Chapter XI: The Halogens: Chlorine, Bromine, Iodine, and Fluorine

Although hydrochloric acid, like water, is one of the most stable substances, it is nevertheless decomposed not only by the action of a galvanic current,[1] but also by a high temperature. Sainte-Claire Deville showed that decomposition already occurs at 1,300°, because a cold tube (as with CO, Chapter IX.) covered with an amalgam of silver absorbs chlorine from hydrochloric acid in a red-hot tube, and the escaping gas contains hydrogen. V. Meyer and Langer (1885) observed the decomposition of hydrochloric acid at 1,690° in a platinum vessel; the decomposition in this instance was proved not only from the fact that hydrogen diffused through the platinum (p. 142), owing to which the volume was diminished, but also from chlorine being obtained in the residue (the hydrogen chloride was mixed with nitrogen), which liberated iodine from potassium iodide.[2] The usual method for the preparation of chlorine consists in the abstraction of the hydrogen by oxidising agents.[2 bis]

[1] The decomposition of fused sodium chloride by an electric current
has been proposed in America and Russia (N. N. Beketoff) as a means
for the preparation of chlorine and sodium. A strong solution of
hydrochloric acid is decomposed into equal volumes of chlorine and
hydrogen by the action of an electric current. If sodium chloride
and lead be melted in a crucible, the former being connected with
the cathode and a carbon anode immersed in the lead, then the lead
dissolves sodium and chlorine is disengaged as gas. This
electrolytic method has not yet been practised on a large scale,
probably because gaseous chlorine has not many applications, and
because of the difficulty there is in dealing with it.

[2] To obtain so high a temperature (at which the best kinds of
porcelain soften) Langer and Meyer employed the dense graphitoidal
carbon from gas retorts, and a powerful blast. They determined the
temperature by the alteration of the volume of nitrogen in the
platinum vessel, for this gas does not permeate through platinum,
and is unaltered by heat.

[2 bis] The acid properties of hydrochloric acid were known when
Lavoisier pointed out the formation of acids by the combination of
water with the oxides of the non-metals, and therefore there was
reason for thinking that hydrochloric acid was formed by the
combination of water with the oxide of some element. Hence when
Scheele obtained chlorine by the action of hydrochloric acid on
manganese peroxide he considered it as the acid contained in common
salt. When it became known that chlorine gives hydrochloric acid
with hydrogen, Lavoisier and Berthollet supposed it to be a
compound with oxygen of an anhydride contained in hydrochloric
acid. They supposed that hydrochloric acid contained water and the
oxide of a particular radicle, and that chlorine was a higher
degree of oxidation of this radicle _muvias_ (from the Latin neme
of hydrochloric acid, _acidum muriaticum_). It was only in 1811
that Gay-Lussac and Thénard in France and Davy in England arrived
at the conclusion that the substance obtained by Scheele does not
contain oxygen, nor under any conditions give water with hydrogen,
and that there is no water in hydrochloric acid gas, and therefore
concluded that chlorine is an elementary substance. They named it
'chlorine' from the Greek word [Greek: chlôros], signifying a green
colour, because of the peculiar colour by which this gas is
characterised.

An aqueous solution of hydrochloric acid is generally employed for t he evolution of chlorine. The hydrogen has to be abstracted from the hydrochloric acid. This is accomplished by nearly all oxidising substances, and especially by those which are able to evolve oxygen at a red heat (besides bases, such as mercury and silver oxides, which are able to give salts with hydrogen chloride); for example, manganese peroxide, potassium chlorate, chromic acid, &c. The decomposition essentially consists in the oxygen of the oxidising substance displacing the chlorine from 2HCl, forming water, H_{2}O, and setting the chlorine free, 2HCl + O (disengaged by the oxidising substances) = H_{2}O + Cl_{2}. Even nitric acid partially produces a like reaction; but as we shall afterwards see its action is more complicated, and it is therefore not suitable for the preparation of pure chlorine.[3] But other oxidising substances which do not give any other volatile products with hydrochloric acid may be employed for the preparation of chlorine. Among these may be mentioned: potassium chlorate, acid potassium chromate, sodium manganate, manganese peroxide, &c. Manganese peroxide is commonly employed in the laboratory, and on a large scale, for the preparation of chlorine. The chemical process in this case may be represented as follows: an exchange takes place between 4HCl and MnO_{2}, in which the manganese takes the place of the four atoms of hydrogen, or the chlorine and oxygen exchange places--that is, MnCl_{4} and 2H_{2}O are produced. The chlorine compound, MnCl_{4}, obtained is very unstable; it splits up into chlorine, which as a gas passes from the sphere of action, and a lower compound containing less chlorine than the substance first formed, which remains in the apparatus in which the mixture is heated, MnCl_{4} = MnCl_{2} + Cl_{2}.[3 bis] The action of hydrochloric acid requires a temperature of about 100°. In the laboratory the _preparation of chlorine_ is carried on in flasks, heated over a water-bath, by acting on manganese peroxide with hydrochloric acid or a mixture of common salt and sulphuric acid[4] and washing the gas with water to remove hydrochloric acid.[5] Chlorine cannot be collected over mercury, because it combines with it as with many other metals, and it is soluble in water; however, it is but slightly soluble in hot water or brine. Owing to its great weight, chlorine may be directly collected in a dry vessel by carrying the gas-conducting tube down to the bottom of the vessel. The chlorine will lie in a heavy layer at the bottom of the vessel, displace the air, and the extent to which it fills the vessel may be followed by its colour.[6]

[3] However, nitric acid has been proposed as a means for obtaining
chlorine, but by methods which have the drawback of being very
complicated

[3 bis] This representation of the process of the reaction is most
natural. However, this decomposition is generally represented as if
chlorine gave only one degree of combination with manganese,
MnCl_{2}, and therefore directly reacts in the following
manner--MnO_{2} + 4HCl = MnCl_{2} + 2H_{2}O + Cl_{2}, in which case
it is supposed that manganese peroxide, MnO_{2}, breaks up, as it
were, into manganous oxide, MnO and oxygen, both of which react
with hydrochloric acid, the manganous oxide acting upon HCl as a
base, giving MnCl_{2} and at the same time 2HCl + O = H_{2}O +
Cl_{2}. In reality, a mixture of oxygen and hydrochloric acid does
give chlorine at a red heat, and this reaction may also take place
at the moment of its evolution in this case.

All the oxides of manganese (Mn_{2}O_{3}, MnO_{2}, MnO_{3},
Mn_{2}O_{7}), with the exception of manganous oxide, MnO, disengage
chlorine from hydrochloric acid, because manganous chloride,
MnCl_{2}, is the only compound of chlorine and manganese which
exists as a stable compound, all the higher chlorides of manganese
being unstable and evolving chlorine. Hence we here take note of
two separate changes: (1) an exchange between oxygen and chlorine,
and (2) the instability of the higher chlorine compounds. As
(according to the law of substitution) in the substitution of
oxygen by chlorine, Cl_{2} takes the place of O, the chlorine
compounds will contain more atoms than the corresponding oxygen
compounds. It is not surprising, therefore, that certain of the
chlorine compounds corresponding with oxygen compounds do not
exist, or if they are formed are very unstable. And furthermore, an
atom of chlorine is heavier than an atom of oxygen, and therefore a
given element would have to retain a large mass of chlorine if in
the higher oxides the oxygen were replaced by chlorine. For this
reason equivalent compounds of chlorine do not exist for all oxygen
compounds. Many of the former are immediately decomposed, when
formed, with the evolution of chlorine. From this it is evident
that there should exist such chlorine compounds as would evolve
chlorine as peroxides evolve oxygen, and indeed a large number of
such compounds are known. Amongst them may be mentioned antimony
pentachloride, SbCl_{5}, which splits up into chlorine and antimony
trichloride when heated. Cupric chloride, corresponding with copper
oxide, and having a composition CuCl_{2}, similar to CuO, when
heated parts with half its chlorine, just as barium peroxide
evolves half its oxygen. This method may even be taken advantage of
for the preparation of chlorine and cuprous chloride, CuCl. The
latter attracts oxygen from the atmosphere, and in so doing is
converted from a colourless substance into a green compound whose
composition is Cu_{2}Cl_{2}O. With hydrochloric acid this substance
gives cupric chloride (Cu_{2}Cl_{2}O + 2HCl = H_{2}O + 2CuCl_{2}),
which has only to be dried and heated in order again to obtain
chlorine. Thus, in solution, and at the ordinary temperature, the
compound CuCl_{2} is stable, but when heated it splits up. On this
property is founded Deacon's process for the preparation of
chlorine from hydrochloric acid with the aid of air and copper
salts, by passing a mixture of air and hydrochloric acid at about
440° over bricks saturated with a solution of a copper salt (a
mixture of solutions of CuSO_{4} and Na_{2}SO_{4}). CuCl_{2} is
then formed by the double decomposition of the salt of copper and
the hydrochloric acid; the CuCl_{2} liberates chlorine, and the
CuCl forms Cu_{2}Cl_{2}O with the oxygen of the air, which again
gives CuCl_{2} with 2HCl, and so on.

Magnesium chloride, which is obtained from sea-water, carnallite,
&c., may serve not only as a means for the preparation of
hydrochloric acid, but also of chlorine, because its basic salt
(magnesium oxychloride) when heated in the air gives magnesium
oxide and chlorine (Weldon-Pechiney's process, 1888). Chlorine is
now prepared on a large scale by this method. Several new methods
based upon this reaction have been proposed for procuring chlorine
from the bye-products of other chemical processes. Thus, Lyte and
Tattars (1891) obtained up to 67 p.c. of chlorine from CaCl_{2} in
this manner. A solution of CaCl_{2}, containing a certain amount of
common salt, is evaporated and oxide of magnesium added to it. When
the solution attains a density of 1·2445 (at 15°), it is treated
with carbonic acid, which precipitates carbonate of calcium, while
chloride of magnesium remains in solution. After adding ammonium
chloride, the solution is evaporated to dryness and the double
chloride of magnesium and ammonium formed is ignited, which drives
off the chloride of ammonium. The chloride of magnesium which
remains behind is used in the Weldon-Pechiney process. The De
Wilde-Reychler (1892) process for the manufacture of chlorine
consists in passing alternate currents of hot air and hydrochloric
acid gas through a cylinder containing a mixture of the chlorides
of magnesium and manganese. A certain amount of sulphate of
magnesium which does not participate in any way in the reaction, is
added to the mixture to prevent its fusing. The reactions may be
expressed by the following equations: (1) 3MgCl_{2} + 3MnCl_{2} +
8O = Mg_{3}Mn_{3}O_{8} + 12Cl; (2) Mg_{3}Mn_{3}O_{8} + 16HCl =
3MgCl_{2} + 3MnCl_{2} + 8H_{2}O + 4Cl. As nitric acid is able to
take up the hydrogen from hydrochloric acid, a heated mixture of
these acids is also employed for the preparation of chlorine. The
resultant mixture of chlorine and lower oxides of nitrogen is mixed
with air and steam which regenerates the HNO_{3}, while the
chlorine remains as a gas together with nitrogen, in which form it
is quite capable of bleaching, forming chloride of lime, &c.
Besides these, Solvay and Mond's methods of preparing chlorine must
be mentioned. The first is based upon the reaction CaCl_{2} +
SiO_{2} + O(air) = CaOSiO_{2} + Cl_{2}, the second on the action of
the oxygen of the air (heated) upon MgCl_{2} (and certain similar
chlorides) MgCl_{2} + O = MgO + Cl_{2} The remaining MgO is treated
with sal-ammoniac to re-form MgCl_{2} (MgO + 2NH_{4}Cl = MgCl_{2} +
H_{2}O + 2NH_{3}) and the resultant NH_{3} again converted into
sal-ammoniac, so that hydrochloric acid is the only substance
consumed. The latter processes have not yet found much application.

[4] The following proportions are accordingly taken by weight: 5 parts
of powdered manganese peroxide, 11 parts of salt (best fused, to
prevent its frothing), and 14 parts of sulphuric acid previously
mixed with an equal volume of water. The mixture is heated in a
salt bath, so as to obtain a temperature above 100°. The corks in
the apparatus must be soaked in paraffin (otherwise they are
corroded by the chlorine), and black india-rubber tubing smeared
with vaseline must be used, and not vulcanised rubber (which
contains sulphur, and becomes brittle under the action of the
chlorine).

The reaction which proceeds may be expressed thus: MnO_{2} + 2NaCl
+ 2H_{2}SO_{4} = MnSO_{4} + Na_{2}SO_{4} + 2H_{2}O + Cl_{2}. The
method of preparation of Cl_{2} from manganese peroxide and
hydrochloric acid was discovered by Scheele, and from sodium
chloride by Berthollet.

[5] The reaction of hydrochloric acid upon bleaching powder gives
chlorine without the aid of heat, CaCl_{2}O_{2} + 4HCl = CaCl_{2} +
2H_{2}O + 2Cl_{2} and is therefore also used for the preparation of
chlorine. This reaction is very violent if all the acid be added at
once; it should be poured in drop by drop (Mermé, Kämmerer). C.
Winkler proposed to mix bleaching powder with one quarter of burnt
and powdered gypsum, and having damped the mixture with water, to
press and cut it up into cubes and dry at the ordinary temperature.
These cubes can be used for the preparation of chlorine in the same
apparatus as that used for the evolution of hydrogen and carbonic
anhydride--the disengagement of the chlorine proceeds uniformly.

A mixture of potassium dichromate and hydrochloric acid evolves
chlorine perfectly free from oxygen (V. Meyer and Langer).

[6]

Chlorine is manufactured on a _large scale_ from manganese peroxide
and hydrochloric acid. It is most conveniently prepared in the
apparatus shown in fig. 66, which consists of a three-necked
earthenware vessel whose central orifice is the largest. A clay or
lead funnel, furnished with a number of orifices, is placed in the
central wide neck of the vessel. Roughly-ground lumps of natural
manganese peroxide are placed in the funnel, which is then closed
by the cover N, and luted with clay. One orifice is closed by a
clay stopper, and is used for the introduction of the hydrochloric
acid and withdrawal of the residues. The chlorine disengaged passes
along a leaden gas-conducting tube placed in the other orifice. A
row of these vessels is surrounded by a water-bath to ensure their
being uniformly heated. Manganese chloride is found in the residue.
In Weldon's process lime is added to the acid solution of manganese
chloride. A double decomposition takes place, resulting in the
formation of manganous hydroxide and calcium chloride. When the
insoluble manganous hydroxide has settled, a further excess of milk
of lime is added (to make a mixture 2Mn(OH)_{2} + CaO +
_x_CaCl_{2}, which is found to be the best proportion, judging from
experiment), and then air is forced through the mixture. The
hydroxide is thus converted from a colourless to a brown substance,
containing peroxide, MnO_{2}, and oxide of manganese, Mn_{2}O_{3}.
This is due to the manganous oxide absorbing oxygen from the air.
Under the action of hydrochloric acid this mixture evolves
chlorine, because of all the compounds of chlorine and manganese
the chloride MnCl_{2} is the only one which is stable (_see_ Note
3). Thus one and the same mass of manganese may be repeatedly used
for the preparation of chlorine. The same result is attained in
other ways. If manganous oxide be subjected to the action of oxides
of nitrogen and air (Coleman's process), then manganese nitrate is
formed, which at a red heat gives oxides of nitrogen (which are
again used in the process) and manganese peroxide, which is thus
renewed for the fresh evolution of chlorine.

Chlorine is a _gas_ of a yellowish green colour, and has a very suffocating and characteristic odour. On lowering the temperature to -50° or increasing the pressure to six atmospheres (at 0°) chlorine condenses[7] into a liquid which has a yellowish-green colour, a density of 1·3, and boils at -34°. The density and atomic weight of chlorine is 35·5 times greater than that of hydrogen, hence the molecule contains Cl_{2}[8]. At 0° one volume of water dissolves about 1-1/2 volume of chlorine, at 10° about 3 volumes, at 50° again 1-1/2 volume.[9] Such a solution of chlorine is termed 'chlorine water;' and is employed in a diluted form in medicine and as a laboratory reagent. It is prepared by passing chlorine through a series of Woulfe's bottles or into an inverted retort filled with water. Under the action of light, chlorine water gives oxygen and hydrochloric acid. At 0° a saturated solution of chlorine yields a crystallo-hydrate, Cl_{2},8H_{2}O, which easily splits up into chlorine and water when heated, so that if it be sealed up in a tube and heated to 35°, two layers of liquid are formed--a lower stratum of chlorine containing a small quantity of water, and an upper stratum of water containing a small quantity of chlorine.[10]

[7] Davy and Faraday liquefied chlorine in 1823 by heating the
crystallo-hydrate Cl_{2}8H_{2}O in a bent tube (as with NH_{3}),
surrounded by warm water, while the other end of the tube was
immersed in a freezing mixture. Meselan condensed chlorine in
freshly-burnt charcoal (placed in a glass tube), which when cold
absorbs an equal weight of chlorine. The tube was then fused up,
the bent end cooled, and the charcoal heated, by which means the
chlorine was expelled from the charcoal, and the pressure
increased.

[8] Judging from Ludwig's observations (1868), and from the fact that
the coefficient of expansion of gases increases with their
molecular weight (Chapter II., Note 26, for hydrogen = 0·367,
carbonic anhydride = 0·373, hydrogen bromide = 0·386), it might be
expected that the expansion of chlorine would be greater than that
of air or of the gases composing it. V. Meyer and Langer (1885)
having remarked that at 1,400° the density of chlorine (taking its
expansion as equal to that of nitrogen) = 29, consider that the
molecules of chlorine split up and partially give molecules Cl, but
it might be maintained that the decrease in density observed only
depends on the increase of the coefficient of expansion.

[9] Investigations on the solubility of chlorine in water (the
solutions evolve all their chlorine on boiling and passing air
through them) show many different peculiarities. First Gay-Lussac,
and subsequently Pelouze, determined that the solubility increases
between 0° and 8°-10° (from 1-1/2 to 2 vols. of chlorine per 100
vols. of water at 0° up to 3 to 2-3/4 at 10°). In the following
note we shall see that this is not due to the breaking-up of the
hydrate at about 8° to 10°, but to its formation below 9°. Roscoe
observed an increase in the solubility of chlorine in the presence
of hydrogen--even in the dark. Berthelot determined an increase of
solubility with the progress of time. Schönbein and others suppose
that chlorine acts on water, forming hypochlorous and hypochloric
acids, (HClO + HCl).

The equilibrium between chlorine and steam as gases and between
water, liquid chlorine, ice, and the solid crystallo-hydrate of
chlorine is evidently very complex. Gibbs, Guldberg (1870) and
others gave a theory for similar states of equilibrium, which was
afterwards developed by Roozeboom (1887), but it would be
inopportune here to enter into its details. It will be sufficient
in the first place to mention that there is now no doubt (according
to the theory of heat, and the direct observations of Ramsay and
Young) that the vapour tensions at one and the same temperature are
different for the liquid and solid states of substances; secondly,
to call attention to the following note; and, thirdly, to state
that, in the presence of the crystallo-hydrate, water between O°·24
and +28°·7 (when the hydrate and a solution may occur
simultaneously) dissolves a different amount of chlorine than it
does in the absence of the crystallo-hydrate.

[10] According to Faraday's data the hydrate of chlorine contains
Cl_{2},10H_{2}O, but Roozeboom (1885) showed that it is poorer in
water and = Cl_{2},8H_{2}O. At first small, almost colourless,
crystals are obtained, but they gradually form (if the temperature
be below their critical point 28°·7, above which they do not
exist) large yellow crystals, like those of potassium chromate.
The specific gravity is 1·23. The hydrate is formed if there be
more chlorine in a solution than it is able to dissolve under the
dissociation pressure corresponding with a given temperature. _In
the presence of the hydrate_ the percentage amount of chlorine at
0° = 0·5, at 9° = 0·9, and at 20° = 1·82. At temperatures below 9°
the solubility (determined by Gay-Lussac and Pelouze, _see_ Note
9) is dependent on the formation of the hydrate; whilst at higher
temperatures under the ordinary pressure the hydrate cannot be
formed, and the solubility of chlorine falls, as it does for all
gases (Chapter I.). If the crystallo-hydrate is not formed, then
below 9° the solubility follows the same rule (6° 1·07 p.c. Cl, 9°
0·95 p.c.). According to Roozeboom, the chlorine evolved by the
hydrate presents the following tensions of dissociation: at 0° =
249 mm., at 4° = 398, at 8° = 620, at 10° = 797, at 14° = 1,400
mm. In this case a portion of the crystallo-hydrate remains solid.
At 9°·6 the tension of dissociation is equal to the atmospheric
pressure. At a higher pressure the crystallo-hydrate may form at
temperatures above 9° up to 28°·7, when the vapour tension of the
hydrate equals the tension of the chlorine. It is evident that the
equilibrium which is established is on the one hand a case of a
complex heterogeneous system, and on the other hand a case of the
solution of solid and gaseous substances in water.

The crystallo-hydrate or chlorine water must be kept in the dark,
or the access of light be prevented by coloured glass, otherwise
oxygen is evolved and hydrochloric acid formed.

Chlorine explodes _with hydrogen_, if a mixture of equal volumes be exposed to the direct action of the sun's rays[11] or brought into contact with spongy platinum, or a strongly heated substance, or when subjected to the action of an electric spark. The explosion in this case takes place for exactly the same reasons--_i.e._ the evolution of heat and expansion of the resultant product--as in the case of detonating gas (Chapter III.) Diffused light acts in the same way, but slowly, whilst direct sunlight causes an explosion.[12] The hydrochloric acid gas produced by the reaction of chlorine on hydrogen occupies (at the original temperature and pressure) a volume equal to the sum of the original volumes; that is, a reaction of substitution here takes place: H_{2} + Cl_{2} = HCl + HCl. In this reaction twenty-two thousand heat units are evolved for one part by weight [1 gram] of hydrogen.[13]

[11] The chemical action of light on a mixture of chlorine and
hydrogen was discovered by Gay-Lussac and Thénard (1809). It has
been investigated by many savants, and especially by Draper,
Bunsen, and Roscoe. Electric or magnesium light, or the light
emitted by the combustion of carbon bisulphide in nitric oxide,
and actinic light in general, acts in the same manner as sunlight,
in proportion to its intensity. At temperatures below -12° light
no longer brings about reaction, or at all events does not give an
explosion. It was long supposed that chlorine that had been
subjected to the action of light was afterwards able to act on
hydrogen in the dark, but it was shown that this only takes place
with moist chlorine, and depends on the formation of oxides of
chlorine. The presence of foreign gases, and even of excess of
chlorine or of hydrogen, very much enfeebles the explosion, and
therefore the experiment is conducted with a detonating mixture
prepared by the action of an electric current on a strong solution
(sp. gr. 1·15) of hydrochloric acid, in which case the water is
not decomposed--that is, no oxygen becomes mixed with the
chlorine.

[12] The quantity of chlorine and hydrogen which combine is
proportional to the intensity of the light--not of all the rays,
but only those so-termed chemical (actinic) rays which produce
chemical action. Hence a mixture of chlorine and hydrogen, when
exposed to the action of light in vessels of known capacity and
surface, may be employed as an actinometer--that is, as a means
for estimating the intensity of the chemical rays, the influence
of the heat rays being previously destroyed, which may be done by
passing the rays through water. Investigations of this kind
(photo-chemical) showed that chemical action is chiefly limited to
the violet end of the spectrum, and that even the invisible
ultra-violet rays produce this action. A colourless gas flame
contains no chemically active rays; the flame coloured green by a
salt of copper evinces more chemical action than the colourless
flame, but the flame brightly coloured yellow by salts of sodium
has no more chemical action than that of the colourless flame.

As the chemical action of light becomes evident in plants,
photography, the bleaching of tissues, and the fading of colours
in the sunlight, and as a means for studying the phenomenon is
given in the reaction of chlorine on hydrogen, this subject has
been the most fully investigated in _photo-chemistry_. The
researches of Bunsen and Roscoe in the fifties and sixties are the
most complete in this respect. Their actinometer contains hydrogen
and chlorine, and is surrounded by a solution of chlorine in
water. The hydrochloric acid is absorbed as it forms, and
therefore the variation in volume indicates the progress of the
combination. As was to be expected, the action of light proved to
be proportional to the time of exposure and intensity of the
light, so that it was possible to conduct detailed photometrical
investigations respecting the time of day and season of the year,
various sources of light, its absorption, &c. This subject is
considered in detail in special works, and we only stop to mention
one circumstance, that a small quantity of a foreign gas decreases
the action of light; for example, 1/330 of hydrogen by 38 p.c.,
1/200 of oxygen by 10 p.c., 1/100 of chlorine by 60 p.c., &c.
According to the researches of Klimenko and Pekatoros (1889), the
photo-chemical alteration of chlorine water is retarded by the
presence of traces of metallic chlorides, and this influence
varies with different metals.

As much heat is evolved in the reaction of chlorine on hydrogen,
and as this reaction, being exothermal, may proceed by itself, the
action of light is essentially the same as that of heat--that is,
it brings the chlorine and hydrogen into the condition necessary
for the reaction--it, as we may say, disturbs the original
equilibrium; this is the work done by the luminous energy. It
seems to me that the action of light on the mixed gases should be
understood in this sense, as Pringsheim (1877) pointed out.

[13] In the formation of steam (from one part by weight [1 gram]
of hydrogen) 29,000 heat units are evolved. The following are the
quantities of heat (thousands of units) evolved in the formation
of various other _corresponding_ compounds of oxygen and of
chlorine (from Thomsen's, and, for Na_{2}O, Beketoff's results):

{2NaCl, 195; CaCl_{2}, 170; HgCl_{2}, 63; 2AgCl, 59.
{ Na_{2}O, 100; CaO, 131; HgO, 42; Ag_{2}O, 6.
{2AsCl_{3}, 143; 2PCl_{5}, 210; CCl_{4}, 21; 2HCl, 44
(gas).
{ As_{2}O_{3}, 155; P_{2}O_{5}, 370; CO_{2}, 97; H_{2}O, 58
(gas).

With the first four elements the formation of the chlorine
compound gives the most heat, and with the four following the
formation of the oxygen compound evolves the greater amount of
heat. The first four chlorides are true salts formed from HCl and
the oxide, whilst the remainder have other properties, as is seen
from the fact that they are not formed from hydrochloric acid and
the oxide, but give hydrochloric acid with water.

These relations show that the affinity of chlorine for hydrogen is very great and analogous to the affinity between hydrogen and oxygen. Thus[14] on the one hand by passing a mixture of steam and chlorine through a red-hot tube, or by exposing water and chlorine to the sunlight, oxygen is disengaged, whilst on the other hand, as we saw above, oxygen in many cases displaces chlorine from its compound with hydrogen, and therefore the reaction H_{2}O + Cl_{2} = 2HCl + O belongs to the number of reversible reactions, and hydrogen will distribute itself between oxygen and chlorine. This determines the relation of Cl to substances containing hydrogen and its reactions in the presence of water, to which we shall turn our attention after having pointed out the relation of chlorine to other elements.

[14] This has been already pointed out in Chapter III., Note 5.

Many _metals_ when brought into contact with chlorine immediately combine with it, and form those metallic chlorides which correspond with hydrogen chloride and with the oxide of the metal taken. This combination may proceed rapidly with the evolution of heat and light; that is, metals are able to burn in chlorine. Thus, for example, sodium[15] burns in chlorine, synthesising common salt. Metals in the form of powders burn without the aid of heat, and become highly incandescent in the process; for instance, antimony, which is a metal easily converted into a powder.[16] Even such metals as gold and platinum,[17] which do not combine directly with oxygen and give very unstable compounds with it, unite directly with chlorine to form metallic chlorides. Either chlorine water or aqua regia may be employed for this purpose instead of gaseous chlorine. These dissolve gold and platinum, converting them into metallic chlorides. _Aqua regia_ is a mixture of 1 part of nitric acid with 2 to 3 parts of hydrochloric acid. This mixture converts into soluble chlorides not only those metals which are acted on by hydrochloric and nitric acids, but also gold and platinum, which are insoluble in either acid separately. This action of aqua regia depends on the fact that nitric acid in acting on hydrochloric acid evolves chlorine. If the chlorine evolved be transferred to a metal, then a fresh quantity is formed from the remaining acids and also combines with the metal.[18] Thus the aqua regia acts by virtue of the chlorine which it contains and disengages.

[15] Sodium remains unaltered in perfectly dry chlorine at the ordinary
temperature, and even when slightly warmed; but the combination is
exceedingly violent at a red heat.

[16] An instructive experiment on combustion in chlorine may be
conducted as follows: leaves of Dutch metal (used instead of gold
for gilding) are placed in a glass globe, and a gas-conducting
tube furnished with a glass cock is placed in the cork closing it,
and the air is pumped out of the globe. The gas-conducting tube is
then connected with a vessel containing chlorine, and the cock
opened; the chlorine rushes in, and the metallic leaves are
consumed.

[17] The behaviour of platinum to chlorine at a high temperature
(1,400°) is very remarkable, because platinous chloride, PtCl_{2},
is then formed, whilst this substance decomposes at a much lower
temperature into chlorine and platinum. Hence, when chlorine comes
into contact with platinum at such high temperatures, it forms
fumes of platinous chloride, and they on cooling decompose, with
the liberation of platinum, so that the phenomenon appears to be
dependent on the volatility of platinum. Deville proved the
formation of platinous chloride by inserting a cold tube inside a
red-hot one (as in the experiment on carbonic oxide). However, V.
Meyer was able to observe the density of chlorine in a platinum
vessel at 1,690°, at which temperature chlorine does not exert
this action on platinum, or at least only to an insignificant
degree.

[18] When left exposed to the air aqua regia disengages chlorine, and
afterwards it no longer acts on gold. Gay-Lussac, in explaining
the action of aqua regia, showed that when heated it evolves,
besides chlorine, the vapours of two chloranhydrides--that of
nitric acid, NO_{2}Cl (nitric acid, NO_{2}OH, in which HO is
replaced by chlorine; _see_ Chapter on Phosphorus), and that of
nitrous acid, NOCl--but these do not act on gold. The formation of
aqua regia may therefore be expressed by 4NHO_{3} + 8HCl =
2NO_{2}Cl + 2NOCl + 6H_{2}O + 2Cl_{2}. The formation of the
chlorides NO_{2}Cl and NOCl is explained by the fact that the
nitric acid is deoxidised, gives the oxides NO and NO_{2}, and
they directly combine with chlorine to form the above anhydrides.

The majority of _non-metals_ also react directly on chlorine; hot sulphur and phosphorus burn in it and combine with it at the ordinary temperature. Only nitrogen, carbon, and oxygen do not combine directly with it. The chlorine compounds formed by the non-metals--for instance, phosphorus trichloride, PCl_{3}, and sulphurous chloride, &c., do not have the properties of salts, and, as we shall afterwards see more fully, correspond to acid anhydrides and acids; for example, PCl_{3}--to phosphorous acid, P(OH)_{3}:

NaCl FeCl_{2} SnCl_{4} PCl_{3} HCl
Na(HO) Fe(HO)_{2} Sn(HO)_{4} P(HO)_{3} H(HO)

As the above-mentioned relation in composition--_i.e._ substitution of Cl by the aqueous residue--exists between many chlorine compounds and their corresponding hydrates, and as furthermore some (acid) hydrates are obtained from chlorine compounds by the action of water, for instance,

PCl_{3} + 3H_{2}0 = P(HO)_{3} + 3HCl
Phosphorus Water Phosphorous Hydrochloric
trichloride acid acid

whilst other chlorine compounds are formed from hydroxides and hydrochloric acid, with the liberation of water, for example,

NaHO + HCl = NaCl + H_{2}O

we endeavour to express this intimate connection between the hydrates and chlorine compounds by calling the latter _chloranhydrides_. In general terms, if the hydrate be basic, then,

M(HO) + HCl = MCl + H_{2}O
hydrate + hydrochloric acid = chloranhydride + water

and if the hydrate ROH be acid, then,

RCl + H_{2}O = R(HO) + HCl
Chloranhydride + water = hydrate + hydrochloric acid

The chloranhydrides MCl corresponding to the bases are evidently metallic chlorides or salts corresponding to HCl. In this manner a distinct equivalency is marked between the compounds of chlorine and the so-called hydroxyl radicle (HO), which is also expressed in the analogy existing between chlorine, Cl_{2}, and hydrogen peroxide, (HO)_{2}.

As regards the chloranhydrides corresponding to acids and non-metals, they bear but little resemblance to metallic salts. They are nearly all volatile, and have a powerful suffocating smell which irritates the eyes and respiratory organs. They react on water like many anhydrides of the acids, with the evolution of heat and liberation of hydrochloric acid, forming acid hydrates. For this reason they cannot usually be obtained from hydrates--that is, acids--by the action of hydrochloric acid, as in that case water would be formed together with them, and water decomposes them, converting them into hydrates. There are many intermediate chlorine compounds between true saline metallic chlorides like sodium chloride and true acid chloranhydrides, just as there are all kinds of transitions between bases and acids. Acid chloranhydrides are not only obtained from chlorine and non-metals, but also from many lower oxides, by the aid of chlorine. Thus, for example, CO, NO, NO_{2}, SO_{2}, and other lower oxides which are capable of combining with oxygen may also combine with a corresponding quantity of chlorine. Thus COCl_{2}, NOCl, NO_{2}Cl, SO_{2}Cl_{2}, &c., are obtained. They correspond with the hydrates CO(OH)_{2}, NO(OH), NO_{2}(OH), SO_{2}(OH)_{2}, &c., and to the anhydrides CO_{2}, N_{2}O_{3}, N_{2}O_{5}, SO_{3}, &c. Here we should notice two aspects of the matter: (1) chlorine combines with that with which oxygen is able to combine, because it is in many respects equally if not more energetic than oxygen and replaces it in the proportion Cl_{2} : O; (2) that highest limit of possible combination which is proper to a given element or grouping of elements is very easily and often attained by combination with chlorine. If phosphorus gives PCl_{3} and PCl_{5}, it is evident that PCl_{5} is the higher form of combination compared with PCl_{3}. To the form PCl_{5}, or in general PX_{5}, correspond PH_{4}I, PO(OH)_{3}, POCl_{3}, &c. If chlorine does not always directly give compounds of the highest possible forms for a given element, then generally the lower forms combine with it in order to reach or approach the limit. This is particularly clear in hydrocarbons, where we see the limit C_{_n_}H_{2_n_+2} very distinctly. The unsaturated hydrocarbons are sometimes able to combine with chlorine with the greatest ease and thus reach the limit. Thus ethylene, C_{2}H_{4}, combines with Cl_{2}, forming the so-called Dutch liquid or ethylene chloride, C_{2}H_{4}Cl_{2}, because it then reaches the limit C_{_n_}X_{2_n_+2}. In this and all similar cases the combined chlorine is able by reactions of substitution to give a hydroxide and a whole series of other derivatives. Thus a hydroxide called glycol, C_{2}H_{4}(OH)_{2}, is obtained from C_{2}H_{4}Cl_{2}.

Chlorine _in the presence of water_ very often acts directly _as an oxidising agent_. A substance A combines with chlorine and gives, for example, ACl_{2}, and this in turn a hydroxide, A(OH)_{2}, which on losing water forms AO. Here the chlorine has oxidised the substance A. This frequently happens in the simultaneous action of water and chlorine: A + H_{2}O + Cl_{2} = 2HCl + AO. Examples of this oxidising action of chlorine may frequently be observed both in practical chemistry and technical processes. Thus, for instance, chlorine in the presence of water oxidises sulphur and metallic sulphides. In this case the sulphur is converted into sulphuric acid, and the chlorine into hydrochloric acid, or a metallic chloride if a metallic sulphide be taken. A mixture of carbonic oxide and chlorine passed into water gives carbonic anhydride and hydrochloric acid. Sulphurous anhydride is oxidised by chlorine in the presence of water into sulphuric acid, just as it is by the action of nitric acid: SO_{2} + 2H_{2}O + Cl_{2} = H_{2}SO_{4} + 2HCl.

The oxidising action of chlorine in the presence of water is taken advantage of in practice for the rapid bleaching of tissues and fibres. The colouring matter of the fibres is altered by oxidation and converted into a colourless substance, but the chlorine afterwards acts on the tissue itself. Bleaching by means of chlorine therefore requires a certain amount of technical skill in order that the chlorine should not act on the fibres themselves, but that its action should be limited to the colouring matter only. The fibre for making writing paper, for instance, is bleached in this manner. The bleaching property of chlorine was discovered by Berthollet, and forms an important acquisition to the arts, because it has in the majority of cases replaced that which before was the universal method of bleaching--namely, exposure to the sun of the fabrics damped with water, which is still employed for linens, &c. Time and great trouble, and therefore money also, have been considerably saved by this change.[19]

[19] Ozone and peroxide of hydrogen also bleach tissues. As the action
of peroxide of hydrogen is easily controlled by taking a weak
solution, and as it has hardly any action upon the tissues
themselves, it is replacing chlorine more and more as a bleaching
agent. The oxidising property of chlorine is apparent in
destroying the majority of organic tissues, and proves fatal to
organisms. This action of chlorine is taken advantage of in
quarantine stations. But the simple fumigation by chlorine must be
carried on with great care in dwelling places, because chlorine
disengaged into the atmosphere renders it harmful to the health.

The power of chlorine for combination is intimately connected with its capacity for substitution, because, according to the law of substitution, if chlorine combines with hydrogen, then it also replaces hydrogen, and furthermore the combination and substitution are accomplished in the same quantities. Therefore _the atom of chlorine_ which combines with the atom of hydrogen is also able _to replace the atom of hydrogen_. We mention this property of chlorine not only because it illustrates the application of the law of substitution in clear and historically important examples, but more especially because reactions of this kind explain those _indirect methods_ of the formation of many substances which we have often mentioned and to which recourse is had in many cases in chemistry. Thus chlorine does not act on carbon,[20] oxygen, or nitrogen, but nevertheless its compounds with these elements may be obtained by the indirect method of the substitution of hydrogen by chlorine.

[20] A certain propensity of carbon to attract chlorine is evidenced in
the immense absorption of chlorine by charcoal (Note 7), but, so
far as is at present known (if I am not mistaken, no one has tried
the aid of light), no combination takes place between the chlorine
and carbon.

As chlorine easily combines with hydrogen, and does not act on carbon, it decomposes hydrocarbons (and many of their derivatives) at a high temperature, depriving them of their hydrogen and liberating the carbon, as, for example, is clearly seen when a lighted candle is placed in a vessel containing chlorine. The flame becomes smaller, but continues to burn for a certain time, a large amount of soot is obtained, and hydrochloric acid is formed. In this case the gaseous and incandescent substances of the flame are decomposed by the chlorine, the hydrogen combines with it, and the carbon is disengaged as soot.[21] This action of chlorine on hydrocarbons, &c., proceeds otherwise at lower temperatures, as we will now consider.

[21] The same reaction takes place under the action of oxygen, with
the difference that it burns the carbon, which chlorine is not
able to do. If chlorine and oxygen compete together at a high
temperature, the oxygen will unite with the carbon, and the
chlorine with the hydrogen.

A very important epoch in the history of chemistry was inaugurated by the discovery of Dumas and Laurent that chlorine is able to displace and _replace hydrogen_. This discovery is important from the fact that chlorine proved to be an element which combines with great ease simultaneously with both the hydrogen and the element with which the hydrogen was combined. This clearly proved that there is no opposite polarity between elements forming stable compounds. Chlorine does not combine with hydrogen because it has opposite properties, as Dumas and Laurent stated previously, accounting hydrogen to be electro-positive and chlorine electro-negative; this is not the reason of their combining together, for the same chlorine which combines with hydrogen is also able to replace it without altering many of the properties of the resultant substance. This substitution of hydrogen by chlorine is termed _metalepsis_. The mechanism of this substitution is very constant. If we take a hydrogen compound, preferably a hydrocarbon, and if chlorine acts directly on it, then there is produced on the one hand hydrochloric acid and on the other hand a compound containing chlorine in the place of the hydrogen--so that the chlorine divides itself into two equal portions, one portion is evolved as hydrochloric acid, and the other portion takes the place of the hydrogen thus liberated. _Hence this metalepsis is always accompanied by the formation of hydrochloric acid._[22] The scheme of the process is as follows:

C_{n}H_{m}X + Cl_{2} = C_{n}H_{m-1}ClX + HCl
Hydrocarbon Free Product of Hydrochloric
chlorine metalepsis acid

Or, in general terms--

RH + Cl_{2} = RCl + HCl.

[22] This division of chlorine into two portions may at the same time
be taken as a clear confirmation of the conception of molecules.
According to Avogadro-Gerhardt's law, the molecule of chlorine (p.
310) contains two atoms of this substance; one atom replaces
hydrogen, and the other combines with it.

The conditions under which metalepsis takes place are also very constant. In the dark chlorine does not usually act on hydrogen compounds, but the action commences under the influence of light. The direct action of the sun's rays is particularly propitious to metalepsis. It is also remarkable that the presence of traces of certain substances,[23] especially of iodine, aluminium chloride, antimony chloride, &c., promotes the action. A trace of iodine added to the substance subjected to metalepsis often produces the same effect as sunlight.[24]

[23] Such carriers or media for the transference of chlorine and the
halogens in general were long known to exist in iodine and
antimonious chloride, and have been most fully studied by
Gustavson and Friedel, of the Petroffsky Academy--the former with
respect to aluminium bromide, and the latter with respect to
aluminium chloride. Gustavson showed that if a trace of metallic
aluminium be dissolved in bromine (it floats on bromine, and when
combination takes place much heat and light are evolved), the
latter becomes endowed with the property of entering into
metalepsis, which it is not able to do of its own accord. When
pure, for instance, it acts very slowly on benzene, C_{6}H_{6},
but in the presence of a trace of aluminium bromide the reaction
proceeds violently and easily, so that each drop of the
hydrocarbon gives a mass of hydrobromic acid, and of the product
of metalepsis. Gustavson showed that the _modus operandi_ of this
instructive reaction is based on the property of aluminium bromide
to enter into combination with hydrocarbons and their derivatives.
The details of this and all researches concerning the metalepsis
of the hydrocarbons must be looked for in works on organic
chemistry.

[24] As small admixtures of iodine, aluminium bromide, &c., aid the
metalepsis of large quantities of a substance, just as nitric
oxide aids the reaction of sulphurous anhydride on oxygen and
water, so the principle is essentially the same in both cases.
Effects of this kind (which should also be explained by a chemical
reaction proceeding at the surfaces) only differ from true contact
phenomena in that the latter are produced by solid bodies and are
accomplished at their surfaces, whilst in the former all is in
solution. Probably the action of iodine is founded on the
formation of iodine chloride, which reacts more easily than
chlorine.

If marsh gas be mixed with chlorine and the mixture ignited, then the hydrogen is entirely taken up from the marsh gas and hydrochloric acid and carbon formed, but there is no metalepsis.[25] But if a mixture of equal volumes of chlorine and marsh gas be exposed to the action of diffused light, then the greenish yellow mixture gradually becomes colourless, and hydrochloric acid and the first product of metalepsis--namely, methyl chloride--are formed:

CH_{4} + Cl_{2} = CH_{3}Cl + HCl
Marsh gas Chlorine Methyl chloride Hydrochloric acid

[25] Metalepsis belongs to the number of delicate reactions--if it may
be so expressed--as compared with the energetic reaction of
combustion. Many cases of substitution are of this kind. Reactions
of metalepsis are accompanied by an evolution of heat, but in a
less quantity than that evolved in the formation of the resulting
quantity of the halogen acids. Thus the reaction C_{2}H_{6} +
Cl_{2} = C_{2}H_{5}Cl + HCl, according to the data given by
Thomsen, evolves about 20,000 heat units, whilst the formation of
hydrochloric acid evolves 22,000 units.

The volume of the mixture remains unaltered. The methyl chloride which is formed is a gas. If it be separated from the hydrochloric acid (it is soluble in acetic acid, in which hydrochloric acid is but sparingly soluble) and be again mixed with chlorine, then it may be subjected to a further metalepsical substitution--the second atom of hydrogen may be substituted by chlorine, and a liquid substance, CH_{2}Cl_{2}, called methylene chloride, will be obtained. In the same manner the substitution may be carried on still further, and CHCl_{3}, or chloroform, and lastly carbon tetrachloride, CCl_{4}, will be produced. Of these substances the best known is chloroform, owing to its being formed from many organic substances (by the action of bleaching powder) and to its being used in medicine as an anæsthetic; chloroform boils at 62° and carbon tetrachloride at 78°. They are both colourless odoriferous liquids, heavier than water. The progressive substitution of hydrogen by chlorine is thus evident, and it can be clearly seen that the double decompositions are accomplished between molecular quantities of the substance--that is, between equal volumes in a gaseous state.

_Carbon tetrachloride_, which is obtained by the metalepsis of marsh gas, cannot be obtained directly from chlorine and carbon, but it may be obtained from certain compounds of carbon--for instance, from carbon bisulphide--if its vapour mixed with chlorine be passed through a red-hot tube. Both the sulphur and carbon then combine with the chlorine. It is evident that by ultimate metalepsis a corresponding carbon chloride may be obtained from any hydrocarbon--indeed, the number of chlorides of carbon C_{_n_}Cl_{2_m_} already known is very large.

As a rule, the fundamental chemical characters of hydrocarbons are not changed by metalepsis; that is, if a neutral substance be taken, then the product of metalepsis is also a neutral substance, or if an acid be taken the product of metalepsis also has acid properties. Even the crystalline form not unfrequently remains unaltered after metalepsis. The metalepsis of acetic acid, CH_{3}·COOH, is historically the most important. It contains three of the atoms of the hydrogen of marsh gas, the fourth being replaced by carboxyl, and therefore by the action of chlorine it gives three products of metalepsis (according to the amount of the chlorine and conditions under which the reaction takes place), mono-, di-, and tri-chloracetic acids--CH_{2}Cl·COOH, CHCl_{2}·COOH, and CCl_{3}·COOH; they are all, like acetic acid, monobasic. The resulting products of metalepsis, in containing an element which so easily acts on metals as chlorine, possess the possibility of attaining a further complexity of molecules of which the original hydrocarbon is often in no way capable. Thus on treating with an alkali (or first with a salt and then with an alkali, or with a basic oxide and water, &c.) the chlorine forms a salt with its metal, and the hydroxyl radicle takes the place of the chlorine--for example, CH_{3}·OH is obtained from CH_{3}Cl. By the action of metallic derivatives of hydrocarbons--for example, CH_{3}Na--the chlorine also gives a salt, and the hydrocarbon radicle--for instance, CH_{3}--takes the place of the chlorine. In this, or in a similar manner, CH_{3}·CH_{3}, or C_{2}H_{6} is obtained from CH_{3}Cl and C_{6}H_{5}·CH_{3} from C_{6}H_{6}. The products of metalepsis also often react on ammonia, forming hydrochloric acid (and thence NH_{4}Cl) and an amide; that is, the product of metalepsis, with the ammonia radicle NH_{2}, &c. in the place of chlorine. Thus by means of metalepsical substitution methods were found in chemistry for an artificial and general means of the formation of complex carbon compounds from more simple compounds which are often totally incapable of direct reaction. Besides which, this key opened the doors of that secret edifice of complex organic compounds into which man had up to then feared to enter, supposing the hydrocarbon elements to be united only under the influence of those mystic forces acting in organisms.[26]

[26] With the predominance of the representation of compound radicles
(this doctrine dates from Lavoisier and Gay-Lussac) in organic
chemistry, it was a very important moment in its history when it
became possible to gain an insight into the structure of the
radicles themselves. It was clear, for instance, that ethyl,
C_{2}H_{5}, or the radicle of common alcohol, C_{2}H_{5}·OH,
passes, without changing, into a number of ethyl derivatives, but
its relation to the still simpler hydrocarbons was not clear, and
occupied the attention of science in the 'forties' and 'fifties.'
Having obtained ethyl hydride, C_{2}H_{5}H = C_{2}H_{6}, it was
looked on as containing the same ethyl, just as methyl hydride,
CH_{4} = CH_{3}H, was considered as existing in methane. Having
obtained free methyl, CH_{3}CH_{3} = C_{2}H_{6}, from it, it was
considered as a derivative of methyl alcohol, CH_{3}OH, and as
only isomeric with ethyl hydride. By means of the products of
metalepsis it was proved that this is not a case of isomerism but
of strict identity, and it therefore became clear that ethyl is
methylated methyl, C_{2}H_{5} = CH_{2}CH_{3}. In its time a still
greater impetus was given by the study of the reactions of
monochloracetic acid, CH_{2}Cl·COOH, or CO(CH_{2}Cl)(OH). It
appeared that metalepsical chlorine, like the chlorine of
chloranhydrides--for instance, of methyl chloride, CH_{3}Cl, or
ethyl chloride, C_{2}H_{5}Cl--is capable of substitution; for
example, glycollic acid, CH_{2}(OH)(CO_{2}H), or
CO(CH_{2}·OH)(OH), was obtained from it, and it appeared that the
OH in the group CH_{2}(OH) reacted like that in alcohols, and it
became clear, therefore, that it was necessary to examine the
radicles themselves by analysing them from the point of view of
the bonds connecting the constituent atoms. Whence arose the
present doctrine of the structure of the carbon compounds. (_See_