Chapter IV: Ozone and Hydrogen Peroxide--Dalton's Law (1)

VAN MARUM, during the last century, observed that oxygen in a glass tube, when subjected to the action of a series of electric sparks, acquired a peculiar smell, and the property of combining with mercury at the ordinary temperature. This was afterwards confirmed by a number of fresh experiments. Even in the simple revolution of an electrical machine, when electricity diffuses into the air or passes through it, the peculiar and characteristic smell of ozone, proceeding from the action of the electricity on the oxygen of the atmosphere, is recognised. In 1840 Prof. Schönbein, of Basle, turned his attention to this odoriferous substance, and showed that it is also formed, with the oxygen evolved at the positive pole, in the decomposition of water by the action of a galvanic current; in the oxidation of phosphorus in damp air, and also in the oxidation of a number of substances, although it is distinguished for its instability and capacity for oxidising other substances. The characteristic smell of this substance gave it its name, from the Greek [Greek: ozô], 'I emit an odour.' Schönbein pointed out that _ozone_ is capable of oxidising many substances on which oxygen does not act at the ordinary temperature. It will be sufficient to point out for instance that it oxidises silver, mercury, charcoal, and iron with great energy at the ordinary temperature. It might be thought that ozone was some new compound substance, as it was at first supposed to be; but careful observations made in this direction have long led to the conclusion that ozone is nothing but oxygen altered in its properties. This is most strikingly proved by the complete transformation of oxygen containing ozone into ordinary oxygen when it is passed through a tube heated to 250°. Further, at a low temperature pure oxygen gives ozone when electric sparks are passed through it (Marignac and De la Rive). Hence it is proved both by synthesis and analysis that ozone is that same oxygen with which we are already acquainted, only endowed with particular properties and in a particular state. However, by whatever method it be obtained, the amount of it contained in the oxygen is inconsiderable, generally only a few fractions per cent., rarely 2 per cent., and only under very propitious circumstances as much as 20 per cent. The reason of this must be looked for first in the fact that _ozone in its formation from oxygen absorbs heat_. If any substance be burnt in a calorimeter at the expense of ozonised oxygen, then more heat is evolved than when it is burnt in ordinary oxygen, and Berthelot showed that this difference is very large--namely, 29,600 heat units correspond with every forty-eight parts by weight of ozone. This signifies that the transformation of forty-eight parts of oxygen into ozone is accompanied by the absorption of this quantity of heat, and that the reverse process evolves this quantity of heat. Therefore the passage of ozone into oxygen should take place easily and fully (as an exothermal reaction), like combustion; and this is proved by the fact that at 250° ozone entirely disappears, forming oxygen. Any rise of temperature may thus bring about the breaking up of ozone, and as a rise of temperature takes place in the action of an electrical discharge, there are in an electric discharge the conditions both for the preparation of ozone and for its destruction. Hence it is clear that the transformation of oxygen into ozone _as a reversible reaction_ has a limit when a state of equilibrium is arrived at between the products of the two opposite reactions, that the phenomena of this transformation accord with the phenomena of _dissociation_, and that a fall of temperature should aid the formation of a large quantity of ozone.[1] Further, it is evident, from what has been said, that the best way of preparing ozone is not by electric sparks,[2] which raise the temperature, but by the employment of a continual discharge or flow of electricity--that is, by the action of a _silent discharge_.[3] For this reason all _ozonisers_ (which are of most varied construction), or forms of apparatus for the preparation of ozone from oxygen (or air) by the action of electricity, now usually consist of sheets of metal--for instance, tinfoil--a solution of sulphuric acid mixed with chromic acid, &c. separated by thin glass surfaces placed at short distances from each other, and between which the oxygen or air to be ozonised is introduced and subjected to the action of a silent discharge.[4] Thus in Siemens' apparatus (fig. 37) the exterior of the tube _a_ and the interior of the tube _b c_ are coated with tinfoil and connected with the poles of a source of electricity (with the terminals of a Ruhmkorff's coil). A silent discharge passes through the thin walls of the glass cylinders _a_ and _b c_ over all their surfaces, and consequently, if oxygen be passed through the apparatus by the tube _d_, fused into the side of _a_, it will be ozonised in the annular space between _a_ and _b c_. The ozonised oxygen escapes by the tube _e_, and may be introduced into any other apparatus.[5]

[1] This conclusion, deduced by me as far back as 1878 (_Moniteur
Scientifique_) by conceiving the molecules of ozone (see later)
as more complex than those of oxygen, and ozone as containing
a greater quantity of heat than oxygen, has been proved
experimentally by the researches of Mailfert (1880), who showed
that the passage of a silent discharge through a litre of oxygen
at 0° may form up to 14 milligrams of ozone, and at -30° up to
60 milligrams; but best of all in the determinations of Chappuis
and Hautefeuille (1880), who found that at a temperature of -25° a
silent discharge converted 20 p.c. of oxygen into ozone, whilst
at 20° it was impossible to obtain more than 12 p.c., and at 100°
less than 2 p.c. of ozone was obtained.

[2] A series of electric sparks may be obtained by an ordinary
electrical machine, the electrophorus machines of Holtz and
Teploff, &c., Leyden jars, Ruhmkorff coils, or similar means, when
the opposite electricities are able to accumulate at the terminals
of conductors, and a discharge of sufficient electrical intensity
passes through the non-conductors air or oxygen.

[3] A silent discharge is such a combination of opposite statical
(potential) electricities as takes place (generally between large
surfaces) regularly, without sparks, slowly, and quietly (as in
the dispersion of electricity). The discharge is only luminous
in the dark; there is no observable rise of temperature, and
therefore a larger amount of ozone is formed. But, nevertheless,
on continuing the passage of a silent discharge through ozone it
is destroyed. For the action to be observable a large surface is
necessary, and consequently a source of electricity at a high
potential. For this reason the silent discharge is best produced
by a Ruhmkorff coil, as the most convenient means of obtaining a
considerable potential of statical electricity with the employment
of the comparatively feeble current of a galvanic battery.

[4] _v. Babo's apparatus_ was one of the first constructed for
ozonising oxygen by means of a silent discharge (and it is still
one of the best). It is composed of a number (twenty and more) of
long, thin capillary glass tubes closed at one end. A platinum
wire, extending along their whole length, is introduced into the
other end of each tube, and this end is then fused up round the
wire, the end of which protrudes outside the tube. The protruding
ends of the wires are arranged alternately in two sides in such a
manner that on one side there are ten closed ends and ten wires.
A bunch of such tubes (forty should make a bunch of not more than
1 c.m. diameter) is placed in a glass tube, and the ends of the
wires are connected with two conductors, and are fused to the ends
of the surrounding tube. The discharge of a Ruhmkorff coil is
passed through these ends of the wires, and the dry air or oxygen
to be ozonised is passed through the tube. If oxygen be passed
through, ozone is obtained in large quantities, and free from
oxides of nitrogen, which are partially formed when air is acted
on. At low temperatures ozone is formed in large quantities. As
ozone acts on corks and india-rubber, the apparatus should be made
entirely of glass. With a powerful Ruhmkorff coil and forty tubes
the ozonation is so powerful that the gas when passed through a
solution of iodide of potassium not only sets the iodine free, but
even oxidises it to potassium iodate, so that in five minutes the
gas-conducting tube is choked up with crystals of the insoluble
iodate.

[5] In order to connect the ozoniser with any other apparatus it is
impossible to make use of india-rubber, mercury, or cements, &c.,
because they are themselves acted on by, and act on, ozone. All
connections must, as was first proposed by Brodie, be hermetically
closed by sulphuric acid, which is not acted on by ozone. Thus, a
cork is passed over the vertical end of a tube, over which a wide
tube passes so that the end of the first tube protrudes above the
cork; mercury is first poured over the cork (to prevent its being
acted on by the sulphuric acid), and then sulphuric acid is poured
over the mercury. The protruding end of the first tube is covered
by the lower end of a third tube immersed in the sulphuric acid.

_The properties of ozone_ obtained by such a method[6] distinguish it in many respects from oxygen. Ozone very rapidly decolorises indigo, litmus, and many other dyes by oxidising them. Silver is oxidised by it at the ordinary temperature, whilst oxygen is not able to oxidise silver even at high temperatures; a bright silver plate rapidly turns black (from oxidation) in ozonised oxygen. It is rapidly absorbed by mercury, forming oxide; it transforms the lower oxides into higher--for instance, sulphurous anhydride into sulphuric, nitrous oxide into nitric, arsenious anhydride (As_{2}O_{3}) into arsenic anhydride (As_{2}O_{5}) &c.[7] But what is especially characteristic in ozone is the decomposing action it exerts on potassium iodide. Oxygen does not act on it, but ozone passed into a solution of potassium iodide _liberates iodine_, whilst the potassium is obtained as caustic potash, which remains in solution, 2KI + H_{2}O + O = 2KHO + I_{2}. As the presence of minute traces of free iodine may be discovered by means of starch paste, with which it forms a very dark blue-coloured substance, a mixture of potassium iodide with starch paste will detect the presence of very small traces of ozone.[8] Ozone is destroyed or converted into ordinary oxygen not only by heat, but also by long keeping, especially in the presence of alkalis, peroxide of manganese, chlorine, &c.

[6] The method above described is the only one which has been well
investigated. The admixture of nitrogen, or even of hydrogen,
and especially of silicon fluoride, appears to aid the formation
and preservation of ozone. Amongst other methods for preparing
ozone we may mention the following: 1. In the action of oxygen on
phosphorus at the ordinary temperature a portion of the oxygen
is converted into ozone. At the ordinary temperature a stick of
phosphorus, partially immersed in water and partially in air in
a large glass vessel, causes the air to acquire the odour of
ozone. It must further be remarked that if the air be left for
long in contact with the phosphorus, or without the presence of
water, the ozone formed is destroyed by the phosphorus. 2. By the
action of sulphuric acid on peroxide of barium. If the latter
be covered with strong sulphuric acid (the acid, if diluted
with only one-tenth of water, does not give ozone), then at a
low temperature the oxygen evolved contains ozone, and in much
greater quantities than in that ozone is obtained by the action
of electric sparks or phosphorus. 3. Ozone may also be obtained
by decomposing strong sulphuric acid by potassium manganate
especially with the addition of barium peroxide.

[7] Ozone takes up the hydrogen from hydrochloric acid; chlorine is
liberated, and can dissolve gold. Iodine is directly oxidised
by ozone, but not by oxygen. Ammonia, NH_{3}, is oxidised by
ozone into ammonium nitrite (and nitrate), 2NH_{3} + O_{3} =
NH_{4}NO_{2} + H_{2}O, and therefore a drop of ammonia, on
falling into the gas, gives a thick cloud of the salts formed.
Ozone converts lead oxide into peroxide, and suboxide of
thallium (which is colourless) into oxide (which is brown), so
that this reaction is made use of for detecting the presence of
ozone. Lead sulphide, PbS (black), is converted into sulphate,
PbSO_{4} (colourless), by ozone. A neutral solution of manganese
sulphate gives a precipitate of manganese peroxide, and an acid
solution may be oxidised into permanganic acid, HMnO_{4}. With
respect to the oxidising action of ozone on organic substances,
it may be mentioned that with ether, C_{4}H_{10}O, ozone gives
ethyl peroxide, which is capable of decomposing with explosion
(according to Berthelot), and is decomposed by water into alcohol,
2C_{2}H_{6}O, and hydrogen peroxide, H_{2}O_{2}.

[8] This reaction is the one usually made use of for detecting the
presence of ozone. In the majority of cases paper is soaked in
solutions of potassium iodide and starch. Such _ozonometrical_
or iodised starch-paper when damp turns blue in the presence
of ozone, and the tint obtained varies considerably, according
to the length of time it is exposed and to the amount of ozone
present. The amount of ozone in a given gas may even to a certain
degree he judged by the shade of colour acquired by the paper, if
preliminary tests be made.

Test-paper for ozone is prepared in the following manner:--One
gram of neutral potassium iodide is dissolved in 100 grams of
distilled water; 10 grams of starch are then shaken up in the
solution, and the mixture is boiled until the starch is converted
into a jelly. This jelly is then smeared over blotting-paper and
left to dry. It must always he remembered, however, that the
colour of iodised starch-paper is changed not only by the action
of ozone, but of many other oxidisers; for example, by the oxides
of nitrogen (especially N_{2}O_{3}) and hydrogen peroxide. Houzeau
proposed soaking common litmus-paper with a solution of potassium
iodide, which in the presence of iodine would turn blue, owing to
the formation of KHO. In order to determine if the blue colour
is not produced by an alkali (ammonia) in the gas, a portion of
the paper is not soaked in the potassium iodide, but moistened
with water; this portion will then also turn blue if ammonia be
present. A reagent for distinguishing ozone from hydrogen peroxide
with certainty is not known, and therefore these substances in
very small quantities (for instance, in the atmosphere) may easily
he confounded. Until recent years the mistake has frequently been
made of ascribing the alteration of iodised starch-paper in the
air to the presence of ozone; at the present time there is reason
to believe that it is most often due to the presence of nitrous
acid (Ilosva, 1889).

Hence _ozone_, although it has the same _composition as oxygen_, differs from it in stability, and by the fact that it oxidises a number of substances very energetically at the ordinary temperature. In this respect ozone resembles the oxygen of certain unstable compounds, or oxygen at the moment of its liberation.[8 bis]

[8 bis] Fluorine (Chap. XI.), acting upon water at the ordinary
temperature, takes up the hydrogen, and evolves the oxygen in the
form of ozone (Moissan, 1889), and therefore the reaction must be
expressed thus:--3H_{2}O + 3F_{2} = 6HF + O_{3}.

In ordinary oxygen and ozone we see an example of one and the same substance, in this case an element, appearing in two states. This indicates that the properties of a substance, and even of an element, may vary without its composition varying. Very many such cases are known. Such cases of a chemical transformation which determine a difference in the properties of one and the same element are termed cases of isomerism. The cause of isomerism evidently lies deep within the essential conditions of a substance, and its investigation has already led to a number of results of unexpected importance and of immense scientific significance. It is easy to understand the difference between substances containing different elements or the same elements in different proportions. That a difference should exist in these cases necessarily follows, if, as our knowledge compels us, we admit that there is a radical difference in the simple bodies or elements. But when the composition--_i.e._ the quality and quantity of the elements in two substances is the same and yet their properties are different, then it becomes clear that the conceptions of diverse elements and of the varying composition of compounds, alone, are insufficient for the expression of all the diversity of properties of matter in nature. Something else, still more profound and internal than the composition of substances, must, judging from isomerism, determine the properties and transformation of substances.

On what are the isomerism of ozone and oxygen, and the peculiarities of ozone, dependent? In what, besides the extra store of energy, which is one of the peculiarities of ozone, resides the cause of its difference from oxygen? These questions for long occupied the minds of investigators, and were the motive for the most varied, exact, and accurate researches, which were chiefly directed to the study of the volumetric relations exhibited by ozone. In order to acquaint the reader with the previous researches of this kind, I cite the following from a memoir by Soret, in the 'Transactions of the French Academy of Sciences' for 1866:

'Our present knowledge of the volumetric relations of ozone may be expressed in the following manner:

'1. "Ordinary oxygen in changing into ozone under the action of electricity shows a diminution in volume." This was discovered by Andrews and Tait.

'2. "In acting on ozonised oxygen with potassium iodide and other substances capable of being oxidised, we destroy the ozone, but the volume of the gas remains unchanged." For the researches of Andrews, Soret, v. Babo, and others showed that the proportion of ozonised oxygen absorbed by the potassium iodide is equal to the original contraction of volume of the oxygen--that is, in the absorption of the ozone the volume of the gas remains unchanged. From this it might be imagined that ozone, so to say, does not occupy any space--is indefinitely dense.

'3. "By the action of heat ozonised oxygen increases in volume, and is transformed into ordinary oxygen. This increase in volume corresponds with the quantity of ozonised oxygen which is given up to the potassium iodide in its decomposition" (the same observers).

'4. These unquestionable experimental results lead to the conclusion that ozone is denser than oxygen, and that in its oxidising action it gives off that portion of its substance to which is due its extra density distinguishing it from ordinary oxygen.'

If we imagine (says Weltzien) that _n_ volumes of ozone consist of _n_ volumes of oxygen combined with _m_ volumes of the same substance, and that ozone in oxidising gives up _m_ volumes of oxygen and leaves _n_ volumes of ordinary oxygen gas, then all the above facts can be explained; otherwise it must be supposed that ozone is infinitely dense. 'In order to determine the density of ozone' (we again cite Soret) 'recourse cannot be had to the direct determination of the weight of a given volume of the gas, because ozone cannot be obtained in a pure state. It is always mixed with a very large quantity of oxygen. It was necessary, therefore, to have recourse to such substances as would absorb ozone without absorbing oxygen and without destroying the ozone. Then the density might be deduced from the decrease of volume produced in the gas by the action of this solvent in comparison with the quantity of oxygen given up to potassium iodide. Advantage must also be taken of the determination of the increase of volume produced by the action of heat on ozone, if the volume occupied by the ozone before heating be known.' Soret found two such substances, turpentine and oil of cinnamon. 'Ozone disappears in the presence of turpentine. This is accompanied by the appearance of a dense vapour, which fills a vessel of small capacity (0·14 litre) to such an extent that it is impenetrable to direct solar-rays. On leaving the vessel at rest, it is observed that the cloud of vapour settles; the clearing is first remarked at the upper portion of the vessel, and the brilliant colours of the rainbow are seen on the edge of a cloud of vapour.' Oil of cinnamon--that is, the volatile or essential oil of the well-known spice, cinnamon--gives under similar circumstances the same kind of vapours, but they are much less voluminous. On measuring the gaseous volume before and after the action of both volatile oils, a considerable decrease is remarked. On applying all the necessary corrections (for the solubility of oxygen in the oily liquids named above, for the tension of their vapour, for the change of pressure, &c.) and making a series of comparative determinations, Soret obtained the following result: two volumes of ozone capable of being dissolved, when changed to ordinary (by heating a wire to a red-heat by a galvanic current) increase by one volume. Hence it is evident that in the formation of ozone three volumes of oxygen give two volumes of ozone--that is, its density (referred to hydrogen) = 24.

The observations and determinations of Soret showed that ozone is heavier than oxygen, and even than carbonic anhydride (because ozonised oxygen passes through fine orifices more slowly than oxygen and than its mixtures with carbonic anhydride), although lighter than chlorine (it flows more rapidly through such orifices than chlorine), and they indicated that _ozone is one and a half times denser than oxygen_, which may be expressed by designating a molecule of oxygen by O_{2} and of ozone by O_{3}, and hence ozone OO_{2} is comparable with compound substances[9] formed by oxygen, as for instance CO_{2}, SO_{2}, NO_{2}, &c. This explains the chief differences between ozone and oxygen and the cause of the isomerism, and at the same time leads one to expect[10] that ozone, being a gas which is denser than oxygen, would be liquefied much more easily. This was actually shown to be the case in 1880, by Chappuis and Hautefeuille in their researches on the _physical properties of ozone_. Its boiling point under a pressure of 760 mm. is about -106°, and consequently compressed and refrigerated ozone when rapidly expanded forms drops, _i.e._ is liquefied. Liquid and compressed[11] ozone is blue. In dissolving in water ozone partly passes into oxygen. It explodes violently when suddenly compressed and heated, changing into ordinary oxygen and evolving, like all explosive substances,[12] that extra heat which distinguishes it from oxygen.

[9] Ozone is, so to say, an oxide of oxygen, just as water is an oxide
of hydrogen. Just as aqueous vapour is composed of two volumes of
hydrogen and one volume of oxygen, which on combining condense
into two volumes of aqueous vapour, so also two volumes of oxygen
are combined in ozone with one volume of oxygen to give two
volumes of ozone. In the action of ozone on different substances
it is only that additional portion of its molecule by which it
differs from ordinary oxygen that combines with other bodies, and
that is why, under these circumstances, the volume of the ozonised
oxygen does not change. Starting with two volumes of ozone,
one-third of its weight is parted with, and two volumes of oxygen
remain.

The above observations of Soret on the capacity of turpentine
for dissolving ozone, together with Schönbein's researches on
the formation of ozone in the oxidation of turpentine and of
similar volatile vegetable oils (entering into the composition
of _perfumes_), also explain the action of this ethereal oil on
a great many substances. It is known that turpentine oil, when
mixed with many substances, promotes their oxidation. In this case
it probably not only itself promotes the formation of ozone, but
also dissolves ozone from the atmosphere, and thus acquires the
property of oxidising many substances. It bleaches linen and cork,
decolorises indigo, promotes the oxidation and hardening of boiled
linseed oil, &c. These properties of turpentine oil are made use
of in practice. Dirty linen and many stained materials are easily
cleaned by turpentine, not only because it dissolves the grease,
but also because it oxidises it. The admixture of turpentine with
drying (boiled) oil, oil-colours, and lacs aids their rapid drying
because it attracts ozone. Various oils occurring in plants,
and entering into the composition of perfumes and certain scent
extracts, also act as oxidisers. They act in the same manner as
oil of turpentine and oil of cinnamon. This perhaps explains
the refreshing influence they have in scents and other similar
preparations, and also the salubrity of the air of pine forests.
Water upon which a layer of turpentine oil has been poured
acquires, when left standing in the light, the disinfecting and
oxidising properties in general of ozonised turpentine (is this
due to the formation of H_{2}O_{2}?).

[10] The densest, most complex, and heaviest particles of matter
should, under equal conditions, evidently be less capable of
passing into a state of gaseous motion, should sooner attain a
liquid state, and have a greater cohesive force.

[11] The blue colour proper to ozone may be seen through a tube one
metre long, filled with oxygen, containing 10 p.c. of ozone.
The density of liquid ozone has not, so far as I am aware, been
determined.

[12] All explosive bodies and mixtures (gunpowder, detonating gas,
&c.) evolve heat in exploding--that is, the reactions which
accompany explosions are exothermal. In this manner ozone in
decomposing evolves latent heat, although generally heat is
absorbed in decomposition. This shows the meaning and cause of
explosion.

Thus, judging by what has been said above, ozone should he formed in nature not only in the many processes of oxidation which go on, but also by the condensation of atmospheric oxygen. The significance of ozone in nature has often arrested the attention of observers. There is a series of ozonometrical observations which show the different amounts of ozone in the air at different localities, at different times of the year, and under different circumstances. But the observations made in this direction cannot be considered as sufficiently exact, because the methods in use for determining ozone were not quite accurate. It is however indisputable[13] that the amount of ozone in the atmosphere is subject to variation; that the air of dwellings contains no ozone (it disappears in oxidising organic matter); that the air of fields and forests always contains ozone, or substances (peroxide of hydrogen) which act like it (on iodised starch paper &c.)[13 bis]; that the amount of ozone increases after storms; and that miasms, &c., are destroyed by ozonising the atmosphere. It easily oxidises organic substances, and miasms are produced by organic substances and the germs of organisms, all of which are easily changed and oxidised. Indeed, many miasms--for instance, the volatile substance of decomposing organisms--are clearly destroyed or changed not only by ozone, but also by many other powerfully oxidising substances, such as chlorine water, potassium permanganate, and the like.[14] All that is now known respecting the presence of ozone in the air may be summed up in the following words: A small quantity of an oxidising substance, resembling ozone in its reactions, has undoubtedly been observed and determined in the atmosphere, especially in fresh air, for instance after a storm, and it is very likely that this substance contains a mixture of such oxidising substances as ozone, peroxide of hydrogen, and the lower oxides of nitrogen (especially nitrous acid and its ammonia salt) produced from the elements of the atmosphere by oxidation and by the action of electrical discharges.

[13] In Paris it has been found that the further from the centre of the
town the greater the amount of ozone in the air. The reason of
this is evident: in a city there are many conditions for the
destruction of ozone. This is why we distinguish country air
as being fresh. In spring the air contains more ozone than in
autumn; the air of fields more than the air of towns.

[13 bis] The question of the presence of ozone in the air has not yet
been fully elucidated, as those reactions by which ozone is
generally detected are also common to nitrous acid (and its
ammonia salt). Ilosvay de Ilosva (1889), in order to exclude
the influence of such bodies, passed air through a 40 per cent.
solution of caustic soda, and then through a 20 per cent.
solution of sulphuric acid (these solutions do not destroy
ozone), and tested the air thus purified for the presence of
ozone. As no ozone was then detected the author concludes that
all the effects which were formerly ascribed to ozone should
be referred to nitrous acid. But this conclusion requires more
careful verification, since the researches of Prof. Schönbein on
the presence of peroxide of hydrogen in the atmosphere.

[14] The oxidising action of ozone may be taken advantage of for
technical purposes; for instance, for destroying colouring
matters. It has even been employed for bleaching tissues and for
the rapid preparation of vinegar, although these methods have not
yet received wide application.

Thus in ozone we see (1) the capacity of elements (and it must be all the more marked in compounds) of changing in properties without altering in composition; this is termed isomerism;[15] (2) the capacity of certain elements for condensing themselves into molecules of different densities; this forms a special case of isomerism called _polymerism_; (3) the capacity of oxygen for appearing in a still more active and energetic chemical state than that in which it occurs in ordinary gaseous oxygen; and (4) the formation of unstable equilibria, or chemical states, which are illustrated both by the ease with which ozone acts as an oxidiser and by its capacity for decomposing with explosion.[16]

[15] Isomerism in elements is termed _allotropism_.

[16] A number of substances resemble ozone in one or other of these
respects. Thus cyanogen, C_{2}N_{2}, nitrogen chloride, &c.,
decompose with an explosion and evolution of heat. Nitrous
anhydride, N_{2}O_{3}, forms a blue liquid like ozone, and in a
number of cases oxidises like ozone.

_Hydrogen peroxide._--Many of those properties which we have seen in ozone belong also to a peculiar substance containing oxygen and hydrogen and called hydrogen peroxide or oxygenated water. This substance was discovered in 1818 by Thénard. When heated it is decomposed into water and oxygen, evolving as much oxygen as is contained in the water remaining after the decomposition. That portion of oxygen by which hydrogen peroxide differs from water behaves in a number of cases just like the active oxygen in ozone, which distinguishes it from ordinary oxygen. In H_{2}O_{2}, and in O_{3}, one atom of oxygen acts as a powerful oxidiser, and on separating out it leaves H_{2}O or O_{2}, which do not act so energetically, although they still contain oxygen.[17] Both H_{2}O_{2} and O_{3} contain the oxygen in a compressed state, so to speak, and when freed from pressure by the forces (internal) of the elements in another substance, this oxygen is easily evolved, and therefore acts as oxygen does at the moment of its liberation. Both substances in decomposing, with the separation of a portion of their oxygen, _evolve_ heat, whilst decomposition is usually accompanied by an absorption of heat.

[17] It is evident that there is a want of words here for distinguishing
oxygen, O, as an ultimate _element_, from oxygen, O_{2}, as a
_free element_. The latter should be termed oxygen gas, did not
custom and the length of the expression render it inconvenient.

Hydrogen peroxide is formed under many circumstances by combustion and oxidation, but in very limited quantities; thus, for instance, it is sufficient to shake up zinc with sulphuric acid, or even with water, to observe the formation of a certain quantity of hydrogen peroxide in the water.[18] From this cause, probably, a series of diverse oxidation processes are accomplished in nature, and according to Prof. Schöne of Moscow, hydrogen peroxide occurs in the atmosphere, although in variable and small quantities, and probably its formation is connected with ozone, with which it has much in common. The usual mode of the formation of hydrogen peroxide, and the method by which it may be indirectly obtained,[19] is by the double decomposition of an acid and the peroxides of certain metals, especially those of potassium, calcium, and barium.[20] We saw when speaking of Oxygen (Chap. III.) that it is only necessary to heat the anhydrous oxide of barium to a red heat in a current of air or oxygen (or, better still, to heat it with potassium chlorate, and then to wash away the potassium chloride formed) to obtain peroxide of barium.[21] Barium peroxide gives hydrogen peroxide by the action of acids in the cold.[22] The process of decomposition is very clear in this case; the hydrogen of the acid replaces the barium of the peroxide, a barium salt of the acid being formed, while the hydrogen peroxide formed in the reaction remains in solution.[23]

[18] Schönbein states that the formation of hydrogen peroxide is
to be remarked in every oxidation in water or in the presence
of aqueous vapour. According to Struve, hydrogen peroxide
is contained in snow and in rain-water, and its formation,
together with ozone and ammonium nitrate, is even probable in
the processes of respiration and combustion. A solution of tin
in mercury, or liquid tin amalgam, when shaken up in water
containing sulphuric acid, produces hydrogen peroxide, whilst
iron under the same circumstances does not give rise to its
formation. The presence of small quantities of hydrogen peroxide
in these and similar cases is recognised by many reactions.
Amongst them, its action on _chromic acid_ in the presence of
ether is very characteristic. Hydrogen peroxide converts the
chromic acid into a higher oxide, Cr_{2}O_{7}, which is of a
dark-blue colour and dissolves in ether. This ethereal solution
is to a certain degree stable, and therefore the presence of
hydrogen peroxide may be recognised by mixing the liquid to be
tested with ether and adding several drops of a solution of
chromic acid. On shaking the mixture the ether dissolves the
higher oxide of chromium which is formed, and acquires a blue
colour. The formation of hydrogen peroxide in the combustion and
oxidation of substances containing or evolving hydrogen must
be understood in the light of the conception, to be considered
later, of molecules occupying equal volumes in a gaseous state.
At the moment of its evolution a molecule H_{2} combines with
a molecule O_{2}, and gives H_{2}O_{2}. As this substance is
unstable, a large proportion of it is decomposed, a small amount
only remaining unchanged. If it is obtained, water is easily
formed from it; this reaction evolves heat, and the reverse
action is not very probable. Direct determinations show that
the reaction H_{2}O_{2} = H_{2}O + O evolves 22,000 heat units.
From this it will be understood how easy is the decomposition
of hydrogen peroxide, as well as the fact that a number of
substances which are not directly oxidised by oxygen are oxidised
by hydrogen peroxide and by ozone, which also evolves heat on
decomposition. Such a representation of the origin of hydrogen
peroxide has been developed by me since 1870. Recently (1890)
Traube has pronounced a similar opinion, stating that Zn under
the action of water and air gives, besides ZnH_{2}O_{2}, also
H_{2}O_{2}.

[19] The formation of hydrogen peroxide from barium peroxide by a
method of double decomposition is an instance of a number of
_indirect methods of preparation_. A substance A does not combine
with B, but A B is obtained from A C in its action on B D (see
Introduction) when C D is formed. Water does not combine with
oxygen, but as a hydrate of acids it acts on the compound of
oxygen with barium oxide, because this oxide gives a salt with an
acid anhydride; or, what is the same thing, hydrogen with oxygen
does not directly form hydrogen peroxide, but when combined with
a haloid (for example, chlorine), under the action of barium
peroxide, BaO_{2}, it leads to the formation of a salt of barium
and H_{2}O_{2}. It is to be remarked that the passage of barium
oxide, BaO, into the peroxide, BaO_{2}, is accompanied by the
_evolution_ of 12,100 heat units per 16 parts of oxygen by weight
combined, and the passage of H_{2}O into the peroxide H_{2}O_{2}
does not proceed directly, because it would be accompanied by the
_absorption_ of 22,000 units of heat by 16 parts by weight of
oxygen combined. Barium peroxide, in acting on an acid, evidently
evolves less heat than the oxide, and it is this difference of
heat that is absorbed in the hydrogen peroxide. Its energy is
obtained from that evolved in the formation of the salt of barium.

[20] Peroxides of lead and manganese, and other analogous peroxides
(see Chap. III., Note 9), do not give hydrogen peroxide under
these conditions, but yield chlorine with hydrochloric acid.

[21] The impure barium peroxide obtained in this manner may be easily
purified. For this purpose it is dissolved in a dilute solution
of nitric acid. A certain quantity of an insoluble residue always
remains, from which the solution is separated by filtration.
The solution will contain not only the compound of the barium
peroxide, but also a compound of the barium oxide itself,
a certain quantity of which always remains uncombined with
oxygen. The acid compounds of the peroxide and oxide of barium
are easily distinguishable by their stability. The peroxide
gives an unstable compound, and the oxide a stable salt. By
adding an aqueous solution of barium oxide to the resultant
solution, the whole of the peroxide contained in the solution
may be precipitated as a pure aqueous compound (Kouriloff, 1889,
obtained the same result by adding an excess of BaO_{2}). The
first portions of the precipitate will consist of impurities--for
instance, oxide of iron. The barium peroxide then separates out,
and is collected on a filter and washed; it forms a substance
having a definite composition, BaO_{2},8H_{2}O, and is very
pure. Pure hydrogen peroxide should always be prepared from such
purified barium peroxide.

[22] In the cold, strong sulphuric acid with barium peroxide gives
ozone; when diluted with a certain amount of water it gives
oxygen (see Note 6), and hydrogen peroxide is only obtained
by the action of very weak sulphuric acid. Hydrochloric,
hydrofluoric, carbonic, and hydrosilicofluoric acids, and others,
when diluted with water also give hydrogen peroxide with barium
peroxide. Professor Schöne, who very carefully investigated
hydrogen peroxide, showed that it is formed by the action of many
of the above-mentioned acids on barium peroxide. In preparing
peroxide of hydrogen by means of sulphuric acid, the solution
must be kept cold. A solution of maximum concentration may
be obtained by successive treatments with sulphuric acid of
increasing strength. In this manner a solution containing 2 to 3
grams of pure peroxide in 100 c.c. of water may be obtained (V.
Kouriloff).

[23] With the majority of acids, that salt of barium which is formed
remains in solution; thus, for instance, by employing
hydrochloric acid, hydrogen peroxide and barium chloride remain
in solution. Complicated processes would be required to obtain
pure hydrogen peroxide from such a solution. It is much more
convenient to take advantage of the action of carbonic anhydride
on the pure hydrate of barium peroxide. For this purpose the
hydrate is stirred up in water, and a rapid stream of carbonic
anhydride is passed through the water. Barium carbonate,
insoluble in water, is formed, and the hydrogen peroxide remains
in solution, so that it may be separated from the carbonate
by filtering only. On a large scale hydrofluosilicic acid is
employed, its barium salt being also insoluble in water.

The reaction is expressed by the equation BaO_{2} + H_{2}SO_{4} = H_{2}O_{2} + BaSO_{4}. It is best to take a weak cold solution of sulphuric acid and to almost saturate it with barium peroxide, so that a small excess of acid remains; insoluble barium sulphate is formed. A more or less dilute aqueous solution of hydrogen peroxide is obtained. This solution may be concentrated in a vacuum over sulphuric acid. In this way the water may even be entirely evaporated from the solution of the hydrogen peroxide; only in this case it is necessary to work at a low temperature, and not to keep the peroxide for long in the rarefied atmosphere, as otherwise it decomposes.[23 bis] A solution of peroxide of hydrogen (mixed with the solution of a salt of sodium NaX) is used for bleaching (especially silk and wool) on a large scale, and is now usually prepared from peroxide of sodium Na_{2}O_{2} by the action of acids. Na_{2}O_{2} + 2HX = 2NaX + H_{2}O_{2}[24].

[23 bis] Hydrogen peroxide may be extracted from very dilute solutions
by means of ether, which dissolves it, and when mixed with it
the hydrogen peroxide may even be distilled. A solution of
hydrogen peroxide in water may be strengthened by cooling it to
a low temperature, when the water crystallises out--that is,
is converted into ice--whilst the hydrogen peroxide remains in
solution, as it only freezes at very low temperatures. It must be
observed that hydrogen peroxide, in a strong solution in a pure
state, is exceedingly unstable even at the ordinary temperature,
and therefore it must be preserved in vessels always kept cold,
as otherwise it evolves oxygen and forms water.

[24] Peroxide of sodium (Chap. XII., Note 49) is prepared by burning
sodium in dry air.

When pure, hydrogen peroxide is a colourless liquid, without smell, and having a very unpleasant taste--such as belongs to the salts of many metals--the so-called 'metallic' taste. Water stored in zinc vessels has this taste, which is probably due to its containing hydrogen peroxide. The tension of the vapour of hydrogen peroxide is less than that of aqueous vapour; this enables its solutions to be concentrated in a vacuum. The specific gravity of anhydrous hydrogen peroxide is 1·455. Hydrogen peroxide decomposes, with the evolution of oxygen, when heated even to 20°. But the more dilute its aqueous solution the more stable it is. Very weak solutions may be distilled without decomposing the hydrogen peroxide. It decolorises solutions of litmus and turmeric, and acts in a similar manner on many colouring matters of organic origin (for which reason it is employed for bleaching tissues).[24 bis]

[24 bis] Peroxide of hydrogen should apparently find an industrial
application in the arts, for instance, (1) as a bleaching agent,
it having the important advantage over chloride of lime, SO_{2},
&c., of not acting upon the material under treatment. It may be
used for bleaching feathers, hair, silk, wool, wood, &c., it also
removes stains of all kinds, such as wine, ink, and fruit stains;
(2) it destroys bacteria like ozone without having any injurious
effect upon the human body. It can also be used for washing all
kinds of wounds, for purifying the air in the sick room, &c., and
(3) as a preserving agent for potted meats, &c.

_Many substances decompose hydrogen peroxide_, forming water and oxygen, without apparently suffering any change. In this case substances in a state of fine division show a much quicker action than compact masses, from which it is evident that the action is here based on contact (_see_ Introduction). It is sufficient to bring hydrogen peroxide into contact with charcoal, gold, the peroxide of manganese or lead, the alkalis, metallic silver, and platinum, to bring about the above decomposition.[25] Besides which, hydrogen peroxide forms water and parts with its oxygen with great ease to a number of substances which are capable of being oxidised or of combining with oxygen, and in this respect is very like ozone and other _powerful oxidisers_.[26] To the class of contact phenomena, which are so characteristic of hydrogen peroxide as a substance which is unstable and easily decomposable with the evolution of heat, must be referred the following--that in the presence of many substances containing oxygen it evolves, not only its own oxygen, but also that of the substances which are brought into contact with it--that is, _it acts in a reducing manner_. It behaves thus with ozone, the oxides of silver, mercury, gold and platinum, and lead peroxide. The oxygen in these substances is not stable, and therefore the feeble influence of contact is enough to destroy its position. Hydrogen peroxide, especially in a concentrated form, in contact with these substances, evolves an immense quantity of oxygen, so that an explosion takes place and an exceedingly powerful evolution of heat is observed if hydrogen peroxide in a concentrated form be made to drop upon these substances in dry powder. Slow decomposition also proceeds in dilute solutions.[27]

[25] As the result of careful research, certain of the _catalytic_ or
contact phenomena have been subjected to exact explanation, which
shows the participation of a substance present in the process
of a reaction, whilst, however, it does not alter the series
of changes proceeding from mechanical actions only. Professor
Schöne, of the Petroffsky Academy, has already explained a
number of reactions of hydrogen peroxide which previously were
not understood. Thus, for instance, he showed that with hydrogen
peroxide, alkalis give peroxides of the alkaline metals, which
combine with the remaining hydrogen peroxide, forming unstable
compounds which are easily decomposed, and therefore alkalis
evince a decomposing (catalytic) influence on solutions of
hydrogen peroxide. Only acid solutions of hydrogen peroxide, and
then only dilute ones, can be preserved well.

[26] _Hydrogen peroxide_, as a substance containing much oxygen
(namely, 16 parts to one part by weight of hydrogen), exhibits
many _oxidising reactions_. Thus, it oxidises arsenic, converts
lime into calcium peroxide, the oxides of zinc and copper into
peroxides; it parts with its oxygen to many sulphides, converting
them into sulphates, &c. So, for example, it converts black
lead sulphide, PbS, into white lead sulphate, PbSO_{4}, copper
sulphide into copper sulphate, and so on. The restoration of old
oil paintings by hydrogen peroxide is based on this action. Oil
colours are usually admixed with white lead, and in many cases
the colour of oil-paints becomes darker in process of time. This
is partly due to the sulphuretted hydrogen contained in the
air, which acts on white lead, forming lead sulphide, which is
black. The intermixture of the black colour darkens the rest.
In cleaning a picture with a solution of hydrogen peroxide,
the black lead sulphide is converted into white sulphate, and
the colours brighten owing to the disappearance of the black
substance which previously darkened them. Hydrogen peroxide
oxidises with particular energy substances containing hydrogen
and capable of easily parting with it to oxidising substances.
Thus it decomposes hydriodic acid, setting the iodine free
and converting the hydrogen it contains into water; it also
decomposes sulphuretted hydrogen in exactly the same manner,
setting the sulphur free. Starch paste with potassium iodide
is not, however, directly coloured by peroxide of hydrogen in
the entire absence of free acids; but the addition of a small
quantity of iron sulphate (green vitriol) or of lead acetate to
the mixture is enough to entirely blacken the paste. This is a
very sensitive reagent (test) for peroxide of hydrogen, like the
test with chromic acid and ether (_see_ Note 8).

[27] To explain the phenomenon, an hypothesis has been put forward
by Brodie, Clausius, and Schönbein which supposes ordinary oxygen
to be an electrically neutral substance, composed, so to speak,
of two electrically opposite kinds of oxygen--positive and
negative. It is supposed that hydrogen peroxide contains one kind
of such polar oxygen, whilst in the oxides of the above-named
metals the oxygen is of opposite polarity. It is supposed that
in the oxides of the metals the oxygen is electro-negative, and
in hydrogen peroxide electro-positive, and that on the mutual
contact of these substances ordinary neutral oxygen is evolved
as a consequence of the mutual attraction of the oxygens of
opposite polarity. Brodie admits the polarity of oxygen in
combination, but not in an uncombined state, whilst Schönbein
supposes uncombined oxygen to be polar also, considering ozone as
electro-negative oxygen. The supposition that the oxygen of ozone
is different from that of hydrogen peroxide is contradicted by
the fact that in acting on barium peroxide strong sulphuric acid
forms ozone, and dilute acid forms hydrogen peroxide.

Just as a whole series of metallic compounds, and especially the oxides and their hydrates, correspond with water, so also there are many substances analogous to hydrogen peroxide. Thus, for instance, calcium peroxide is related to hydrogen peroxide in exactly the same way as calcium oxide or lime is related to water. In both cases the hydrogen is replaced by a metal--namely, by calcium.[27 bis] But it is most important to remark that the nearest approach to the properties of hydrogen peroxide is afforded by a non-metallic element, chlorine; its action on colouring matters, its capacity for oxidising, and for evolving oxygen from many oxides, is analogous to that exhibited by hydrogen peroxide. Even the very formation of chlorine is closely analogous to the formation of peroxide of hydrogen; chlorine is obtained from manganese peroxide, MnO_{2}, and hydrochloric acid, HCl, and hydrogen peroxide from barium peroxide, BaO_{2}, and the same acid. The result in one case is essentially water, chlorine, and manganese chloride; and in the other case barium chloride and hydrogen peroxide are produced. Hence water + chlorine corresponds with hydrogen peroxide, and the action of chlorine in the presence of water is analogous to the action of hydrogen peroxide. This analogy between chlorine and hydrogen peroxide is expressed in the conception of an aqueous radicle, which (Chapter III.) has been already mentioned. _This aqueous radicle_ (or hydroxyl) is that which is left from water if it be imagined as deprived of half of its hydrogen. According to this method of expression, caustic soda will be a compound of sodium with the aqueous radicle, because it is formed from water with the evolution of half the hydrogen. This is expressed by the following formulæ: water, H_{2}O, caustic soda, NaHO, just as hydrochloric acid is HCl and sodium chloride NaCl. Hence the aqueous radicle HO is a compound radicle, just as chlorine, Cl, is a simple radicle. They both give hydrogen compounds, HHO, water, and HCl, hydrochloric acid; sodium compounds, NaHO and NaCl, and a whole series of analogous compounds. Free chlorine in this sense will be ClCl, and hydrogen peroxide HOHO, which indeed expresses its composition, because it contains twice as much oxygen as water does.[28]

[27 bis] It should be mentioned that Schiloff (1893) on taking a 3 per
cent. solution of H_{2}O_{2}, adding soda to it, and then
extracting the peroxide of hydrogen from the mixture by shaking
it with ether, obtained a 50 per cent. solution of H_{2}O_{2},
which, although perfectly free from other acids, gave a
distinctly acid reaction with litmus. And here attention should
first of all be turned to the fact that the peroxides of the
metals correspond to H_{2}O_{2}, like salts to an acid, for
instance, Na_{2}O_{2} and BaO_{2}, &c. Furthermore, it must be
remembered that O is an analogue of S (Chapters XV. and XX.),
and sulphur gives H_{2}S, H_{2}SO_{3}, and H_{2}SO_{4}. And
sulphurous acid, H_{2}SO_{3}, is unstable as a hydrate, and
gives water and the anhydride SO_{2}. If the sulphur be replaced
by oxygen, then instead of H_{2}SO_{3} and SO_{2}, we have
H_{2}OO_{3} and OO_{2}. The latter is ozone, while the salt
K_{2}O_{4} (peroxide of potassium) corresponds to the hydrate
H_{2}O_{4} as to an acid. And between H_{2}O and H_{2}O_{4}
there may exist intermediate acid compounds, the first of which
would be H_{2}O_{2}, in which, from analogy to the sulphur
compounds, one would expect acid properties. Besides which we
may mention that for sulphur, besides H_{2}S (which is a feeble
acid), H_{2}S_{2}, H_{2}S_{3}, H_{2}S_{5} are known. Thus in
many respects H_{2}O_{2} offers points of resemblance to acid
compounds, and as regards its qualitative (reactive) analogies,
it not only resembles Na_{2}O_{2}, BaO_{2}, &c., but also
persulphuric acid HSO_{4} (to which the anhydride S_{2}O_{7}
corresponds) and Cu_{2}O_{7}, &c., which will be subsequently
described.

[28] Tamman and Carrara (1892) showed by determining the depression
(fall of the temperature of the formation of ice, Chapters I.
and VII.) that the molecule of peroxide of hydrogen contains
H_{2}O_{2}, and not HO or H_{3}O_{3}.

Thus in ozone and hydrogen peroxide we see examples of very unstable, easily decomposable (by time, spontaneously, and on contact) substances, full of the energy necessary for change,[28 bis] capable of being easily reconstituted (in this case decomposing with the evolution of heat); they are therefore examples of _unstable chemical equilibria_. If a substance exists, it signifies that it already presents a certain form of equilibrium between those elements of which it is built up. But chemical, like mechanical, equilibria exhibit different degrees of stability or solidity.[29]