} Si_{4} O_{16}

It is then evident that on the conversion of albite into anorthite, Na_{2}Si_{2} is replaced by Ca_{2}Al_{2}, and this sum, both in chemical energy and in the form of oxide, may be considered as corresponding with the first, because sodium and silicon are extreme elements in chemical character (from groups I. and IV.), and calcium and aluminium are means between them (from groups II. and III.), and actually both these felspar minerals are not only of one (triclinic) system, but form (Tchermak, Schuster) all possible kinds of definite compounds (isomorphous mixtures) between themselves, as indicated by their composition and all their properties. Thus oligoclase, andesine, labradorite, &c. (plagioclases), are nothing more than mutual combinations of albite and anorthite. Labradorite consists of albite, in combination with 1 to 2 molecules of anorthite. The class of _zeolites_ corresponds to the felspars; they are hydrated compounds of a similar composition to the felspars. Thus _natrolite_ contains Na_{2}O,Al_{2}O_{3},3SiO_{2},2H_{2}O, and _analcime_ presents the same composition, but contains 4SiO_{2} instead of 3SiO_{2}. In general, the felspars and zeolites contain RO,Al_{2}O_{3},_n_SiO_{2}, where _n_ varies considerably.[24]

[24] The majority of the siliceous minerals have now been obtained
artificially under various conditions. Thus N. N. Sokoloff showed
that slags very frequently contain peridote. Hautefeuille,
Chroustchoff, Friedel, and Sarasin obtained felspar identical in
all respects with the natural minerals. The details of the methods
here employed must be looked for in special works on mineralogy;
but, as an example, we will describe the method of the preparation
of felspar employed by Friedel and Sarasin (1881). From the fact
that felspar gives up potassium silicate to water even at the
ordinary temperature (Debray's experiments), they concluded that
the felspar in granites had an aqueous origin (and this may be
supposed to be the case from geological data); then, in the first
place, its formation could not be accomplished unless in the
presence of an excess of a solution of potassium silicate. In
order to render this argument clear I may mention, as an example,
that carnallite is decomposed by water into easily soluble
magnesium chloride and potassium chloride, and therefore if it is
of aqueous origin it could not be formed otherwise than from a
solution containing an excess of magnesium chloride, and, in the
second place, from a strongly-heated solution; again, felspar
itself and its fellow-components in granites are anhydrous. On
these facts were based experiments of heating hydrates of silica
with alumina and a solution of potassium silicate in a closed
vessel. The mixture was placed in a sealed platinum tube, which
was enclosed in a steel tube and heated to dull redness. When the
mixture contained an excess of silica the residue contained many
crystals of rock crystal and tridymite, together with a powder of
felspar, which formed the main product of the reaction when the
proportion of hydrate of silica was decreased, and a mixture of a
solution of potassium silicate with alumina precipitated together
with the silica by mixing soluble glass with aluminium chloride
was employed. The composition, properties, and forms of the
resultant felspar proved it to be identical with that found in
nature. The experiments approach very nearly to the natural
conditions, all the more as felspar and quartz are obtained
together in one mixture, as they so often occur in nature.

Such complex silicates are generally insoluble in water,[25] and if they undergo change in it, it is but very slow, and more often only in the presence of carbonic acid. Some of the silicates which are insoluble in water are easily and directly decomposed by acids; for instance, the zeolites and those fused silicates which contain a large quantity of energetic bases--such as lime. Many of the silicates, like glass,[26] are hardly changed by acids, particularly if they contain much silica, whilst fusion with alkalis leads to the formation of compounds rich in bases, after which acids decompose the alloys formed.[27]

[25] The application of _cements_ is based on this principle; they are
those sorts of 'hydraulic' lime which generally form a stony mass,
which hardens even under water, when mixed with sand and water.

The hydraulic properties of cements are due to their containing
calcareous and silico-aluminous compounds which are able to
combine with water and form hydrates, which are then unacted on by
water. This is best proved, in the first place, by the fact that
certain slags containing lime and silica, and obtained by fusion
(for example, in blast-furnaces), solidify like cements when
finely ground and mixed with water; and, in the second place, by
the method now employed for the manufacture of artificial cements
(formerly only peculiar and comparatively rare natural products
were used). For this purpose a mixture of lime and clay is taken,
containing about 25 p.c. of the latter; this mixture is then
heated, not to fusion, but until both the carbonic anhydride and
water contained in the clay are expelled. This mass when finely
ground forms Portland cement, which hardens under water. The
process of hardening is based on the formation of chemical
compounds between the lime, silica, alumina, and water. These
substances are also found combined together in various natural
minerals--for example, in the zeolites, as we saw above. In all
cases cement which has set contains a considerable amount of
water, and its hardening is naturally due to hydration--that is,
to the formation of compounds with water. Well-prepared and very
finely-ground cement hardens comparatively quickly (in several
days, especially after being rammed down), with 3 parts (and even
more) of coarse sand and with water, into a stony mass which is as
hard and durable as many stones, and more so than bricks and
limestone. Hence not only all maritime constructions (docks,
ports, bridges, &c.), but also ordinary buildings, are made of
Portland cement, and are distinguished for their great durability.
A combination of ironwork (ties, girders) and cement is
particularly suitable for the construction of aqueducts, arches,
reservoirs, &c. Arches and walls made of such cements may be much
less thick than those built up of ordinary stone. Hence the
production and use of cement rapidly increases from year to year.
The origin of accurate data respecting cements is chiefly due to
Vicat. In Russia Professor Schuliachenko has greatly aided the
extension of accurate data concerning Portland cement. Many works
for the manufacture of cement have already been established in
various parts of Russia, and this industry promises a great future
in the arts of construction.

[26] _Glass_ presents a similar complex composition, like that of many
minerals. The ordinary sorts of white glass contain about 75 p.c.
of silica, 13 p.c. of sodium oxide, and 12 p.c of lime; but the
inferior sorts of glass sometimes contain up to 10 p.c. of
alumina. The mixtures which are used for the manufacture of glass
are also most varied. For example, about 300 parts of pure sand,
about 100 parts of sodium carbonate, and 50 of limestone are
taken, and sometimes double the proportion of the latter. Ordinary
_soda-glass_ contains sodium oxide, lime, and silica as the chief
component parts. It is generally prepared from sodium sulphate
mixed with charcoal, silica, and lime (Chapter XII.), in which
case the following reaction takes place at a high temperature:
Na_{2}SO_{4} + C + SiO_{2} = Na_{2}SiO_{3} + SO_{2} + CO.
Sometimes potassium carbonate is taken for the preparation of the
better qualities of glass. In this case a glass, _potash-glass_,
is obtained containing potassium oxide instead of sodium oxide.
The best-known of these glasses is the so-called Bohemian glass or
crystal, which is prepared by the fusion of 50 parts of potassium
carbonate, 15 parts of lime, and 100 parts of quartz. The
preceding kinds of glass contain lime, whilst crystal glass
contains lead oxide instead. Flint glass--that is, the lead glass
used for optical instruments--is prepared in this manner,
naturally from the purest possible materials.
_Crystal-glass_--_i.e._ glass containing lead oxide--is softer
than ordinary glass, more fusible and has a higher index of
refraction. However, although the materials for the preparation of
glass be most carefully sorted, a certain amount of iron oxides
falls into the glass and renders it greenish. This coloration may
be destroyed by adding a number of substances to the vitreous
mass, which are able to convert the ferrous oxide into ferric
oxide; for example, manganese peroxide (because the peroxide is
deoxidised to manganous oxide, which only gives a pale violet tint
to the glass) and arsenious anhydride, which is deoxidised to
arsenic, and this is volatilised. The manufacture of glass is
carried on in furnaces giving a very high temperature (often in
regenerative furnaces, Chapter IX.). Large clay crucibles are
placed in these furnaces, and the mixture destined for the
preparation of the glass, having been first roasted, is charged
into the crucibles. The temperature of the furnace is then
gradually raised. The process takes place in three separate
stages. At first the mass intermixes and begins to react; then it
fuses, evolves carbonic acid gas, and forms a molten mass; and,
lastly, at the highest temperature, it becomes homogeneous and
quite liquid, which is necessary for the ultimate elimination of
the carbonic anhydride and solid impurities, which latter collect
at the bottom of the crucible. The temperature is then somewhat
lowered, and the glass is taken out on tubes and blown into
objects of various shapes. In the manufacture of window-glass it
is blown into large cylinders, which are then cut at the ends and
across, and afterwards bent back in a furnace into the ordinary
sheets. After being worked up, all glass objects have to be
subjected to a slow cooling (_annealing_) in special furnaces,
otherwise they are very brittle, as is seen in the so-called
'Rupert's drops,' formed by dropping molten glass into water;
although these drops preserve their form, they are so brittle that
they break up into a fine powder if a small piece be knocked off
them. Glass objects have frequently to be polished and chased. In
the manufacture of mirrors and many massive objects the glass is
cast and then ground and polished. Coloured glasses are either
made by directly introducing into the glass itself various oxides,
which give their characteristic tints, or else a thin layer of a
coloured glass is laid on the surface of ordinary glass. Green
glasses are formed by the oxides of chromium and copper, blue by
cobalt oxide, violet by manganese oxide, and red glass by cuprous
oxide and by the so-called purple of Cassius--_i.e._ a compound of
gold and tin--which will be described later. A yellow coloration
is obtained by means of the oxides of iron, silver, or antimony,
and also by means of carbon, especially for the brown tints for
certain kinds of bottle-glass.

From what has been said about glass it will be understood that it
is impossible to give a definite formula for it, because it is a
non-crystalline or amorphous alloy of silicates; but such an alloy
can only be formed within certain limits in the proportions
between the component oxides. With a large proportion of silica
the glass very easily becomes clouded when heated; with a
considerable proportion of alkalis it is easily acted on by
moisture, and becomes cloudy in time on exposure to the air; with
a large proportion of lime it becomes infusible and opaque, owing
to the formation of crystalline compounds in it; in a word, a
certain proportion is practically attained among the component
oxides in order that the glass formed may have suitable
properties. Nevertheless, it may be well to remark that the
composition of common glass approaches to the formula
Na_{2}O,CaO,4SiO_{2}.

The coefficient of cubical expansion of glass is nearly equal to
that of platinum and iron, being approximately 0·000027. The
specific heat of glass is nearly 0·18, and the specific gravity of
common soda glass is nearly 2·5, of Bohemian glass 2·4, and of
bottle glass 2·7. Flint glass is much heavier than common glass,
because it contains the heavier oxide of lead, its specific
gravity being 2·9 to 3·2.

[27] It must be recollected that although acids seem to act only feebly
on the majority of silicates, nevertheless a finely-levigated
powder of siliceous compounds is acted on by strong acids,
especially with the aid of heat, the basic oxides being taken up
and gelatinous silica left behind. In this respect sulphuric acid
heated to 200° with finely-divided siliceous compounds in a closed
tube acts very energetically.

According to the periodic law, the nearest analogues of silicon ought to be elements of the uneven series, because silicon, like sodium, magnesium, and aluminium, belongs to the uneven series.[28] Immediately after silicon follows ekasilicon or _germanium_, Ge = 72, whose properties were predicted (1871) before Winkler (1886) in Freiberg, Saxony (Chapter XV. § 5), discovered this element in a peculiar silver ore called _argyrodite_, Ag_{6}GeS_{5}.[29] Easily reduced from the oxide by heating with hydrogen and charcoal, and separated from its solutions by zinc, metallic germanium proved to be greyish white, easily crystallisable (in octahedra), brittle, fusible (under a coating of fused borax) at about 900°, and easily oxidisable; the specific gravity = 5·469, the atomic weight = 72·3, and the specific heat = 0·076,[30] as might be expected for this element according to the periodic law. The corresponding _germanium dioxide_, GeO_{2}, is a white powder having a specific gravity of 4·703; water, especially when boiling, dissolves this dioxide (1 part of GeO_{2} requires for solution 247 parts of water at 20°, 95 parts at 100°). It forms soluble salts with alkalis and is but sparingly soluble in acids.[31] In a stream of chlorine the metal forms _germanium chloride_, GeCl_{4}, which boils at 86°, and has a specific gravity of 1·887 at 18°; water decomposes it, forming the oxide. All these properties[32] of germanium, showing its analogy to silicon and tin, form a most beautiful demonstration of the truth of the periodic law.[33]

[28] Such elements as silicon, tin, and lead were only brought together
under one common group by means of the periodic law, although the
quadrivalency of tin and lead was known much earlier. Generally
silicon was placed among the non-metals, and tin and lead among
the metals.

[29] At first (February 1886) the want of material to work on, the
absence of a spectrum in the Bunsen's flame, and the solubility of
many of the compounds of germanium, presented difficulties in the
researches of Professor Winkler, who, on analysing argyrodite by
the usual method, obtained a constant loss of 7 p.c., and was thus
led to search for a new element. The presence of arsenic and
antimony in the accompanying minerals also impeded the separation
of the new metal. After fusion with sulphur and sodium carbonate,
argyrodite gives a solution of a sulphide which is precipitated by
an _excess_ of hydrochloric acid; germanium sulphide is soluble in
ammonia and then precipitated by hydrochloric acid, as a _white_
precipitate, which is dissolved (or decomposed) by water. After
being oxidised by nitric acid, dried and ignited germanium
sulphide leaves the oxide GeO_{2}, which is reduced to the metal
when ignited in a stream of hydrogen.

[30] G. Kobb determined the spectrum of germanium, when the metal was
taken as one of the electrodes of a powerful Ruhmkorff's coil. The
wave-lengths of the most distinct lines are 602, 583, 518, 513,
481, 474, millionths of a millimetre.

[31] If germanium or germanium sulphide be heated in a stream of
hydrochloric acid, it forms a volatile liquid, boiling at 72°,
which Winkler regarded as germanium chloride, GeCl_{2}, or
germanium chloroform, GeHCl_{3}. It is decomposed by water,
forming a white substance, which may perhaps be the hydrate of
germanious oxide, GeO, and acts as a powerfully reducing agent in
a hydrochloric acid solution.

[32] Under certain circumstances germanium gives a blue coloration like
that of ultramarine, as Winkler showed, which might have been
expected from the analogy of germanium with silicon.

[33] Winkler expressed this in the following words (_Jour. f. pract.
Chemie_, 1886 [2], 34, 182-183): '... es kann keinem Zweifel mehr
unterliegen, dass das neue Element nichts Anderes, als das vor
fünfzehn Jahren von _Mendeléeff_ prognosticirte _Ekasilicium_
ist.'

'Denn einen schlagenderen Beweis für die Richtigkeit der Lehre von
der Periodicität der Elemente, als den, welchen die Verkörperung
des bisher hypothetischen "Ekasilicium" in sich schliesst, kann es
kaum geben, und er bildet in Wahrheit mehr, als die blosse
Bestätigung einer kühn aufgestellten Theorie, er bedeutet eine
eminente Erweiterung des chemischen Gesichtfeldes, einen mächtigen
Schritt in's Reich der Erkenntniss.'

The increase of atomic weight from silicon 28 to germanium 72 is 44--that is, about the same difference as there is in the atomic weights of chlorine and bromine; between germanium and its next analogue, _tin_ (Sn = 118), the difference is 46--that is, almost as much as the amount by which the atomic weight of iodine exceeds that of bromine.

Metallic tin is rarely met with in _nature_; it occurs in the veins of ancient formations, almost exclusively in the form of oxide, SnO_{2}, called _tin-stone_. The best known tin deposits are in Cornwall and in Malacca. In Russia, tin ores have been found in small quantities on the shores of Lake Ladoga, in Pitkarand. The crushed ore may easily be separated from the earthy matter accompanying it by washing on inclined tables, as the tin-stone has a specific gravity of 6·9, whilst the impurities are much lighter. _Tin oxide is very easily reduced_ to metallic tin by heating with charcoal. For this reason tin was known in ancient times, and the Ph[oe]nicians brought it from England. Metallic tin is cast into ingots of considerable weight or into thin sticks or rods. Tin has a white colour, rather duller than that of silver. It fuses easily at 232°, and crystallises on cooling. Its specific gravity is 7·29. The crystalline structure of ordinary tin is noticed in bending tin rods, when a peculiar sound is heard, produced by the fracture of the particles of tin along the surfaces of crystalline structure.

When pure tin is cooled to a low temperature it splits up into separate crystals, the bond between the particles is lost, the tin assumes a grey colour, becomes less brilliant--in a word, its properties become changed, as Fritzsche showed. This depends on the peculiar structure which the tin then acquires, and is particularly remarkable because it is effected by cold in a solid.[33 bis] If such tin be fused, or even simply heated, it becomes like ordinary tin, but is again changed when cooled. When in this condition tin has a specific gravity of 7·19. Similarly, tin is obtained by the action of the galvanic current on a solution of tin chloride; it then appears in crystals of the cubic system, and has a specific gravity of 7·18--that is, the same as when cooled.[34]

[33 bis] Emilianoff (1890) states that in the cold of the Russian
winter 30 out of 200 tin moulds for candles were spoilt through
becoming quite brittle.

[34] The tin deposited by an electric current from a neutral solution
of SnCl_{2} easily oxidises and becomes coated with SnO (Vignon,
1889).

Tin is softer than silver and gold, and is only surpassed by lead in this respect. In addition to this it is very ductile, but its tenacity is very slight, so that wire made from it will bear but little strain. In consequence of its ductility it is easily worked, by forging and rolling into very thin sheets (tin foil), which are used for wrapping many articles to preserve them from moisture, &c. In this case, however, and in many others, lead is mixed with the tin, which, within certain limits, does not alter the ductility. Whilst so soft at the ordinary temperatures tin becomes brittle at 200°, before fusing. Tin powder may be easily obtained if the metal be fused and then stirred whilst cooling. At a white heat tin may be distilled, but with more difficulty than zinc. If molten tin comes into contact with oxygen, it oxidises, forming stannic oxide, SnO_{2}, _and its vapour burns_ with a white flame. _At ordinary temperatures tin does not oxidise_, and this very important property of tin allows it to be applied in many cases for covering other metals to prevent their oxidising. This is termed _tinning_. Iron and copper are frequently tinned. Iron and steel sheets, coated with tin, bear the name of tin plate (for the most part made in England), and are used for numerous purposes. Tin plate is prepared by immersing iron sheets, previously thoroughly cleansed by acid and mechanical means, into molten tin.[34 bis]

[34 bis] If after this the coating of tin be rapidly cooled--for
instance, by dashing water over it--it crystallises into diverse
star-shaped figures, which become visible when the sheets are
first immersed in dilute aqua regia and then in a solution of
caustic soda.

The coating of iron by tin, guards it against the direct access of
air, but it only preserves the iron from oxidation so long as it
forms a perfectly continuous coating. If the iron is left bare in
certain places, it will be powerfully oxidised at these spots,
because the tin is electro-negative with respect to the iron, and
thus the oxidation is confined entirely to the iron in the
presence of tin. Hence a coating of tin over iron objects only
partially preserves them from rusting. In this respect a coating
of zinc is more effectual. However, a dense and invariable alloy
is formed over the surface of contact of the iron and tin, which
binds the coating of tin to the remaining mass of the iron. Tin
may be fused with cast iron, and gives a greyish-white alloy,
which is very easily cast, and is used for casting many objects
for which iron by itself would be unsuitable owing to its ready
oxidisability and porosity. The coating of copper objects by tin
is generally done to preserve the copper from the action of acid
liquids, which would attack the copper in the presence of air and
convert it into soluble salts. Tin is not acted on in this manner,
and therefore copper vessels for the preparation of food should be
tinned.

Tin with copper forms _bronze_, an alloy which is most extensively used in the arts. Bronze has various colours and a variety of physical properties, according to the relative amount of copper and tin which it contains. With an excess of copper the alloy has a yellow colour; the admixture of tin imparts considerable hardness and elasticity to the copper. An alloy containing 78 parts of copper and about 22 per cent. of tin is so elastic that it is used for casting bells, which naturally require a very elastic and hard alloy.[35] For casting statues and various large or small ornamental articles alloys containing 2 to 5 p.c. of tin, 10 to 30 p.c. of zinc, and 65 to 85 p.c. of copper are used.[36] Tin is also often used alloyed with lead, for making various objects--for instance, drinking vessels.

[35] The ancient Chinese alloys, containing about 20 p.c. of tin
(specific gravity of alloys about 8·9), which have been rapidly
cooled, are distinguished for their resonance and elasticity.
These alloys were formerly manufactured in large quantities in
China for the musical instruments known as _tom-toms_. Owing to
their hardness, alloys of this nature are also employed for
casting guns, bearings, &c., and an alloy containing about 11 p.c.
of tin (corresponding with the ratio Cu_{15}Sn) is known as
gun-metal. The addition of a small quantity of phosphorus, up to 2
p.c., renders bronze still harder and more elastic, and the alloy
so formed is now used under the name of phosphor-bronze.

The alloy SnCu_{3} is brittle, of a bluish colour, and has nothing
in common with either copper or tin in its appearance or
properties. It remains perfectly homogeneous on cooling, and
acquires a crystalline structure (Riche). All these signs clearly
indicate that the alloy SnCu_{3} is a product of chemical
combination, which is also seen to be the case from its density,
8·91. Had there been no contraction, the density of the alloy
would be 8·21. It is the heaviest of all the alloys of tin and
copper, because the density of tin is 7·29 and of copper 8·8. The
alloy SnCu_{4}, specific gravity 8·77, has similar properties. All
the alloys except SnCu_{3} and SnCu_{4} split up on cooling; a
portion richer in copper solidifies first (this phenomenon is
termed the _liquation_ of an alloy), but the above two alloys do
not split up on cooling. In these and many similar facts we can
clearly distinguish a _chemical union between the metals_ forming
an alloy. The alloys of tin and copper were known in very remote
ages, before iron was used. The alloys of zinc and tin are less
used, but alloys composed of zinc, tin, and copper frequently
replace the more costly bronze. Concerning the alloys of lead
_see_ Note 46.

[36] An excellent proof of the fact that alloys and solutions are
subject to law is given, amongst others, by the application of
Raoult's method (Chapter I., Note 49) to solutions of different
metals in tin. Thus Heycock and Neville (1889) showed that the
temperature of solidification of molten tin (226°·4) is lowered by
the presence of a small quantity of other metals in proportion to
the concentration of the solution. The following were the
reductions of the temperature of solidification of tin obtained by
dissolving in it atomic proportions of different metals (for
example, 65 parts of zinc in 11,800 parts of tin); Zn 2°·53, Cu
2°·47, Ag 2°·67, Cd 2°·16, Pb 2°·22, Hg 2°·3, Sb 2° [rise], Al
1°·34. As Raoult's method (Chapter VII.) enables the molecular
weight to be determined, the almost perfect identity of the
resultant figures (except for aluminium) shows that the molecules
of copper, silver, lead, and antimony contain _one atom in the
molecule_, like zinc, mercury, and cadmium. They obtained the same
result (1890) for Mg, Na, Ni, Au, Pd, Bi and In. It should here be
mentioned that Ramsay (1889) for the same purpose (the
determination of the molecular weight of metals on the basis of
their mutual solution) took advantage of the variation of the
vapour tension of mercury (_see_ Vol. I., p. 134), containing
various metals in solution, and he also found that the
above-mentioned metals contain but one atom in the molecule.