Chapter XX: Sulphur, Selenium, and Tellurium (5)

A remarkable example[74 bis] of the thio-compounds is found in _thiocyanic acid_--_i.e._ cyanic acid in which the oxygen is replaced by sulphur, HCNS. We know (Chapter IX.) that with oxygen the cyanides of the alkaline metals RCN give cyanates RCNO; but they also combine with sulphur, and therefore if yellow prussiate of potash be treated as in the preparation of potassium cyanide, and sulphur be added to the mass, potassium thiocyanate, KNCS, is obtained in solution. This salt is much more stable than potassium cyanate; it dissolves without change in water and alcohol, forming colourless solutions from which it easily crystallises on evaporation. It may be kept exposed to air even when in solution; in dissolving in water it absorbs a considerable amount of heat, and forms a starting-point for the preparation of all the thiocyanates, RCNS, and organic compounds in which the metals are replaced by hydrocarbon groups. Such, for example, is volatile mustard oil, C_{3}H_{5}CSN (allyl thiocyanate),[75] which gives to mustard its caustic properties. With ferric salts the thiocyanates give an exceedingly brilliant red coloration, which serves for detecting the smallest traces of ferric salts in solution. Thiocyanic acid, HCNS, may be obtained by a method of double decomposition, by distilling potassium thiocyanate with dilute sulphuric acid. It is a volatile colourless liquid, having a smell recalling that of vinegar, is soluble in water, and may be kept in solution without change.[75 bis]

[74 bis] Thorpe and Rodger (1889), by heating a mixture of lead
fluoride and phosphorus pentasulphide to 250° in an atmosphere of
dry nitrogen, obtained gaseous _phosphorus fluosulphide_, or
_thiophosphoryl fluoride_, PSF_{3}, corresponding with POCl_{3}.
This colourless gas is converted into a colourless liquid by a
pressure of eleven atmospheres; it does not act on dry mercury,
and takes fire spontaneously in air or oxygen, forming phosphorus
pentafluoride, phosphoric anhydride, and sulphurous anhydride. It
is soluble in ether, but is decomposed by water: PSF_{3} + 4H_{2}O
= H_{2}S + H_{3}PO_{4} + 3HF (Note 20).

[75] Although mustard oil may be obtained from the thiocyanates, it is
only an isomer of allyl thiocyanate proper, as is explained in
Organic Chemistry.

[75 bis] Sulphur can only replace half the oxygen in CO_{2}, as is seen
in _carbon oxysulphide_, or monothiocarbonic anhydride COS. This
substance was obtained by Than, and is formed in many reactions. A
certain amount is obtained if a mixture of carbonic oxide and the
vapour of sulphur be passed through a red-hot tube. When carbon
tetrachloride is heated with sulphurous anhydride, this substance
is also formed; but it is best obtained in a pure form by
decomposing potassium thiocyanate with a mixture of equal volumes
of water and sulphuric acid. A gas is then evolved containing a
certain amount of hydrocyanic acid, from which it may be freed by
passing it over wool containing moistened mercuric oxide, which
retains the hydrocyanic acid. The reaction is expressed by the
equation: 2KCNS + 2H_{2}SO_{4} + 2H_{2}O = K_{2}SO_{4} +
(NH_{4})_{2}SO_{4} + 2COS. It is also formed by passing the vapour
of carbon bisulphide over alumina or clay heated to redness
(Gautier; silicon sulphide is then formed). COS is also formed by
passing phosgene over a long layer of asbestos mixed with cadmium
sulphide at 270°; CdS + COCl_{3} = CdCl_{2} + COS (Nuricsán,
1892). The pure gas has an aromatic odour, is soluble in an equal
volume of water, which, however, acts on it, so that it must be
collected over mercury. When slightly heated, carbon oxysulphide
decomposes into sulphur and carbonic oxide. It burns in air with a
pale blue flame, explodes with oxygen, and yields potassium
sulphide and carbonate with potassium hydroxide: COS + 4KHO =
K_{2}CO_{3} + K_{2}S + 2H_{2}O.

The sulphur compounds of chlorine Cl_{2}S and Cl_{2}S_{2} may be regarded on the one hand as products of the metalepsis of the sulphides of hydrogen, H_{2}S and H_{2}S_{2}; and on the other hand of the oxygen compounds of chlorine, because chloride of sulphur, Cl_{2}S, resembles chlorine oxide, Cl_{2}O, whilst Cl_{2}S_{2} corresponds with the higher oxide of chlorine; or thirdly, we may see in these compounds the type of the acid chloranhydrides, because they are all decomposed by water, forming hydrochloric acid, and sulphur tetrachloride, SCl_{4}, is decomposed with the formation of sulphurous anhydride.[76]

[76] There is no reason for seeing any contradiction or mutual
incompatibility in these three views, because every analogy is
more or less modified by a change of elements. Thus, for instance,
it cannot be expected that the product of the metalepsis of
hydrogen sulphide would resemble the corresponding products of
water in all respects, because water has not the acid properties
of hydrogen sulphide. In the days of dualism and electrical
polarity it was supposed that the sulphur varied in its nature: in
hydrogen sulphide or potassium sulphide it was considered to be
negative, and in sulphurous anhydride or sulphur dichloride
positive. It then appeared evident that sulphur dichloride would
have no point of analogy with potassium sulphide. But metalepsis,
or its expression in the law of substitution, necessitates such
opinions being laid aside. If we can compare CO_{2}, CH_{4},
CCl_{4}, CHCl_{3}, CH_{3}(OH) with each other, we cannot recognise
any difference in the sulphur in SH_{2}, SCl_{2}, SK_{2}, or in
general SX_{2}, for otherwise we should have to acknowledge as
many different states of sulphur, carbon, or hydrogen as there are
compounds of sulphur, carbon, or hydrogen. The essential truth of
the matter is that all the elements in a molecule play their part
in the reactions into which it enters. Often this appears to be
contradicted in the result--for example, hydrogen alone may be
replaced; but it is not this hydrogen alone that has determined
the reaction; all the elements present have participated in it.
This may be made clearer by the following rough illustration.
Supposing two regiments of soldiers were fighting against each
other, and that several men were lost by one of the regiments; no
one could say that it was only these men who took part in the
engagement. The other men fired and the bullets flew over the
heads of their opponents. It was not only those who fell who
fought, although they only were removed from the field of battle;
the fighting proceeded among the masses, but only those few were
disabled who went forward and were more conspicuous &c.; not that
the remainder did not take part in the action; they also fought
and were an object of attack, only they remained sound and unhurt.
Hydrogen is lighter than other elements and its atoms more mobile;
it subjects itself more frequently and easily to reactions; but it
is not it alone which reacts, it is even less liable to attack
than other elements. It participates in exceedingly diverse
reactions, not indeed because the hydrogen itself varies, but
because one atom of it puts itself forward, another is hidden, one
is united with carbon, another feebly held by sulphur, one stands
or moves in the neighbourhood of oxygen, another is joined to a
hydrocarbon. All hydrogen atoms are equal, and equally serve as an
object of attack for the atoms of molecules encountering them, but
those only are removed from the sphere of action which are nearer
the surface of a molecule, which are more mobile, or held by a
less sum of forces. So also sulphur is one and the same in sulphur
dichloride, in sulphurous or sulphuric anhydride, in hydrogen
sulphide, in potassium sulphide, but it reacts differently, and
those elements which are with it also vary in their reactions
because they are with it, and it varies its reactions because it
is with them. It is possible to seize on a character common to
substances quantitatively and qualitatively analogous to each
other. It may be admitted that an element in certain forms is not
able to enter into reactions into which in other forms it enters
willingly, if only the requisite conditions are encountered; but
it must not therefore be concluded that an element changes its
essential quality in these different cases. The preceding remarks
touch on questions which are subject to much argument among
chemists, and I mention them here in order to show the treatment
of those most important problems of chemistry which lie at the
basis of this treatise.

The compounds of sulphur with chlorine are prepared in the apparatus depicted in fig. 91. As sulphur chloride is decomposed by water, the chlorine evolved in the flask C must be dried before coming into contact with the sulphur. It is therefore first passed through a Woulfe's bottle, B, containing sulphuric acid, and then through the cylinder D containing pumice stone moistened with sulphuric acid, and then led into the retort E, in which the sulphur is heated. The compound which is formed distils over into the receiver R. A certain amount of sulphur passes over with the sulphur chloride, but if the resultant distillate be re-saturated with chlorine and distilled no free sulphur remains, the boiling-point rises to 144°, and pure sulphur chloride, S_{2}Cl_{2}, is obtained. It has this formula because its vapour density referred to hydrogen is 68. It is also obtained by heating certain metallic chlorides (stannous, mercuric) with sulphur; both the metal and chlorine then combine with the sulphur. Sulphur chloride is a yellowish-brown liquid, which boils at 144°, and has a specific gravity of 1·70 at 0°. It fumes strongly in the air, reacting on the moisture contained therein, and has a heavy chloranhydrous odour. It dissolves sulphur, is miscible with carbon bisulphide, and falls to the bottom of a vessel containing water, by which it is decomposed, forming sulphurous anhydride and hydrochloric acid; but it first forms various lower stages of oxidation of sulphur, because the addition of silver nitrate to the solution gives a black precipitate. With hydrogen sulphide it gives sulphur and hydrochloric acid, and it reacts directly with metals--especially arsenic, antimony, and tin--forming sulphides and chlorides. In the cold, it absorbs chlorine and gives _sulphur dichloride_, SCl_{2}. The entire conversion into this substance requires the prolonged passage of dry chlorine through sulphur chloride surrounded by a freezing mixture. The distillation of the dichloride must be conducted in a stream of chlorine, as otherwise it partially decomposes into sulphur chloride and chlorine. Pure sulphur dichloride is a reddish-brown liquid, which resembles the lower chloride in many respects; its specific gravity is 1·62; its odour is more suffocating than that of sulphur chloride; it volatilises at 64°.[77]

[77] The observed vapour density of sulphur dichloride referred to
hydrogen is 53·3, and that given by the formula is 51·5. The
smaller molecular weight explains its boiling point being lower
than that of sulphur chloride, S_{2}Cl_{2}. The reactions of both
these compounds are very similar. Sulphur converts the dichloride,
SCl_{2}, into the monochloride, S_{2}Cl_{2}. In one point the
dichloride differs distinctly from the monochloride--that is, in
its capacity for easily giving up chlorine and decomposing. Even
light decomposes it into chlorine and the monochloride. Hence it
acts on many substances in the same manner as chlorine, or
substances which easily part with the latter, such as phosphoric
or antimonic chloride. In distinction to these, however, sulphur
dichloride would appear to distil without any considerable
decomposition, judging by the vapour density. But this is not a
valid conclusion, for if there be a decomposition, then 2SCl_{2} =
S_{2}Cl_{2} + Cl_{2}; now the density of sulphur chloride = 67·5,
and of chlorine = 35·5, and consequently a mixture of equal
volumes of the two = 51·5, just the same as an equal volume of
sulphur dichloride. _Therefore the distillation of sulphur
dichloride is probably nothing but its decomposition._ Hence the
compound SCl_{2}, which is stable at the ordinary temperature,
decomposes at 64°. In the cold it absorbs a further amount of
chlorine, corresponding to SCl_{4}, but even at -10° a portion of
the absorbed chlorine is given off--that is, dissociation takes
place. Thus the tetrachloride is even less stable than the
dichloride.

_Thionyl chloride_, SOCl_{2}, may be regarded as oxidised sulphur dichloride; it corresponds with sulphur chloride, S_{2}Cl_{2}, in which one atom of sulphur is replaced by oxygen. At the same time it is chlorine oxide (hypochlorous anhydride, Cl_{2}O) combined with sulphur, and also the chloranhydride of sulphurous acid--that is, SO(HO)_{2}, in which the two hydroxyl groups are replaced by two atoms of chlorine, or sulphurous anhydride, SO_{2}, in which one atom of oxygen is replaced by two atoms of chlorine. All these representations are confirmed by reactions of formation, or decompositions; they all agree with our notions of the other compounds of sulphur, oxygen, and chlorine; hence these definitions are not contradictory to each other. Thus, for instance, thionyl chloride was first obtained by Schiff, by the action of dry sulphurous anhydride on phosphorus pentachloride. On distilling the resultant liquid, thionyl chloride comes over first at 80°, and on continuing the distillation phosphorus oxychloride distils over at above 100°, PCl_{5} + SO_{2} = POCl_{3} + SOCl_{2}. This mode of preparation is direct evidence of the oxychloride character of SOCl_{2}. Würtz obtained the same substance by passing a stream of chlorine oxide through a cold solution of sulphur in sulphur chloride; the chlorine oxide then combined directly with the sulphur, S + Cl_{2}O = SOCl_{2}, whilst the sulphur chloride remained unchanged (sulphur cannot be combined directly with chlorine oxide, as an explosion takes place). Thionyl chloride is a colourless liquid, with a suffocating acrid smell; it has a specific gravity at 0° of 1·675, and boils at 78°. It sinks in water, by which it is immediately decomposed, like all chloranhydrides--for example, like carbonyl chloride, which corresponds with it: SOCl_{2} + H_{2}O = SO_{2} + 2HCl.[77 bis]

[77 bis] Hartog and Sims (1893) obtained thionyl bromide, SOBr_{2}, by
treating SOCl_{2} with sodium bromide; it is a red liquid, sp. gr.
2·62, and decomposes at 150°.

Normal _sulphuric acid has two corresponding chloranhydrides_; the first, SO_{2}(OH)Cl, is sulphuric acid, SO_{2}(HO)_{2}, in which one equivalent of HO is replaced by chlorine; the second has the composition SO_{2}Cl_{2}--that is, two HO groups are substituted by two of chlorine. The second chloranhydride, or the compound SO_{2}Cl_{2}, is called sulphuryl chloride, and the first chloranhydride, SO_{2}HOCl, may be called chlorosulphonic acid, because it is really an acid; it still retains one hydroxyl of sulphuric acid, and its corresponding salts are known. Thus, potassium chloride absorbs the vapour of sulphuric anhydride, forming a salt, SO_{3}KCl, corresponding with SO_{3}HCl as acid. In acting on sodium chloride it forms hydrochloric acid and the salt NaSO_{3}Cl. This first chloranhydride of sulphuric acid, SO_{2}HOCl, discovered by Williamson, is obtained either by the action of phosphorus pentachloride on sulphuric acid (PCl_{5} + H_{2}SO_{4} = POCl_{3} + HCl + HSO_{3}Cl), or directly by the action of dry hydrochloric acid on sulphuric anhydride, SO_{3} + HCl = HSO_{3}Cl. The most easy and rapid method of its formation is by direct saturation of cold Nordhausen acid with dry hydrochloric acid gas (SO_{3} + HCl = HSO_{3}Cl), and distillation of the resultant solution; the distillate then contains HSO_{3}Cl. It is a colourless fuming liquid, having an acrid odour; it boils at 153° (according to my determination, confirmed by Konovaloff), and its specific gravity at 19° is 1·776. It is immediately decomposed by water, forming hydrochloric and sulphuric acids, as should be the case with a true chloranhydride. In the reactions of this chloranhydride we find the easiest means of introducing the sulphonic group HSO_{3} into other compounds, because it is here combined with chlorine. The second chloranhydride of sulphuric acid, or _sulphuryl chloride_, SO_{2}Cl_{2}, was obtained by Regnault by the direct action of the sun's ray on a mixture of equal volumes of chlorine and sulphurous oxide. The gases gradually condense into a liquid, combining together as carbonic oxide does with chlorine. It is also obtained when a mixture of the two gases in acetic acid is allowed to stand for some time. The first chloranhydride, SO_{3}HCl, decomposes when heated at 200° in a closed tube into sulphuric acid and sulphuryl chloride. It boils at 70°, its specific gravity is 1·7, it gives hydrochloric and sulphuric acids with water, fumes in the air, and, judging by its vapour density, does not decompose when distilled.[78]

[78] Pyrosulphuryl chloride, S_{2}O_{5}Cl_{2}. See Note 44. Thorpe and
Kirman, by treating SO_{3} with HF, obtained SO_{2}(OH)F, as a
liquid boiling at 163°, but which decomposed with greater facility
and then gave SO_{2}F_{2}.

The acids of sulphur naturally have their corresponding ammonium
salts, and the latter their amides and nitriles. It will be
readily understood how vast a field for research is presented by
the series of compounds of sulphur and nitrogen, if we only
remember that to carbonic and formic acids there corresponds, as
we saw (Chapter IX.), a vast series of derivatives corresponding
with their ammonium salts. To sulphuric acid there correspond two
ammonium salts, SO_{2}(HO)(NH_{4}O) and SO_{2}(NH_{4}O)_{2}; three
amides: the acid amide SO_{2}(HO)(NH_{2}), or sulphamic acid, the
normal saline compound SO_{2}(NH_{4}O)(NH_{2}), or ammonium
sulphamate, and the normal amide SO_{2}(NH_{2})_{2}, or sulphamide
(the analogue of urea); then the acid nitrile, SON(HO), and two
neutral nitriles, SON(NH_{2}) and SN_{2}. There are similar
compounds corresponding with sulphurous acid, and therefore its
nitriles will be, an acid, SN(HO), its salt, and the normal
compound, SN(NH_{2}). Dithionic and the other acids of sulphur
should also have their corresponding amides and nitriles. Only a
few examples are known, which we will briefly describe. Sulphuric
acid forms salts of very great stability with ammonia, and
ammonium sulphate is one of the commonest ammoniacal compounds. It
is obtained by the direct action of ammonia on sulphuric acid, or
by the action of the latter on ammonium carbonate; it separates
from its solutions in an anhydrous state, like potassium sulphate,
with which it is isomorphous. Hence, the composition of crystals
of ammonium sulphate is (NH_{4})_{2}SO_{4}. This salt fuses at
140°, and does not undergo any change when heated up to 180°. At
higher temperatures it does not lose water, but parts with half
its ammonia, and is converted into the acid salt, HNH_{4}SO_{4};
and this acid salt, on further heating, undergoes a further
decomposition, and splits up into nitrogen, water, and acid
ammonium sulphite, HNH_{4}SO_{3}. At the ordinary temperature the
normal salt is soluble in twice its weight of water and at the
boiling-point of water in an equal weight. In its faculty for
combinations this salt exhibits a great resemblance to potassium
sulphate, and, like it, easily forms a number of double salts; the
most remarkable of which are the ammonia alums,
NH_{4}AlS_{2}O_{8},12H_{2}O, and the double salts formed by the
metals of the magnesium group, having, for example, the
composition (NH_{4})_{2}MgS_{2}O_{8},6H_{2}O. Ammonium sulphate
does not give an amide when heated, perhaps owing to the faculty
of sulphuric anhydride to retain the water combined with it with
great force. But the amides of sulphuric acid may be very
conveniently prepared from sulphuric anhydride. Their formation by
this method is very easily understood because an amide is equal to
an ammonium salt less water, and if the anhydride be taken it will
give an amide directly with ammonia. Thus, if dry ammonia be
passed into a vessel surrounded by a freezing mixture and
containing sulphuric anhydride, it forms a white powdery mass
called sulphatammon, having the composition SO_{3},2H_{3}N, and
resembling the similar compound of carbonic acid, CO_{2},2NH_{3}.
This substance is naturally the ammonium salt of sulphamic acid,
SO_{2}(NH_{4}O)NH_{2}. It is slowly acted on by water, and may
therefore be obtained in solution, in which it slowly reacts with
barium chloride, which proves that with water it still forms
ammonium sulphate. If this substance be carefully dissolved in
water and evaporated, it yields well-formed crystals, whose
solution no longer gives a precipitate with barium chloride. This
is not due to the presence of impurities, but to a change in the
nature of the substance, and therefore Rose calls the crystalline
modification _parasulphatammon_. Platinum chloride only
precipitates half the nitrogen as platinochloride from solutions
of sulphat- and parasulphatammon, which shows that they are
ammonium salts, SO_{2}(NH_{4}O)(NH_{2}). It may be that the reason
of the difference in the two modifications is connected with the
fact that two different substances of the composition
N_{2}H_{4}SO_{2} are possible: one is the amide SO_{2}(NH_{2})_{2}
corresponding with the normal salt, and the other is the salt of
the nitrile acid corresponding with acid ammonium sulphate--that
is, SON(ONH_{4}) corresponds with the acid SON(OH) =
SO_{2}(NH_{4}O)OH - 2H_{2}O. Hence there may here be a difference
of the same nature as between urea and ammonium cyanate. Up to the
present, the isomerism indicated above has been but little
investigated, and might be the subject of interesting researches.

If in the preceding experiment the ammonia, and not the sulphuric
anhydride, be taken in excess, a soluble substance of the
composition 2SO_{2},3NH_{3} is formed. This compound, obtained by
Jacqueline and investigated by Voronin, doubtless also contains a
salt of sulphamic acid--that is, of the amide corresponding with
the acid ammonium sulphate = HNH_{4}SO_{4} - H_{2}O =
(NH_{2})SO_{2}(OH). Probably it is a compound of sulphatammon with
sulphamic acid. Thus it has an acid reaction, and does not give a
precipitate with barium chloride.

With normal sulphate of ammonium, an amide of the composition
N_{2}H_{4}SO_{2} should correspond, which should bear the same
relation to sulphuric acid as urea bears to carbonic acid. This
amide, known as _sulphamide_, is obtained by the action of dry
ammonia on the sulphuryl chloride, SO_{2}Cl_{2}, just as urea is
obtained by the action of ammonia on carbonyl chloride,
SO_{2}Cl_{2} + 4NH_{3} = N_{2}H_{4}SO_{2} + 2NH_{4}Cl. The
ammonium chloride is separated from the resultant sulphamide with
great difficulty. Cold water, acting on the mixture, dissolves
them both; the cold solution does not gives precipitate with
barium chloride. Alkalis act on it slowly, as they do on urea; but
on boiling, especially in the presence of alkalis or acids, it
easily recombines with water, and gives an ammonium salt. V.
Traube (1892) obtained sulphamide by the reaction of sulphuryl,
dissolved in chloroform, upon ammonia. The resultant precipitate
dissolves when shaken up with water, and the solution (after
boiling with the oxides or lead or silver) is evaporated, when a
syrupy liquid remains. With nitrate of silver the latter gives a
solid compound, which, when decomposed by hydrochloric acid, gives
free sulphamide in large colourless crystals, having the
composition SO_{2}(NH_{2})_{2}. This substance fuses at 81°,
begins to decompose below 100°, and is entirely decomposed above
250°; it is soluble in water, and the solution has a neutral
reaction and bitter taste. When heated with acids, sulphamide
gradually decomposes, forming sulphuric acid and ammonia. If the
silver compound obtained by the action of sulphamide on nitrate of
silver be heated at 170°-180° until ammonia is no longer evolved,
and the residue be extracted with water acidulated with nitric
acid, a salt separates out from the solution, answering in its
composition to sulphamide, SO_{2}NAg, which = the amide - NH_{3} =
SO_{2}N_{2}H_{4} - NH_{3} = SO_{2}NH. The action of sulphuryl
chloride (and of the other chloranhydrides of sulphur) on ammonium
carbonate always, as Mente showed (1888), results in the formation
of the salt NH(SO_{3}NH_{4})_{2}.

The nitriles corresponding with sulphuric acid are not as yet
known with any certainty. The most simple nitrile corresponding
with sulphuric acid should have the composition N_{2}H_{8}SO_{4} -
4H_{2}O = N_{2}S. This would be a kind of cyanogen corresponding
with sulphuric acid. On comparing sulphurous acid with carbonic
acid, we saw that they present a great analogy in many respects,
and therefore it might be expected that nitrile compounds having
the composition NHS and N_{2}S_{2} would be found. The latter of
these compounds is well known, and was obtained by Soubeiron, by
the action of dry ammonia on sulphur chloride. This substance
corresponds with cyanogen (paracyanogen), and is known as
_nitrogen sulphide_, N_{2}S_{2}. It is formed according to the
equation 3SCl_{2} + 8NH_{3} = N_{2}S_{2} + S + 6NH_{4}Cl. The free
sulphur and nitrogen sulphide are dissolved by acting on the
product with carbon bisulphide, the nitrogen sulphide being much
less soluble than the sulphur. It is a yellow substance, which is
excessively irritating to the eyes and nostrils. It explodes when
rubbed with a hard substance, being naturally decomposed with the
evolution of nitrogen; but when heated it fuses without
decomposing, and only decomposes with explosion at 157°. It is
insoluble in water, and only slightly so in alcohol, ether, and
carbon bisulphide; 100 parts of the latter dissolve 1·5 part of
nitrogen sulphide at the boiling point. This solution on cooling
deposits it in minute transparent prisms of a golden yellow
colour.

In the group of the halogens we saw four closely analogous elements--fluorine, chlorine, bromine, and iodine--and we meet with the same number of closely allied analogues in the oxygen group; for besides sulphur this group also includes _selenium_ and _tellurium_: O, S, Se, Te. These two groups are very closely allied, both in respect to the magnitudes of their atomic weights and also in the faculty of the elements of both groups for combining with metals. The distinct analogy and definite degree of variance known to us for the halogens, also repeat themselves in the same degree for the elements of the oxygen group. Amongst the halogens fluorine has many peculiarities compared to Cl, Br and I which are more closely analogous, whilst oxygen differs in many respects from S, Se, Te, which possess greater similarities. The analogy in a quantitative respect is perfect in both cases. Thus the halogens combine with H, and the elements of the oxygen group with H_{2}, forming H_{2}O, H_{2}S, H_{2}Se, H_{2}Te. The hydrogen compounds of selenium and tellurium are acids like hydrogen sulphide. Selenium, by simple heating in a stream of hydrogen, partially combines with it directly, but seleniuretted hydrogen is more readily decomposable by heat than sulphuretted hydrogen, and this property is still more developed in telluretted hydrogen. Hydrogen selenide and telluride are gases like sulphuretted hydrogen, and, like it, are soluble in water, form saline compounds with alkalis, precipitate metallic salts, are obtained by the action of acids on their compounds with metals, &c. Selenium and tellurium, like sulphur, give two normal grades of combination with oxygen, both of an acid character, of which only the forms corresponding to sulphurous anhydride--namely, selenious anhydride, SeO_{2}, and tellurous anhydride, TeO_{2}[79]--are formed directly. These are both solids, obtained by the combustion of the elements themselves and by the action of oxidising agents on them. They form feebly energetic acids, having distinct bibasic properties; however, a characteristic difference from SO_{2} is observable both in the physical properties of these compounds and in their stability and capacity for further oxidation, just as in the series of the halogens already known to us, only in an inverse order; in the latter we saw that iodine combines more easily than bromine or chlorine with oxygen, forming more stable oxygen compounds, whereas here, on the contrary, sulphurous anhydride, as we know, is difficultly decomposed, parts with its sulphur with difficulty, and is easily oxidised and especially in its salts, while selenious and tellurous anhydrides are oxidised with difficulty and easily reduced, even by means of sulphurous acid.

[79] _Selenious anhydride_, SeO_{2}, is a volatile solid, which
crystallises in prisms soluble in water. It is best procured by
the action of nitric acid on selenium. The well-known researches
of Nilson (1874) showed that the salts of selenious acid easily
form acid salts, and are so characteristic in many respects that
they may even serve for judging the analogy of types of oxides.
Thus the oxides of the composition RO give normal salts of the
composition RSeO_{3},2H_{2}O, where R = Mn, Co, Ni, Cu, Zn. The
salts of magnesium, barium, and calcium contain a different
quantity of water, as do also the salts of the oxides R_{2}O_{3}.
We here turn attention to the fact that beryllium gives a normal
salt, BeSeO_{3},2H_{2}O, and not a salt analogous to those of
aluminium, scandium, Sc_{2}(SeO_{3})_{3},H_{2}O, yttrium,
Y_{2}(SeO_{3})_{2},12H_{2}O, and other oxides of the form
R_{2}O_{3}, which speaks in favour of the formula BeO.

_Tellurous anhydride_ is also a colourless solid, which
crystallises in octahedra; it also, when heated, first fuses and
then volatilises. It is insoluble in water, and the decomposition
of its salts gives a hydrate, H_{2}TeO_{3}, which is insoluble.

It is a very characteristic circumstance that selenious and
tellurous anhydrides are very easily _reduced_ to selenium and
tellurium. This is not only effected by metals like zinc, or by
sulphuretted hydrogen, which are powerful deoxidisers, but even by
sulphurous anhydride, which is able to precipitate selenium and
tellurium from solutions of the selenites and tellurites, and even
of the acids themselves, which is taken advantage of in obtaining
these elements and separating them from sulphur.

Sulphuric acid, as we know, rarely acts as an oxidising agent. It
is otherwise with selenic and telluric acids, H_{2}SeO_{4} and
H_{2}TeO_{4}, which are powerful oxidising agents--that is, are
easily reduced in many circumstances either into the lower oxide
or even to selenium and tellurium. A powerful oxidising agent is
required in order to convert selenious and tellurous anhydrides
into selenic and telluric anhydrides, and, moreover, it must be
employed in excess. If chlorine be passed through a solution of
potassium selenide, K_{2}Se, telluride, K_{2}Te, selenite,
K_{2}SeO_{3}, or tellurite, K_{2}TeO_{3}, it acts as an oxidiser
in the presence of the water, forming potassium selenate,
K_{2}SeO_{4}, or tellurate, K_{2}TeO_{4}. The same salts are
formed by fusing the lower oxides with nitre. These salts are
isomorphous with the corresponding sulphates, and cannot therefore
be separated from them by crystallisation. The salts of potassium,
sodium, magnesium, copper, cadmium, &c. are soluble like the
sulphates, but those of barium and calcium are insoluble, in
perfect analogy with the sulphates. When copper selenate,
CuSeO_{4}, is treated with sulphuretted hydrogen (CuS is
precipitated), _selenic acid_ remains in solution. On evaporation
and drying in vacuo at 180° it gives a syrupy liquid, which may be
concentrated to almost the pure acid, H_{2}SeO_{4}, having a
specific gravity of 2·6. Cameron and Macallan (1891) showed that
pure H_{2}SeO_{4} only remains liquid in a state of superfusion
whilst the solidified acid melts at +58°, the solid acid
crystallises well, its sp. gr. is then 2·95. The hydrate
H_{2}SeO_{4},H_{2}O melts at +25°. The acid in a superfused state
has a sp. gr. 2·36 and the solid 2·63. Like sulphuric acid strong
selenic acid attracts moisture from the atmosphere; it is not
decomposed by sulphurous acid, but oxidises hydrochloric acid
(like nitric, chromic, and manganic acids), evolving chlorine and
forming selenious acid, H_{2}SeO_{4} + 2HCl = H_{2}SeO_{3} +
H_{2}O + Cl_{2}. _Telluric acid_, H_{2}TeO_{4}, is obtained by
fusing tellurous anhydride with potassium hydroxide and chlorate;
the solution, containing potassium tellurate, is then precipitated
with barium chloride, and the barium tellurate, BaTeO_{4} obtained
in the precipitate is decomposed by sulphuric acid. A solution of
telluric acid is thus obtained, which on evaporation yields
colourless prisms, soluble in water, and containing
TeH_{2}O_{4},2H_{2}O. Two equivalents of water are driven off at
160°; on further heating the last equivalent of water is expelled,
and then oxygen is given off. It also gives chlorine with
hydrochloric acid, like selenic acid. Its salts also correspond
with those of sulphuric acid. It must, however, be remarked that
telluric and selenic acids are able to give poly-acid salts with
much greater ease than sulphuric acid. Thus, for example, there
are known for telluric acid not only K_{2}TeO_{4},5H_{2}O and
KHTeO_{4},3H_{2}O, but also KHTeO_{4},H_{2}TeO_{4},H_{2}O =
K_{2}TeO_{4},3H_{2}TeO_{4},2H_{2}O. This salt is easily obtained
from acid solutions of the preceding salts and is less soluble in
water. As selenious anhydride is volatile and gives similar
poly-salts, it may be surmised that selenious, tellurous, selenic,
and telluric anhydrides are polymeric as compared with sulphurous
and sulphuric anhydrides, for which reason it would be desirable
to determine the vapour density of selenious anhydride. It would
probably correspond with Se_{2}O_{4} or Se_{3}O_{6}.

In order to show the very close analogy of selenium to sulphur, I
will quote two examples. Potassium cyanide dissolves selenium, as
it does sulphur, forming potassium selenocyanate, KCNSe,
corresponding with potassium thiocyanate. Acids precipitate
selenium from this solution, because selenocyanic acid, H_{2}CNSe,
when in a free state is immediately decomposed. A boiling solution
of sodium sulphite dissolves selenium, just as it would sulphur,
forming a salt analogous to thiosulphate of sodium, namely, sodium
selenosulphate, Na_{2}SSeO_{3}. Selenium is separated from a
solution of this salt by the action of acid.

_Selenium_ was obtained in 1817 by Berzelius from the sublimate which collects in the first chamber in the preparation of sulphuric acid from Fahlun pyrites. Certain other pyrites also contain small quantities of selenium. Some native selenides, especially those of lead, mercury, and copper, have been found in the Hartz Mountains, but only in small quantities. Pyrites and blendes, in which the sulphur is partially replaced by selenium, still remain the chief source for its extraction. When these pyrites are roasted they evolve selenious anhydride, which condenses in the cooler portions of the apparatus in which the pyrites are roasted, and is partially or wholly reduced by the sulphurous anhydride simultaneously formed. The presence of selenium in ores and sublimates is most simply tested by heating them before the blowpipe, when they evolve the characteristic odour of garlic. Selenium exhibits two modifications, like sulphur: one amorphous and insoluble in carbon bisulphide, the other crystalline and slightly soluble in carbon bisulphide (in 1,000 parts at 45° and 6,000 at 0°), and separating from its solutions in monoclinic prisms. If the red precipitate obtained by the action of sulphurous anhydride on selenious anhydride be dried, it gives a brown powder, having a specific gravity of 4·26, which when heated changes colour and fuses to a metallic mass, which gains lustre as it cools. The selenium acquires different properties according to the rate at which it is cooled from a fused state; if rapidly cooled, it remains amorphous and has the same specific gravity (4·28) as the powder, but if slowly cooled it becomes crystalline and opaque, soluble in carbon bisulphide, and has a specific gravity of 4·80. In this form it fuses at 214° and remains unchanged, whilst the amorphous form, especially above 80°, gradually passes into the crystalline variety. The transition is accompanied by the evolution of heat, as in the case of sulphur; thus the analogy between sulphur and selenium is clearly shown here. In the fused amorphous form selenium presents a brown mass, slightly translucent, with a vitreous fracture, whilst in the crystalline form it has the appearance of a grey metal, with a feeble lustre and a crystalline fracture.[79 bis] Selenium boils at 700°, forming a vapour whose density is only constant at a temperature of about 1,400°, when it is equal to 79·4 (referred to hydrogen)--that is, the molecular formula is then Se_{2}, like sulphur at an equally high temperature.

[79 bis] Muthmann, in his researches upon the allotropic forms of
selenium, pointed out (1889) a peculiar modification, which
appears, as it were, as a transition between crystalline and
amorphous selenium. It is obtained together with the crystalline
variety by slowly evaporating a solution of selenium in bisulphide
of carbon, and differs from the crystalline variety in the form of
its crystals; it passes into the latter modification when heated.
Schultz also obtained selenium (like Ag, _see_ Chapter XXIV.) in a
soluble form, but these researches are not so conclusive as those
upon soluble silver, and we shall therefore not consider them more
fully.

_Tellurium_ is met with still more rarely than selenium (it is known in Saxony) in combination with gold, silver, lead, and antimony in the so-called foliated tellurium ore. Bismuth telluride and silver telluride have been found in Hungary and in the Altai. Tellurium is extracted from bismuth telluride by mixing the finely-powdered ore with potassium and charcoal in as intimate a mixture as possible, and then heating in a covered crucible. Potassium telluride, K_{2}Te, is then formed, because the charcoal reduces potassium tellurite. As potassium telluride is soluble in water, forming a red-brown solution which is decomposed by the oxygen of the atmosphere (K_{2}Te + O + H_{2}O = 2KHO + Te), the mass formed in the crucible is treated with boiling water and filtered as rapidly as possible, and the resultant solution exposed to the air, by which means the tellurium is precipitated.[80] In a free state tellurium has a perfectly _metallic appearance_; it is of a silver-white colour, crystallises very easily in long brilliant needles; is very brittle, so that it can be easily reduced to powder; but it is a bad conductor of heat and electricity, and in this respect, as in many others, it forms a transition from the metals to the non-metals. Its specific gravity is 6·18, it melts at an incipient red heat, and takes fire when heated in air, like selenium and sulphur, burning with a blue flame, evolving white fumes of tellurous anhydride, TeO_{2}, and emitting an acrid smell if no selenium be present; but if it be, the odour of the latter preponderates. Alkalis dissolve tellurium when boiled with it, potassium telluride, K_{2}Te, and potassium tellurite, K_{2}TeO_{3}, being formed. The solution is of a red colour, owing to the presence of the telluride, K_{2}Te; but the colour disappears when the solution is cooled or diluted, the tellurium being all precipitated: 2K_{2}Te + K_{2}TeO_{3} + 3H_{2}O = 6KHO + 3Te.[81]

[80] The tellurium thus prepared is impure, and contains a large amount
of selenium. The latter may be removed by converting the mixture
into the salts of potassium, and treating this with nitric acid
and barium nitrate, when barium selenate only is precipitated,
whilst the barium tellurate remains in solution. This method does
not, however, give a pure product, and it appears to be best to
separate the selenium from the tellurium in a metallic form; this
is done by boiling the impure potassium tellurate with
hydrochloric acid, which converts it into potassium tellurite,
from which the tellurium is reduced by sulphurous anhydride. The
metal thus obtained is then fused and distilled in a stream of
hydrogen; the selenium volatilises first, and then the tellurium,
owing to its being much less volatile than the former.
Nevertheless, tellurium is also volatile, and may be separated in
this manner from less volatile metals, such as antimony. Brauner
determined the atomic weight of pure tellurium, and found it to be
125, but showed (1889) that tellurium purified by the usual
method, even after distillation, contains a large amount of
impurities.

[81] The decomposition proceeds in the above order in the cold, but in
a hot solution with an excess of potassium hydroxide it proceeds
inversely. A similar phenomenon takes place when tellurium is
fused with alkalis, and it is therefore necessary in order to
obtain potassium telluride to add charcoal.

Selenium and tellurium form higher compounds with chlorine with
comparative ease. For selenium, SeCl_{2} and SeCl_{4} are known,
and for tellurium TeCl_{2} and TeCl_{4}. The tetrachlorides of
selenium and tellurium are formed by passing chlorine over these
elements. Selenium tetrachloride, SeCl_{4}, is a crystalline,
volatile mass which gives selenious anhydride and hydrochloric
acid with water. Tellurium tetrachloride is much less volatile,
fuses easily, and is also decomposed by water. Both elements form
similar compounds with bromine. Tellurium tetrabromide is red,
fuses to a brown liquid, volatilises, and gives a crystalline
salt, K_{2}TeBr_{6},3H_{2}O, with an aqueous solution of potassium
bromide.