Chapter XVIII: Silicon and the Other Elements of the Fourth Group (1)

Carbon, which gives the compounds CH_{4}, and CO_{2}, belongs to the fourth group of elements. The nearest element to carbon is silicon, which forms the compounds SiH_{4} and SiO_{2}; its relation to carbon is like that of aluminium to boron or phosphorus to nitrogen. As carbon composes the principal and most essential part of animal and vegetable substances, so is silicon almost an invariable component part of the rocky formations of the earth's crust. Silicon hydride, SiH_{4}, like CH_{4}, has no acid properties, but silica, SiO_{2}, shows feeble acid properties like carbonic anhydride. In a free state silicon is also a non-volatile, slightly energetic non-metal, like carbon. Therefore the form and nature of the compounds of carbon and silicon are very similar. In addition to this resemblance, silicon presents one exceedingly important distinction from carbon: namely, the nature of the higher degree of oxidation. That is, silica, silicon dioxide, or silicic anhydride, SiO_{2} is a solid, non-volatile, and exceedingly infusible substance, very unlike carbonic anhydride, CO_{2}, which is a gas. This expresses the essential peculiarity of silicon. The cause of this distinction may be most probably sought for in the polymeric composition of silica compared with carbonic anhydride. The molecule of carbonic anhydride contains CO_{2}, as seen by the density of this gas. The molecular weight and vapour density of silica, were it volatile, would probably correspond with the formula SiO_{2}, but it might be imagined that it would correspond to a far higher atomic weight of Si_{_n_}O_{2_n_}, principally from the fact that SiH_{4} is a gas like CH_{4}, and SiCl_{4} is a liquid and volatile, boiling at 57°--that is, even lower than CCl_{4}, which boils at 76°. In general, analogous compounds of silicon and carbon have nearly the same boiling points if they are liquid and volatile.[1] From this it might be expected that silicic anhydride, SiO_{2}, would be a gas like carbonic anhydride, whilst in reality silica is a hard non-volatile substance,[1 bis] and therefore it may with great certainty be considered that in this condition it is polymeric with SiO_{2}, as on polymerisation--for instance, when cyanogen passes into paracyanogen, or hydrocyanic acid into cyanuric acid (Chapter IX.)--very frequently gaseous or volatile substances change into solid, non-volatile, and physically denser and more complex substances.[2] We will first make acquaintance with free silicon and its volatile compounds, as substances in which the analogy of silicon with carbon is shown, not only in a chemical but also in a physical sense.[3]

[1] Chloroform, CHCl_{3}, boils at 60°, and silicon chloroform,
SiHCl_{3}, at 34°; silicon ethyl, Si(C_{2}H_{5})_{4}, boils at
about 150°, and its corresponding carbon compound,
C(C_{2}H_{5})_{4}, at about 120°; ethyl orthosilicate,
Si(OC_{2}H_{5})_{4}, boils at 160°, and ethyl orthocarbonate,
C(OC_{2}H_{5})_{4}, at 158°. The specific volumes in a liquid
state--that is, those of the silicon compounds--generally are
slightly greater than those of the carbon compounds; for example,
the volumes of CCl_{4} = 94, SiCl_{4} = 112, CHCl_{3} = 81,
SiHCl_{3} = 82, of C(OC_{2}H_{5})_{4} = 186, and
Si(OC_{2}H_{5})_{4} = 201. The corresponding salts have also nearly
equal specific volumes; for example, CaCO_{3} = 37, CaSiO_{3} = 41.
It is impossible to compare SiO_{2} and CO_{2}, because their
physical states are so widely different.

[1 bis] But silica fuses and volatilises (Moissan) in the heat of the
electric furnace, about 3000°, SiO_{2} is also partially volatile
at the temperature attained in the flame of detonating gas (Cremer,
1892).

[2] A property of intercombination is observable in the atoms of
carbon, and a faculty for intercombination, or polymerisation, is
also seen in the unsaturated hydrocarbons and carbon compounds in
general. In silicon a property of the same nature is found to be
particularly developed in silica, SiO_{2}, which is not the case
with carbonic anhydride. The faculty of the molecules of silica for
combining both with other molecules and among themselves is
exhibited in the formation of most varied compounds with bases, in
the formation of hydrates with a gradually decreasing proportion of
water down to anhydrous silica, in the colloid nature of the
hydrate (the molecules of colloids are always complex), in the
formation of polymeric ethereal salts, and in many other properties
which will be considered in the sequel. Having come to this
conclusion as to the polymeric state of silica since the years
1850-1860, I have found it to be confirmed by all subsequent
researches on the compounds of silica, and, if I mistake not, this
view has now been very generally accepted.

[3] It was only after Gerhardt, and in general subsequently to the
establishment of the true atomic weights of the elements (Chapter
VII.), that a true idea of the atomic weight of silicon and of the
composition of silica was arrived at from the fact that the
molecules of SiCl_{4}, SiF_{4}, Si(OC_{2}H_{5})_{4}, &c., never
contain less than 28 parts of silicon.

The question _of the composition of silica_ was long the subject of
the most contradictory statements in the history of science. In the
last century Pott, Bergmann, and Scheele distinguished silica from
alumina and lime. In the beginning of the present century Smithson
for the first time expressed the opinion that silica was an acid,
and the minerals of rocks salts of this acid. Berzelius determined
the presence of oxygen in silica--namely, that 8 parts of oxygen
were united with 7 of silicon. The composition of silica was first
expressed as SiO (and for the sake of shortness S only was
sometimes written instead). An investigation in the amount of
silica present in crystalline minerals showed that the amount of
oxygen in the bases bears a very varied proportion to the amount of
oxygen in the silica, and that this ratio varies from 2 : 1 to 1 :
3. The ratio 1 : 1 is also met with, but the majority of these
minerals are rare. Other more common minerals contain a larger
proportion of silica, the ratio between the oxygen of the bases and
the oxygen of the silica being equal to 1 : 2, or thereabouts; such
are the augites, labradorites, oligoclase, talc, &c. The higher
ratio 1 : 3 is known for a widely distributed series of natural
silicates--for example, the felspars. Those silicates in which the
amount of oxygen in the bases is equal to that in the silica are
termed _monosilicates_; their general formula will be
(RO)_{2}SiO_{2} or (R_{2}O_{3})_{2}(SiO_{2})_{3}. Those in which
the ratio of the oxygen is equal to 1 : 2 are termed _bisilicates_,
and their general formula will be ROSiO_{2} or
R_{2}O_{3}(SiO_{2})_{3}. Those in which the ratio is 1 : 3 will be
_trisilicates_, and their general formula (RO)_{2}(SiO_{2})_{3} or
(R_{2}O_{3})_{2}(SiO_{2})_{9}.

In these formulæ the now established composition of SiO_{2}--that
is, that in which the atom of Si = 28--is employed. Berzelius, who
made an accurate analysis of the composition of felspar, and
recognised it as a trisilicate formed by the union of potassium
oxide and alumina with silica, in just the same manner as the alums
are formed by sulphuric acid, gave silica the same formula as
sulphuric anhydride--that is, SiO_{3}. In this case the formula of
felspar would be exactly similar to that of the alums--that is,
KAl(SiO_{4})_{2}, like the alums, KAl(SO_{4})_{2}. If the
composition of silica be represented as SiO_{3}, the atom of
silicon must be recognised as equal to 42 (if O = 16; or if O = 8,
as it was before taken to be, Si = 21).

The former formulæ of silica, SiO (Si = 14) and SiO_{3} (Si = 42),
were first changed into the present one, SiO_{2} (Si = 28), on the
basis of the following arguments:--An excess of silica occurs in
nature, and in siliceous rocks free silica is generally found side
by side with the silicates, and one is therefore led to the
conclusion that it has formed acid salts. It would therefore be
incorrect to consider the trisilicates as normal salts of silica,
for they contain the largest proportion of silica; it is much
better to admit another formula with a smaller proportion of oxygen
for silica, and it then appears that the majority of minerals are
normal or slightly basic salts, whilst some of the minerals
predominating in nature contain an excess of silica--that is,
belong to the order of acid salts.

At the present time, when there is a general method (Chapter VII.)
for the determination of atomic weights, the volumes of the
volatile compounds of silica show that its atomic weight Si = 28,
and therefore silica is SiO_{2}. Thus, for example, the vapour
density of silicon chloride with respect to air is, as Dumas showed
(1862), 5·94, and hence with respect to hydrogen it is 85·5, and
consequently its molecular weight will be 171 (instead of 170 as
indicated by theory). This weight contains 28 parts of silicon and
142 parts of chlorine, and as an atom of the latter is equal to
35·5, the molecule of silicon chloride contains SiCl_{4}. As two
atoms of chlorine are equivalent to one of oxygen, the composition
of silica will be SiO_{2}--that is, the same as stannic oxide,
SnO_{2}, or titanic oxide, TiO_{2}, and the like, and also as
carbonic and sulphurous anhydrides, CO_{2} and SO_{2}. But silica
bears but little physical resemblance to the latter compounds,
whilst stannic and titanic oxides resemble silica both physically
and chemically. They are non-volatile, crystalline insoluble, are
colloids, also form feeble acids like silica, &c., and they might
therefore be expected to form analogous compounds, and be
isomorphous with silica, as Marignac (1859) found actually to be
the case. He obtained stannofluorides, for example an easily
soluble strontium salt, SrSnF_{6},2H_{2}O, corresponding with the
already long known silicofluorides, such as SrSiF_{6},2H_{2}O.
These two salts are almost identical in crystalline form
(monoclinic; angle of the prism, 83° for the former and 84° for the
latter; inclination of the axes, 103° 46´ for the latter and 103°
30´ for the former), that is, they are isomorphous. We may here add
that the specific volume of silica in a solid form is 22·6, and of
stannic oxide 21·5.

Free silicon can be obtained in an amorphous or crystalline state. Amorphous silicon is produced, like aluminium, by decomposing the double fluoride of sodium and silicon (sodium silicofluoride) by means of sodium: Na_{2}SiF_{6} + 4Na = 6NaF + Si. By treating the mass thus obtained with water the sodium fluoride may be extracted and the residue will consist of brown, powdery silicon. In order to free it from any silica which might be formed, it is treated with hydrofluoric acid. This silicon powder is not lustrous; when heated it easily ignites, but does not completely burn. It fuses when very strongly heated, and has then the appearance of carbon.[4] Crystalline silicon is obtained in a similar way, but by substituting an excess of aluminium for the sodium: 3Na_{2}SiF_{6} + 4Al = 6NaF + 4AlF_{3} + 3Si. The part of the aluminium remaining in the metallic state dissolves the silicon, and the latter separates from the solution on cooling in a crystalline form. The excess of aluminium after the fusion is removed by means of hydrochloric and hydrofluoric acid. The best silicon crystals are obtained from molten zinc; 15 parts of sodium silicofluoride are mixed with 20 parts of zinc and 4 parts of sodium, and the mixture is thrown into a strongly heated crucible, a layer of common salt being used to cover it; when the mass fuses it is stirred, cooled, treated with hydrochloric acid, and then washed with nitric acid. Silicon, especially when crystalline, like graphite and charcoal, does not in any way act on the above-mentioned acids. It forms black, very brilliant, regular octahedra having a specific gravity of 2·49; it is a bad conductor of electricity, and does not burn even in pure oxygen (but it burns in gaseous fluorine). The only acid which acts on it is a mixture of hydrofluoric and nitric acids; but caustic alkalis dissolve in it like aluminium, with evolution of hydrogen, thus showing its acid character. In general silicon strongly resists the action of reagents, as do also boron and carbon. Crystalline silicon was obtained in 1855 by Deville, and amorphous silicon in 1826 by Berzelius.[4 bis]

[4] A similar form of silicon is obtained by fusing SiO_{2} with
magnesium, when an alloy of Si and Mg is also formed (Gattermann).
Warren (1888) by heating magnesium in a stream of SiF_{4} obtained
silicon and its alloy with magnesium. Winkler (1890) found that
Mg_{5}Si_{3} and Mg_{2}Si are formed when SiO_{2} and Mg are heated
together at lower temperatures, whilst at a high temperature Si
only is formed.

[4 bis] It is very remarkable that silicon decomposes carbonic
anhydride at a white heat, forming a white mass which, after being
treated with potassium hydroxide and hydrofluoric acid, leaves a
very stable yellow substance of the formula SiCO, which is formed
according to the equation, 3Si + 2CO_{2} = SiO_{2} + 2SiCO. It is
also slowly formed when silicon is heated with carbonic oxide. It
is not oxidised when heated in oxygen. A mixture of silicon and
carbon when heated in nitrogen gives the compound Si_{2}C_{2}N,
which is also very stable. On this basis Schützenberger recognises
a group, C_{2}Si_{2}, as capable of combining with O_{2} and N,
like C.

We may add that Troost and Hautefeuille, by heating amorphous
silicon in the vapour of SiCl_{4}, obtained crystalline silicon,
and probably at the same time lower compounds of Si and Cl were
temporarily formed. In the vapour of TiCl_{4} under the same
conditions crystalline titanium is formed (Levy, 1892).

Silicon hydride, SiH_{4}, analogous to marsh gas was obtained first of all in an impure state, mixed with hydrogen, by two methods: by the action of an alloy of silicon and magnesium on hydrochloric acid,[5] and by the action of the galvanic current on dilute sulphuric acid, using electrodes of aluminium, containing silicon. In these cases silicon hydride is set free, together with hydrogen, and the presence of the hydride is shown by the fact that the hydrogen separated ignites spontaneously on coming into contact with the air, forming water and silica. The formation of silicon hydride by the action of hydrochloric acid on magnesium silicide is perfectly akin to the formation of phosphuretted hydrogen by the action of hydrochloric acid on calcium phosphide, to the formation of hydrogen sulphide by the action of acids on many metallic sulphides, and to the formation of hydrocarbons by the action of hydrochloric acid on white cast iron. On heating silicon hydride--that is, on passing it through an incandescent tube, it is decomposed into silicon and hydrogen, just like the hydrocarbons, but the caustic alkalis, although without action on the latter, react with silicon hydride according to the equation: SiH_{4} + 2KHO + H_{2}O = SiK_{2}O_{3} + 4H_{2}.

[5] This alloy, as Beketoff and Cherikoff showed, is easily obtained by
directly heating finely divided silica (the experiment may be
conducted in a test tube) with magnesium powder (Chapter XIV.,
Notes 17, 18). The substance formed, when thrown into a solution of
hydrochloric acid, evolves spontaneously inflammable and impure
silicon hydride, so that the self-inflammability of the gas is
easily demonstrated by this means.

In 1850-60 Wöhler and Buff obtained an alloy of silicon and
magnesium by the action of sodium on a molten mixture of magnesium
chloride, sodium silicofluoride, and sodium chloride. The sodium
then simultaneously reduces the silicon and magnesium.

Friedel and Ladenburg subsequently prepared silicon hydride in a
pure state, and showed that it is not spontaneously inflammable in
air, at the ordinary pressure, but that, like PH_{3}, and like the
mixture prepared by the above methods, it easily takes fire in air
under a lower pressure or when mixed with hydrogen. They prepared
the pure compound in the following manner: Wöhler showed that when
dry hydrochloric acid gas is passed through a slightly heated tube
containing silicon it forms a very volatile colourless liquid,
which fumes strongly in air; this is a mixture of silicon chloride,
SiCl_{4}, and _silicon chloroform_, SiHCl_{3}, which corresponds
with ordinary chloroform, CHCl_{3}. This mixture is easily
separated by distillation, because silicon chloride boils at 57°,
and silicon chloroform at 36°. The formation of the latter will be
understood from the equation Si + 3HCl = H_{2} + SiHCl_{3}. It is
an anhydrous inflammable liquid of specific gravity 1·6. It forms a
transition product between SiH_{4} and SiCl_{4}, and may be
obtained from silicon hydride by the action of chlorine and
SbCl_{5}, and is itself also transformed into silicon chloride by
the action of chlorine. Gattermann obtained SiHCl_{3} by heating
the mass obtained after the action (Note 4) of Mg upon SiO_{2}, in
a stream of chlorine (with HCl) at about 470°. Friedel and
Ladenburg, by acting on anhydrous alcohol with silicon chloroform,
obtained an ethereal compound having the composition
SiH(OC_{2}H_{5})_{3}. This ether boils at 136°, and when acted on
by sodium disengages silicon hydride, and is converted into ethyl
orthosilicate, Si(OC_{2}H_{5})_{4}, according to the equation
4SiH(OC_{2}H_{5})_{3} = SiH_{4} + 3Si(OC_{2}H_{5})_{4} (the sodium
seems to be unchanged), which is exactly similar to the
decomposition of the lower oxides of phosphorus, with the evolution
of phosphuretted hydrogen. If we designate the group C_{2}H_{5},
contained in the silicon ethers by Et, the parallel is found to be
exact:

4PHO(OH)_{2} = PH_{3} + 3PO(OH)_{3};
4SiH(OEt)_{3} = SiH_{4} + 3Si(OEt)_{4}.

_Silicon chloride_, SiCl_{4}, is obtained from amorphous anhydrous silica (made by igniting the hydrate) mixed with charcoal,[6] heated to a white heat in a stream of dry chlorine--that is, by that general method by which many other chloranhydrides having acid properties are obtained. Silicon chloride is purified from free chlorine by distillation over metallic mercury. Free silicon forms the same substance when treated with dry chlorine. It is a volatile colourless liquid, which boils at 59° and has a specific gravity of 1·52. It fumes strongly in air, has a pungent smell, and in general has the characteristic properties of the acid chloranhydrides. It is completely decomposed by water, forming hydrochloric acid and silicic acid, according to the equation: SiCl_{4} + 4H_{2}O = Si(OH)_{4} + 4HCl.[7]

[6] The amorphous silica is mixed with starch, dried, and then charred
by heating the mixture in a closed crucible. A very intimate
mixture of silica and charcoal is thus formed. In Chapter XI., Note
13, we saw that elements like silicon disengage more heat with
oxygen than with chlorine, and therefore their oxygen compounds
cannot be directly decomposed by chlorine, but that this can be
effected when the affinity of carbon for oxygen is utilised to aid
the action. When the mass obtained by the action of Mg upon SiO_{2}
is heated to 300° in a current of chlorine, it easily forms
SiCl_{4} (Gattermann): besides which two other compounds,
corresponding to SiCl_{4}, are formed, namely: Si_{2}Cl_{6}, which
boils at 145° and solidifies at -1°, and Si_{3}Cl_{8}, which boils
at about 212°. These substances, which answer to corresponding
carbon compounds (C_{2}H_{6} and C_{3}H_{8}), act upon water and
form corresponding oxygen compounds; for instance, Si_{2}Cl_{6} +
4H_{2}O = (SiO_{2}H)_{2} + 6HCl gives the analogue of oxalic acid
(CO_{2}H)_{2}. This substance is insoluble in water, decomposes
under the action of friction and heat with an explosion, and should
be called _silico-oxalic acid_, Si_{2}H_{2}O_{4} (_see_ later, Note
11 ^{bis}).

[7] Silicon chloride shows a similar behaviour with alcohol. This is
accompanied by a very characteristic phenomenon; on pouring silicon
chloride into anhydrous alcohol a momentary evolution of heat is
observed, owing to a reaction of double decomposition, but this is
immediately followed by a powerful cooling effect, due to the
disengagement of a large amount of hydrochloric acid--that is,
there is an absorption of heat from the formation of gaseous
hydrochloric acid. This is a very instructive example in this
respect; here two processes occurring simultaneously--one chemical
and the other physical--are divided from each other by time, the
latter process showing itself by a distinct fall in temperature. In
the majority of cases the two processes proceed simultaneously, and
we only observe the difference between the heat developed and
absorbed. In acting on alcohol, silicon chloride forms ethyl
orthosilicate, SiCl_{4} + 4HOC_{2}H_{5} = 4HCl +
Si(OC_{2}H_{5})_{4}. This substance boils at 160°, and has a
specific gravity 0·94. Another salt, ethyl metasilicate,
SiO(OC_{2}H_{5})_{2}, is also formed by the action of silicon
chloride on anhydrous alcohol; it volatilises above 300°, having a
sp. gr. 1·08. It is exceedingly interesting that these two ethereal
salts are both volatile, and both correspond with silica, SiO_{2}:
the first ether corresponds to the hydrate Si(OH)_{4}, orthosilic
acid, and the second to the hydrate SiO(OH)_{2}, metasilicic acid.
As the nature of hydrates may be judged from the composition of
salts, so also, with equal right, can ethereal salts serve the same
purpose. The composition of an ethereal salt corresponds with that
of an acid in which the hydrogen is replaced by a hydrocarbon
radicle--for instance, by C_{2}H_{5}. And, therefore, it may be
truly said that there exist at least the two silicic acids above
mentioned. We shall afterwards see that there are really several
such hydrates; that these ethereal salts actually correspond with
hydrates of silica is clearly shown from the fact that they are
decomposed by water, and that in moist air they give alcohol and
the corresponding hydrate, although the hydrate which is obtained
in the residue always corresponds with the second ethereal salt
only--that is, it has the composition SiO(OH)_{2}; this form
corresponds also to carbonic acid in its ordinary salts. This
hydrate is formed as a vitreous mass when the ethyl silicates are
exposed to air, owing to the action of the atmospheric moisture on
them. Its specific gravity is 1·77.

_Silicon bromide_, SiBr_{4}, as well as silicon bromoform,
SiHBr_{3}, are substances closely resembling the chlorine compounds
in their reactions, and they are obtained in the same manner.
Silicon iodoform, SiHI_{3}, boils at about 220°, has a specific
gravity of 3·4, reacts in the same manner as silicon chloroform,
and is formed, together with silicon iodide, SiI_{4}, by the action
of a mixture of hydrogen and hydriodic acid on heated silicon.
Silicon iodide is a solid at the ordinary temperature, fusing at
about 120°; it may be distilled in a stream of carbonic anhydride,
but easily takes fire in air, and behaves with water and other
reagents just like silicon chloride. It may be obtained by the
direct action of the vapour of iodine on heated silicon. Besson
(1891) also obtained SiCl_{3}I (boils at 113°), SiCl_{2}I_{2}
(172°), and SiClI_{3} (220°), and the corresponding bromine
compounds. All the halogen compounds of Si are capable of absorbing
6NH_{3} and more. Besides which Besson obtained SiSCl_{2} by
heating Si in the vapour of chloride of sulphur; this compound
melts at 74°, boils at 185°, and gives with water the hydrate of
SiO_{2}, HCl, and H_{2}S.

The most remarkable of the haloid compounds of silicon is _silicon fluoride_, SiF_{4}. It is a gaseous substance only liquefied by intense cold, -100°, and is obtained (Chapter XI.) directly by the action of hydrofluoric acid on silica and its compounds (SiO_{2} + 4HF = 2H_{2}O + SiF_{4}), and also by heating fluorspar with silica (2CaF_{2} + 3SiO_{2} = 2CaSiO_{3} + SiF_{4}).[8] In order to prepare silicon fluoride, sand or broken glass is mixed with an equal quantity by weight of fluorspar and 6 parts by weight of strong sulphuric acid, and the mixture is gently heated. It fumes strongly in air, reacting with the aqueous vapours, although it is produced from silica and hydrofluoric acid with the separation of water. It is evident that a reverse reaction occurs here; that is to say, the water reacts with the silicon fluoride, but the reaction is not complete. This phenomenon is similar to that which occurs when water decomposes aluminium chloride, but at the same time hydrochloric acid dissolves aluminium hydroxide and forms the same aluminium chloride. The relative amount of water present (together with the temperature) determines the limit and direction of the reaction. The faculty which silicon fluoride has of reacting with water is so great that it takes up the elements of water from many substances--for instance, like sulphuric acid, it chars paper. Water dissolves about 300 volumes of this gas, but in this case it is not a common dissolution which takes place, but a reaction. During the first absorption of silicon fluoride by water, silicic acid is separated in the form of a jelly, but a certain quantity of the silicon fluoride also remains in the liquid, because the hydrofluoric acid formed dissolves the other part of the silica[9] and forms the so-called _hydrofluosilicic acid_: H_{2}SiF_{6} = SiF_{4} + 2HF = SiH_{2}O_{3} + 6HF - 3H_{2}O. That is to say, a metasilicic acid, SiH_{2}O_{3}, in which O_{3} is replaced by F_{6}. This view of the composition of hydrofluosilicic acid may be admitted, because it forms a whole series of crystallisable and well defined salts. In general, the whole reaction of water on silicon fluoride may be expressed by the equation: 3SiF_{4} + 3H_{2}O = SiO(OH)_{2} + 2SiH_{2}F_{6}. Hydrofluosilicic acid and silicic acid resemble each other as much, and differ as much, in their chemical character as water and hydrofluoric acid. For this reason silicic acid is a feebler acid than hydrofluosilicic acid, and in addition to this the former is insoluble, and the latter soluble, in water.[10] Hydrofluosilicic acid is also formed if silicic acid be dissolved in a solution of hydrofluoric acid. It is incapable of volatilising without decomposition, and on heating the concentrated acid silicon fluoride is evolved, leaving an aqueous solution of hydrofluoric acid. This is the reason why solutions of hydrofluosilicic acid corrode glass. This decomposition may be further accelerated by the addition of sulphuric acid, or even of other acids. Hydrofluosilicic acid, when acting on potassium and barium salts, gives precipitates, because the salts of these metals are but sparingly soluble in water: thus 2KX + H_{2}SiF_{6} = 2HX + K_{2}SiF_{6}. The potassium salt is obtained in the form of very fine octahedra, but the precipitate does not form quickly, and at first appears as a jelly. Nevertheless, the decomposition is complete, and it is taken advantage of for obtaining their corresponding acids from salts of potassium.[10 bis]

[8] This property of calcium fluoride of converting silica into a gas
and a vitreous fusible slag of calcium silicate is frequently taken
advantage of in the laboratory and in practice in order to remove
silica. The same reaction is employed for preparing silicon
fluoride on a large scale in the manufacture of hydrofluosilicic
acid (see sequel).

[9] The amount of heat developed by the solution of silicic acid,
SiO_{2}_n_H_{2}O, in aqueous hydrofluoric acid, _x_HF_n_H_{2}O,
increases with the magnitude of _x_ and normally equals _x_5,600
heat units, where _x_ varies between 1 and 8. However, when _x_ =
10 the maximum amount of heat is developed (= 49,500 units), and
beyond that the amount decreases (Thomsen).

[10] In reality, however, it would seem that the reaction is still more
complex, because the aqueous solution of silicon fluoride does not
yield a hydrate of silica, but a fluo-hydrate (Schiff),
Si_{2}O_{3}(OH)F, corresponding to the (pyro) hydrate
Si_{2}O_{3}(OH)_{2}, equal to SiO(OH)_{2}SiO_{2}, so that the
reaction of silicon fluoride on water is expressed by the
equation: 5SiF_{4} + 4H_{2}O = 3SiH_{2}F_{6} + Si_{2}O_{3}(OH)F +
HF. However, Berzelius states that the hydrate, when well washed
with water, contains no fluorine, which is probably due to the
fact that an excess of water decomposes Si_{2}O_{3}(OH)F, forming
hydrofluoric acid and the compound Si_{2}O_{3}(OH)_{2}. Water
saturated with silicon fluoride disengages silicon fluoride and
hydrofluoric acid when treated with hydrochloric acid, the
gelatinous precipitate being simultaneously dissolved. It may be
further remarked that hydrofluosilicic acid has been frequently
regarded as SiO_{2},6HF, because it is formed by the solution of
silica in hydrofluoric acid, but only two of these six hydrogens
are replaced by metals. On concentration, solutions of the acid
begin to decompose when they reach a strength of 6H_{2}O per
H_{2}SiF_{6}, and therefore the acid may be regarded as
Si(OH)_{4},2H_{2}O,6HF, but the corresponding salts contain less
water, and there are even anhydrous salts, R_{2}SiF_{6}, so that
the acid itself is most simply represented as H_{2}SiF_{6}.

If gaseous silicon fluoride be passed directly into water, the
gas-conducting tube becomes clogged with the precipitated silicic
acid. This is best prevented by immersing the end of the tube
under mercury, and then pouring water over the mercury; the
silicon fluoride then passes through the mercury, and only comes
into contact with the water at its surface, and consequently the
gas-conducting tube remains unobstructed. The silicic acid thus
obtained soon settles, and a colourless solution with a pleasant
but distinctly acid taste is procured.

Mackintosh, by taking 9 p.c. of hydrofluoric acid, observed that
in the course of an hour its action on opal attained 77 p.c. of
the possible, and did not exceed 1-1/2 p.c. of its possible action
on quartz during the same time. This shows the difference of the
structure of these two modifications of silica, which will be more
fully described in the sequel.

[10 bis] The sodium salt is far more soluble in water, and crystallises
in the hexagonal system. The magnesium salt, MgSiF_{6}, and
calcium salt are soluble in water. The salts of hydrofluosilicic
acid may be obtained not only by the action of the acid on bases
or by double decompositions, but also by the action of
hydrofluoric acid on metallic silicates. Sulphuric acid decomposes
them, with evolution of hydrofluoric acid and silicon fluoride,
and the salts when heated evolve silicon fluoride, leaving a
residue of metallic fluoride, R_{2}F_{2}.

Silicon, having so much in common with carbon, is also able to combine with it in the proportion given by the law of substitution, that is, it forms a carbide of silicon CSi, called _carborundum_ and obtained by Mühlhäuser and Acheson in the United States, and by Moissan in France (1891), and others, by reducing silica with carbon in the electrical furnace at a temperature of about 2500°[11], _i.e._ by the action of an electrical current upon a mixture of carbon and SiO_{2} with NaCl. After treating the resultant mass with acids and washing with water, carborundum is obtained in transparent, lustrous grains of a greenish color, possessing great hardness (greater than corundum) and therefore used for polishing the hardest kinds of steel and stones. The specific gravity is about 3·1. Carborundum does not alter at a red heat, does not burn, and apparently approaches the diamond in its properties. (Moissan obtained, 1894, a similar very hard compound for boron, B_{6}C, sp. gr. 2·5.)

[11] _See_ Note 4 bis. Probably Schützenberger had already obtained CSi
in his researches together with other silicon compounds. An
amorphous, less hard compound of the same alloy is also obtained
together with the hard crystalline CSi.

According to the principle of substitution, if silicon forms SiH_{4}, a series of hydrates, or hydroxyl derivatives, ought to exist corresponding to it. The first hydrate of an alcoholic character ought to have the composition SiH_{3}(OH); the second hydrate SiH_{2}(OH)_{2}; the third, SiH(OH)_{3};[11 bis] and the last, Si(OH)_{4}. The last is a hydrate of silica, because it is equal to SiO_{2} + 2H_{2}O); and it is formed by the action of water on silicon chloride, when all four atoms of chlorine are replaced by four hydroxyl groups. It does not, however, remain in this state, but easily loses part of its water.

[11 bis] The following consideration is very important in explaining
the nature of the lower hydrates which are known for silicon. If
we suppose water to be taken up from the first hydrates (just as
formic acid is CH(OH)_{3}, _minus_ water), we shall obtain the
various lower hydrates corresponding with silicon hydride. When
ignited they should, like phosphorous and hypophosphorous acids,
disengage silicon hydride, and leave a residue of silica
behind--_i.e._ of the oxide corresponding to the highest
hydrate--just as organic hydrates (for example, formic acid with
an alkali) form carbonic anhydride as the highest oxygen compound.
Such imperfect hydrates of silicon, or, more correctly speaking,
of silicon hydride, were first obtained by Wöhler (1863) and
studied by Geuther (1865), and were named after their
characteristic colours. (_See_ Note 6).

_Leucone_ is a white hydrate of the composition SiH(OH)_{3}. It is
obtained by slowly passing the vapour of silicon chloroform into
cold water: SiHCl_{3} + 3H_{2}O = SiH(OH)_{3} + 3HCl. But this
hydrate, like the corresponding hydrate of phosphorus or carbon,
does not remain in this state of hydration, but loses a portion of
its water. The carbon hydrate of this nature, CH(OH)_{3}, loses
water and forms formic acid, CHO(OH); but the silicon hydrate
loses a still greater proportion of water, 2SiH(OH)_{3}, parting
with 3H_{2}O, and consequently leaving Si_{2}H_{2}O_{3}. This
substance must be an anhydride; all the hydrogen previously in the
form of hydroxyl has been disengaged, two remaining hydrogens
being left from SiH_{4}. The other similar hydrate is also white,
and has the composition Si_{3}H_{2}O (nearly). It may be regarded
as the above white hydrate + SiO_{2}. A yellow hydrate, known as
_chryseone_ (silicone), is obtained by the action of hydrochloric
acid on an alloy of silicon and calcium; its composition is about
Si_{6}H_{4}O_{3}. Most probably, however, chryseone has a more
complex composition, and stands in the same relation to the
hydrate SiH_{2}(OH)_{3} as leucone does to the hydrate
SiH(OH)_{3}, because this very simply expresses the transition of
the first compound into the second with the loss of water,
SiH_{2}(OH)_{3} - H_{2} + H_{2}O = SiH(OH)_{3}. When these lower
hydrates are ignited without access of air, they are decomposed
into hydrogen, silicon, and silica--that is, it may be supposed
that they form silicon hydride (which decomposes into silicon and
hydrogen) and silica (just as phosphorous and hypophosphorous
acids give phosphoric acid and phosphuretted hydrogen). When
ignited in air, they burn, forming silica. They are none of them
acted on by acids, but when treated with alkalis they evolve
hydrogen and give silicates; for example, leucone: SiH_{2}O_{3} +
4KHO = 2SiK_{2}O_{3} + H_{2}O + 2H_{2}. They have no acid
properties.

Silica or silicic anhydride, both in the free state and in combination with other oxides, enters into the composition of most of the rocky formations of the earth's crust. These silicious compounds are substances varying so much in their properties, crystalline forms, and relations to one another that they are comprised in a special branch of natural science (like the carbon compounds), and are treated of in works on mineralogy; so that, in dealing with them further, we shall only give a short description of these various compounds. It is first of all necessary to turn to the description of silica itself, especially as it is not unfrequently met with in nature in a separate state, and often forms whole masses of rocky formations, called 'quartz.' In an anhydrous condition silica appears in the greatest variety of natural forms--sometimes in well-formed crystals, hexagonal prisms, terminated by hexagonal pyramids. If the crystals are colourless and transparent, they are called _rock crystal_. This is the purest form of silica. Prismatic crystals of rock crystal sometimes attain considerable size, and as they are remarkable for their unchangeability, great hardness, and high index of refraction, they are used for ornaments, for seals, making necklaces, &c.[12] Rock crystal coloured with organic matter in contact with which it has been produced has a brown or greyish colour, and then bears the name of _cairngorm_ or _smoky quartz_. In this form it has the same uses as rock crystal, especially as it is often found in large masses. The same mineral, frequently occurs, coloured red or pink by manganese or iron oxides, especially in aqueous formations, and is then known as _amethyst_. When finely coloured the amethyst is used as a precious stone, but amethysts most frequently occur as small crystals in the cavities formed in other rocky formations, and especially in those formed in silica itself. A similar anhydrous silica is often found in transparent non-crystalline masses, having the same specific gravity as rock crystal itself (2·66). In this case it is called _quartz_. Sometimes it forms complete rocky formations, but more often penetrates or is interspersed through other rocky formations, together with other siliceous compounds. Thus, in granite, quartz is mixed with felspar and similar substances. Sometimes the colouring of quartz is so considerable that it is hardly transparent in thin sheets, but it is often found in transparent masses slightly coloured with various tints. The existence in nature of enormous masses of quartz proves that it resists the action of water. When water destroys rocky formations, the siliceous minerals which they contain are partly dissolved and partly transformed into clay, &c. But the quartz remains untouched, in the form of grains in which it existed in the rocky formation; sometimes, when crushed, it is carried away by the water and deposited. This is the nature of _sand_. Naturally, sometimes other rocky substances which are not changed by water, or only slightly acted on by it, are found in sand; but as these latter are more or less changed by the continuous action of water, it is not unusual to find sand which consists almost entirely of pure quartz. Common sand is generally coloured yellow or reddish-brown by foreign mineral matter, consisting principally of ferruginous minerals and clays. The purest or so-called quartz sand is, however, rarely found, and is recognised by the absence of colour, and also by the test that when shaken in water it does not form any turbidity: this shows the absence of clay; when fused with bases it forms a colourless glass, and on this account is a valuable material for the manufacture of glass. Sands were formed at all periods of the earth's existence; the ancient ones, compressed by strata of more recent formation and permeated with various substances (deposited from the infiltrating water), are sometimes solidified into rock, called _sandstone_, composing, in some places, whole mountain chains, and serviceable as a most excellent building material, on account of the slight change it undergoes under the influence of atmospheric agencies, and on account of the facility with which it may be wrought from rocky formations into immense regularly-shaped flags--the latter property is due to the primary laminar structure of the sand formations deposited, as above-mentioned, by water. Many grindstones and whetstones are made from such rocks.

[12] Two modifications of rock crystal are known. They are very easily
distinguished from each other by their relation to polarised
light; one rotates the plane of polarisation to the right and the
other to the left--in the one the hemihedral faces are right and
in the other they are left; this opposite rotatory power is taken
advantage of in the construction of polarisers. But, with this
physical difference--which is naturally dependent on a certain
difference in the distribution of the molecules--there is not only
no observable difference in the chemical properties, but not even
in the density of the mass. Perfectly pure rock crystal is a
substance which is most invariable with respect to its specific
gravity. The numerous and accurate determinations made by
Steinheil on the specific gravity of rock crystal show that (if
the crystal be free from flaws) it is very constant and is equal
to 2·66.

Perfectly pure anhydrous silica is not only known in the condition of rock crystal and quartz having a specific gravity of 2·6, but also in another special form, having other chemical and physical properties. This variety of silica has a specific gravity of 2·2, and is formed by fusing rock crystal or heating silicic acid.[12 bis] Silicic acid, when heated to a dull red heat, parts entirely with the water it contains, and leaves an exceedingly fine amorphous mass of silica (easily levigated, but difficult to moisten); it is characterised by such excessive friability that, when lightly blown on, a large mass of it rises into the air like a cloud of dust. A mass of anhydrous silica maybe poured in this way from one vessel to another like a liquid, and like the latter it takes a horizontal position in the vessel containing it.[13] Anhydrous silica, like quartz, does not fuse in the heat of a furnace, but it fuses in the oxyhydrogen flame to a colourless glassy mass exactly similar to that formed in the same way from rock crystal. In this condition silica has a specific gravity of 2·2.[13 bis] Both forms of silica are insoluble in ordinary acids, and even when they are in the state of powder, alkalis in solution act very slowly and feebly on them; rock crystal offers much greater resistance to the action of alkalis than the powder obtained by heating the hydrate. The latter is quite soluble, although but slowly, in hot alkaline solutions. This last property appertains in a greater degree to anhydrous silica having a specific gravity of 2·2 than to that which has a specific gravity of 2·6. Hydrofluoric acid more easily transforms the former into silicon fluoride than it does the latter. Both varieties of silica, when taken in the form of powder, easily combine with bases, forming, on being fused with an alkali, a vitreous slag, which is a salt corresponding with silica. Glass is such a salt, formed of alkalis and alkaline earthy bases; if the glass does not contain any of the latter--that is, if only alkaline glass be taken--a mass soluble in water is obtained. In order to obtain such _soluble glass_, potassium or sodium carbonates, or better a mixture of the two (fusion mixture), is fused with fine sand. A still better and further saturation of the alkalis with silica is effected by the action of alkaline solutions on the silicon hydrate met with in nature; for instance, an alkaline solution is often made use of to act on the so-called _tripoli_, or collection of siliceous skeletons of the lowest microscopical infusoria, which is sometimes found in considerable layers in the form of a sandy mass. Tripoli is used for polishing, not only on account of the considerable hardness of the silica, but also because the microscopic bodies of the infusoria have a pointed shape, which, however, is not angular, so that they do not scratch metals like sand.[14] The alkaline solutions of silica obtained by boiling tripoli with caustic soda under pressure contain various proportions of silica and alkali.[14 bis] In order that it may contain the greatest amount of silica, silicic acid should be added to the heated solution. Silicic acid is formed by taking any solution containing silica and alkali, and adding to it, by degrees, some acid--for instance, sulphuric or hydrochloric; if the experiment be carried on carefully and the solution be concentrated, the whole mass thickens to a jelly, due to the gelatinous form of the _silicic acid_ separated from the salt by the action of the acid. The decomposition may be expressed by the following equation: Si(ONa)_{4} + 4HCl = 4NaCl + Si(OH)_{4}. The hydrate separated, Si(OH)_{4}, easily loses part of the water and forms a jelly, the whole mass gelatinising if the solution be strong enough.[15]

[12 bis] Several other modifications are known as minute crystals. For
example, there is a particular mineral first found in Styria and
known as _tridymite_. Its specific gravity 2·3 and form of
crystals clearly distinguish it from rock crystal; its hardness is
the same as that of quartz--that is, slightly below that of the
ruby and diamond.

[13] There is a distinct rise of temperature (about 4°) when amorphous
silica is moistened with water. Benzene and amyl alcohol also give
an observable rise of temperature. Charcoal and sand give the same
result, although to a less extent.

[13 bis] Silica also occurs in nature in two modifications. The opal
and tripoli (infusorial earth) have a specific gravity of about
2·2, and are comparatively easily soluble in alkalis and
hydrofluoric acid. Chalcedony and flint (tinted quartzose
concretions of aqueous origin), agate and similar forms of silica
of undoubted aqueous origin, although still containing a certain
amount of water, have a specific gravity of 2·6, and correspond
with quartz in the difficulty with which they dissolve. This form
of silica sometimes permeates the cellulose of wood, forming one
of the ordinary kinds of petrified wood. The silica may be
extracted from it by the action of hydrofluoric acid, and the
cellulose remains behind, which clearly shows that silica in a
soluble form (see sequel) has permeated into the cells, where it
has deposited the hydrate, which has lost water, and given a
silica of sp. gr. 2·6. The quartzose stalactites found in certain
caves are also evidently of a similar aqueous origin; their sp.
gr. is also 2·6. As crystals of amethyst are frequently found
among chalcedonies, and as Friedau and Sarrau (1879) obtained
crystals of rock crystal by heating soluble glass with an excess
of hydrate of silica in a closed vessel, there is no doubt but
that rock crystal itself is formed in the wet way from the
gelatinous hydrate. Chroustchoff obtained it directly from soluble
silica. Thus this hydrate is able to form not only the variety
having the specific gravity 2·2 but also the more stable variety
of sp. gr. 2·6; and both exist with a small proportion of water
and in a perfectly anhydrous state in an amorphous and crystalline
form. All these facts are expressed by recognising silica as
dimorphous, and their cause must be looked for in a difference in
the degree of polymerisation.

[14] Deposits of perfectly white tripoli have been discovered near
Batoum, and might prove of some commercial importance.

[14 bis] Alkaline solutions, saturated with silica and known as _soluble
glass_, are prepared on a large scale for technical purposes by
the action of potassium (or sodium) hydroxide in a steam boiler on
tripoli or infusorial earth, which contains a large proportion of
amorphous silica. All solutions of the alkaline silicates have an
alkaline reaction, and are even decomposed by carbonic acid. They
are chiefly used by the dyer, for the same purposes as sodium
aluminate, and also for giving a hardness and polish to stucco and
other cements, and in general to substances which contain lime. A
lump of chalk when immersed in soluble glass, or better still when
moistened with a solution and afterwards washed in water (or
better in hydrofluosilicic acid, in order to bind together the
free alkali and make it insoluble), becomes exceedingly hard,
loses its friability, is rendered cohesive, and cannot be
levigated in water. This transformation is due to the fact that
the hydrate of silica present in the solution acts upon the lime,
forming a stony mass of calcium silicate, whilst the carbonic acid
previously in combination with the lime enters into combination
with the alkali and is washed away by the water.

[15] The equation given above does not express the actual reaction, for
in the first place silica has the faculty of forming compounds
with bases, and therefore the formula SiNa_{4}O_{4} is not rightly
deduced, if one may so express oneself. And, in the second place,
silica gives several hydrates. In consequence of this, the hydrate
precipitated does not actually contain so high a proportion of
water as Si(OH)_{4}, but always less. The insoluble gelatinous
hydrate which separates out is able (before, but not after, having
been dried) to dissolve in a solution of sodium carbonate. When
dried in air its composition corresponds with the ordinary salts
of carbonic acid--that is, SiH_{2}O_{3}, or SiO(OH)_{2}. If
gradually heated it loses water by degrees, and, in so doing,
gives various degrees of combination with it. The existence of
these degrees of hydration, having the composition
SiH_{2}O_{3}_n_SiO_{2}, or, in general, _n_SiO_{2}_m_H_{2}O, where
_m_ < _n_, must be recognised, because most varied degrees of
combination of silica with bases are known. The hydrate of silica,
when not dried above 30°, has a composition of nearly
H_{4}Si_{3}O_{8} = (H_{2}SiO_{3})_{2}SiO_{2}, but at 60° contains
a greater proportion of silica--that is, it loses still more
water; and at 100° a hydrate of the composition
SiH_{2}O_{3}2SiO_{2}, and at 250° a hydrate having approximately a
composition SiH_{2}O_{3}7SiO_{2} is obtained.

These data show the complexity of the molecules of anhydrous
silica. The hydrates of silica easily lose water and give the
hydrates (SiO_{2})_{_n_}(H_{2}O)_{_m_}, where _m_ becomes smaller
and smaller than _n_. In the natural hydrates, this decrement of
water proceeds quite consecutively, and, so to say, imperceptibly,
until _n_ becomes incomparably greater than _m_, and when the
ratio becomes very large, anhydrous silica of the two
modifications 2·6 and 2·2 is obtained. The composition
(SiO_{2})_{10},H_{2}O still corresponds with 2·9 p.c. of water,
and natural hydrates often contain still less water than this.
Thus some opals are known which contain only 1 p.c. of water,
whilst others contain 7 and even 10 p.c. As the artificially
prepared gelatinous hydrate of silica when dried has many of the
properties of native opals, and as this hydrate always loses water
easily and continually, there can be no doubt that the transition
of (SiO_{2})_{_n_}(H_{2}O)_{_m_} into anhydrous silica, both
amorphous and crystalline (in nature, chalcedony), is accomplished
gradually. This can only be the case if the magnitude of _n_ be
considerable, and therefore the molecule of silica in the hydrate
is undoubtedly complex, and hence the anhydrous silica of sp. gr.
2·2 and 2·6 does not contain SiO_{2}, but a complex molecule,
Si_{_n_}O_{2_n_}--that is, the structure of silica is polymeric
and complex, and not simple as represented above by the formula
SiO_{2}.

Neither of the two varieties of anhydrous silica, nor the various natural gelatinous hydrates, are directly soluble in water. There is, however, a condition of silica known which is soluble in water, _soluble silica_, and silica is found in this state in nature. Small quantities of soluble silica are met with in all waters. Certain mineral springs, and especially hot springs--of which the best known are the Geysers of Iceland and those in the North American National Park (Yellowstone Valley)--contain a considerable amount of silica in solution. Such water, permeating the objects it meets with--for instance, wood--penetrates into them and deposits silica inside them, that is, transforms them into a petrified condition. Siliceous stalactites, and also many (if not all) forms of silica are formed by such water. The absorption of silica by plants by means of their roots, and also by the lower organisms having siliceous bodies, is due also to their nourishing themselves with the solutions containing silica continually formed in nature. Thus, in plants, in the straws of the grasses, in hard shave-grass, and especially in the knots of bamboo and other straw-like plants, a considerable quantity of silica is deposited, which must previously have been absorbed by the plants.

Silicic acid is a colloid. The gelatinous silicon hydrate is its hydrogel, the soluble hydrate is the hydrosol (Chapter XII.) Both varieties may be easily obtained from the alkaline silicates and from water-glass. The very same substances--that is, aqueous solutions of soluble glass and acid--taken in the same proportion, may produce either the gelatinous or the soluble silica, according to the way these solutions are mixed together. If the acid be added little by little to the _alkaline silicate_, with continuous stirring, a moment arrives when the whole mass thickens to a jelly, hydrogel; in this case the silicic acid is formed in the midst of the alkaline solution and becomes insoluble. But if the mixing be done in the reverse order--that is, if the soluble glass be added to the acid, or if a quantity of acid be rapidly poured into the solution of the salt--then the separation of the silica takes place in the midst of the acid liquid, and it is obtained in the form of the soluble hydrate, the hydrosol.[16]

[16] The presence of an excess of acid aids the retention of the silica
in the solution, because the gelatinous silica obtained in the
above manner, but not heated to 60°--that is, containing more
water than the hydrate H_{2}SiO_{3}--is more soluble in water
containing acid than in pure water. This would seem to indicate a
feeble tendency of silica to combine with acids, and it might even
have been imagined that in such a solution the hydrate of silica
is held in combination by an excess of acid, had Graham not
obtained soluble silica perfectly free from acid, and if there
were not solutions of silica free from any acid in nature. At all
events a tolerably strong solution of free silica or silicic acid
may be obtained from soluble glass diluted with water. The
solution, besides silica, will contain sodium chloride and an
excess of the acid taken. If this solution remains for some time
exposed to the air, or in a closed vessel, and under various other
conditions, it is found that, after a time, insoluble gelatinous
silica separates out--that is, the soluble form of silica is
unstable, like the soluble form of alumina. The analogous forms of
molybdic or tungstic acids may be heated, evaporated, and kept for
a long period of time without the soluble form being converted
into the insoluble.