Chapter III: Oxygen and the Chief Aspects of Its Saline Combinations (3)

[49] It must be remarked that certain elements form oxides of all three
kinds--_i.e._ indifferent, basic, and acid; for example,
manganese forms manganous oxide, manganic oxide, peroxide of
manganese, red oxide of manganese, and manganic anhydride,
although some of them are not known in a free state but only
in combination. The basic oxides contain less oxygen than the
peroxides, and the peroxides less than the acid anhydrides.
Thus they must be placed in the following general normal
order with respect to the amount of oxygen entering into
their composition--(1) basic oxides, suboxides, and oxides;
(2) peroxides; (3) acid anhydrides. The majority of elements,
however, do not give all three kinds of oxides, some giving only
one degree of oxidation. It must further be remarked that there
are oxides formed by the combination of acid anhydrides with
basic oxides, or, in general, of oxides with oxides. For every
oxide having a higher and a lower degree of oxidation, it might
be said that the intermediate oxide was formed by the combination
of the higher with the lower oxide. But this is not true in all
cases--for instance, when the oxide under consideration forms
a whole series of independent compounds--for oxides which are
really formed by the combination of two other oxides do not give
such independent compounds, but in many cases decompose into the
higher and lower oxides.

The oxides themselves rarely undergo chemical transformations, and in the few cases where they are subject to such changes a particularly important part is played by their combinations with water. The majority of, if not all, basic and acid oxides combine with water, either by a direct or an indirect method forming _hydrates_--that is, compounds which split up into water and an oxide of the same kind only. It is well known that many substances are capable of combining with water. Oxides possess this property in the highest degree. We have already seen examples of this (Chapter I.) in the combination of lime, and of sulphuric and phosphoric anhydrides, with water. The resulting combinations are basic and acid hydrates. Acid hydrates are called _acids_ because they have an acid taste when dissolved in water (or saliva), for then only can they act on the palate. Vinegar, for example, has an acid taste because it contains acetic acid dissolved in water. Sulphuric acid, to which we have frequently referred, because it is the acid of the greatest importance both in practical chemistry and for its technical applications, is really a hydrate formed by the combination of sulphuric anhydride with water. Besides their acid taste, dissolved acids or acid hydrates have the property of changing the blue colour of certain vegetable dyes to red. Of these dyes _litmus_ is particularly remarkable and much used. It is the blue substance extracted from certain lichens, and is used for dyeing tissues blue; it gives a blue infusion with water. This infusion, on the addition of an acid, _changes from blue to red_.[50]

[50] Blotting or unsized paper, soaked in a solution of litmus, is
usually employed for detecting the presence of acids. This paper
is cut into strips, and is called _test paper_; when dipped into
acid it immediately turns red. This is a most sensitive reaction,
and may be employed for testing for the smallest traces of
acids. If 10,000 parts by weight of water be mixed with 1 part
of sulphuric acid, the coloration is distinct, and it is even
perceptible on the addition of ten times more water. Certain
precautions must, however, be taken in the preparation of such
very sensitive litmus paper. Litmus is sold in lumps. Take, say,
100 grams of it; powder it, and add it to cold pure water in
a flask; shake and decant the water. Repeat this three times.
This is done to wash away easily-soluble impurities, especially
alkalis. Transfer the washed litmus (it is washed with absolute
alcohol to remove the non-sensitive reddish colouring matter) to
a flask, and pour in 600 c.c. of water, heat, and allow the hot
infusion to remain for some hours in a warm place. Then filter,
and divide the filtrate into two parts. Add a few drops of nitric
acid to one portion, so that a faint red tinge is obtained, and
then mix the two portions. Add spirit to the mixture, and keep
it in a stoppered bottle (it soon spoils if left open to the
air). This infusion may be employed directly; it reddens in the
presence of acids, and turns blue in the presence of alkalis. If
evaporated, a solid mass is obtained which is soluble in water,
and may be kept unchanged for any length of time. The test paper
may be prepared as follows:--Take a strong infusion of litmus,
and soak blotting-paper with it; dry it, and cut it into strips,
and use it as test-paper for acids. For the detection of alkalis,
the paper must be soaked in a solution of litmus just reddened by
a few drops of acid; if too much acid be taken, the paper will
not be sensitive. Such acids as sulphuric acid colour litmus,
and especially its infusion, a brick-red colour, whilst more
feeble acids, such as carbonic, give a faint red-wine tinge.
Test-paper of a yellow colour is also employed; it is dyed by
an infusion of turmeric roots in spirit. In alkalis it turns
brown, but regains its original hue in acids. Many blue and other
vegetable colouring matters may be used for the detection of
acids and alkalis; for example, infusions of cochineal, violets,
log-wood, &c. Certain artificially prepared substances and dyes
may also be employed. Thus rosolic acid, C_{20}H_{16}O_{3} and
phenolphthaleïn, C_{20}H_{14}O_{4} (it is used in an alcoholic
solution, and is not suitable for the detection of ammonia), are
colourless in an acid, and red in an alkaline, solution. Cyanine
is also colourless in the presence of acids, and gives a blue
coloration with alkalis. Methyl-orange (yellow in an aqueous
solution) is not altered by alkalis but becomes pink with acids
(weak acids have no action), &c. These are very sensitive tests.
Their behaviour in respect to various acids, alkalis, and salts
sometimes give the means of distinguishing substances from each
other.

Basic oxides, in combining with water, form hydrates, of which, however, very few are soluble in water. Those which are soluble in water have an alkaline taste like that of soap or of water in which wood ashes have been boiled, and are called _alkalis_. Further, alkalis have the property of restoring the blue colour to litmus which has been reddened by the action of acids. The hydrates of the oxides of sodium and potassium, NaHO and KHO, are examples of basic hydrates easily soluble in water. They are true alkalis, and are termed _caustic_, because they act very powerfully on the skin of animals and plants. Thus NaHO is called 'caustic' soda.

The saline oxides are capable of combining together and with water. Water itself is an oxide, and not an indifferent one, for it can, as we have seen, combine with basic and acid oxides; it is a representative of a whole series of saline oxides, _intermediate oxides_, capable of combining with both basic and acid oxides. There are many such oxides, which, like water, combine with basic and acid anhydrides--for instance, the oxides of aluminium and tin, &c. From this it may be concluded that all oxides might be placed, in respect to their capacity for combining with one another, in one uninterrupted series, at one extremity of which would stand those oxides which do not combine with the bases--that is, the alkalis--while at the other end would be the acid oxides, and in the interval those oxides which combine with one another and with both the acid and basic oxides. The further apart the members of this series are, the more stable are the compounds they form together, the more energetically do they act on each other, the greater the quantity of heat evolved in their reaction, and the more marked is their saline chemical character.

We said above that basic and acid oxides combine together, but rarely react on each other; this depends on the fact that the majority of them are solids or gases--that is, they occur in the state least prone to chemical reaction. The gaseo-elastic state is with difficulty destroyed, because it necessitates overcoming the elasticity proper to the gaseous particles. The solid state is characterised by the immobility of its particles; whilst chemical action requires contact, and hence a displacement and mobility. If solid oxides be heated, and especially if they be melted, then reaction proceeds with great ease. But such a change of state rarely occurs in nature or in practice. Only in a few furnace processes is this the case. For example; in the manufacture of glass, the oxides contained in it combine together in a molten state. But when oxides combine with water, and especially when they form hydrates soluble in water, then the mobility of their particles increases to a considerable extent, and their reaction is greatly facilitated. Reaction then takes place at the ordinary temperature--easily and rapidly; so that this kind of reaction belongs to the class of those which take place with unusual facility, and are, therefore, very often taken advantage of in practice, and also have been and are going on in nature at every step. We will now consider the reactions of oxides in the state of hydrates, not losing sight of the fact that water is itself an oxide with definite properties, and has, therefore, no little influence on the course of those changes in which it takes part.

If we take a definite quantity of an acid, and add an infusion of litmus to it, it turns red; the addition of an alkaline solution does not immediately alter the red colour of the litmus, but on adding more and more of the alkaline solution a point is reached when the red colour changes to violet, and then the further addition of a fresh quantity of the alkaline solution changes the colour to blue. This change of the colour of the litmus is a consequence of the formation of a new compound. This reaction is termed the _saturation_ or _neutralisation_ of the acid by the base, or _vice versâ_. The solution in which the acid properties of the acid are saturated by the alkaline properties of the base is termed a _neutral_ solution. Such a solution, although derived from the mixture of a base with an acid, does not exhibit either the acid or basic reaction on litmus, yet it preserves many other signs of the acid and alkali. It is observed that in such a definite admixture of an acid with an alkali, besides the changes in the colour of litmus there is a heating effect--_i.e._ an evolution of heat--which is alone sufficient to prove that there was chemical action. And, indeed, if the resultant violet solution be evaporated, there separates out, not the acid or the alkali originally taken, but a substance which has neither acid nor alkaline properties, but is usually solid and crystalline, having a saline appearance; this is a _salt_ in the chemical sense of the word. Hence a salt is derived from the reaction of an acid on an alkali, in a certain definite proportion. The water here taken for solution plays no other part than merely facilitating the progress of the reaction. This is seen from the fact that the anhydrides of the acids are able to combine with basic oxides, and give the same salts as do the acids with the alkalis or hydrates. Hence, a salt is a compound of definite quantities of an acid with an alkali. In the latter reaction, water is separated out if the substance formed be the same as is produced by the combination of anhydrous oxides together.[51] Examples of the formation of salts from acids and bases are easily observed, and are very often applied in practice. If we take, for instance, insoluble magnesium oxide (magnesia) it is easily dissolved in sulphuric acid, and on evaporation gives a saline substance, bitter, like all the salts of magnesium, and familiar to all under the name of Epsom salts, used as a purgative. If a solution of caustic soda--which is obtained, as we saw, by the action of water on sodium oxide--be poured into a flask in which charcoal has been burnt; or if carbonic anhydride, which is produced under so many circumstances, be passed through a solution of caustic soda, then sodium carbonate or soda, Na_{2}CO_{3}, is obtained, of which we have spoken several times, and which is prepared on a large scale and often used in manufactures. This reaction is expressed by the equation, 2NaHO + CO_{2} = Na_{2}CO_{3} + H_{2}O. Thus, the various bases and acids form an innumerable number of different salts.[52] Salts constitute an example of definite chemical compounds, and both in the history and practice of science are most often cited as confirming the conception of definite chemical compounds. Indeed, all the indications of a definite chemical combination are clearly seen in the formation and properties of salts. Thus, they are produced with a definite proportion of oxides, heat is evolved in their formation,[53] and the chemical character of the oxides and many of the physical properties become hidden in their salts. For example, when gaseous carbonic anhydride combines with a base to form a solid salt, the elasticity of the gas quite disappears in its passage into the salt.[54]

[51] That water really is separated in the reaction of acid on alkaline
hydrates, may be shown by taking some other intermediate
hydrate--for example, alumina--instead of water. Thus, if a
solution of alumina in sulphuric acid be taken, it will have,
like the acid, an acid reaction, and will therefore colour
litmus red. If, on the other hand, a solution of alumina in an
alkali--say, potash--be taken, it will have an alkaline reaction,
and will turn red litmus blue. On adding the alkaline to the
acid solution until neither an alkaline nor an acid reaction is
produced, a salt is formed, consisting of sulphuric anhydride
and potassium oxide. In this, as in the reaction of hydrates,
an intermediate oxide is separated out--namely, alumina. Its
separation will be very evident in this case, as alumina is
insoluble in water.

[52] The mutual interaction of hydrates, and their capacity of forming
salts, may be taken advantage of for determining the character
of those hydrates which are insoluble in water. Let us imagine
that a given hydrate, whose chemical character is unknown, is
insoluble in water. It is therefore impossible to test its
reaction on litmus. It is then mixed with water, and an acid--for
instance, sulphuric acid--is added to the mixture. If the hydrate
taken be basic, reaction will take place, either directly or by
the aid of heat, with the formation of a salt. In certain cases,
the resultant salt is soluble in water, and this will at once
show that combination has taken place between the insoluble basic
hydrate and the acid, with the formation of a soluble saline
substance. In those cases where the resultant salt is insoluble,
still the water loses its acid reaction, and therefore it may he
ascertained, by the addition of an acid, whether a given hydrate
has a basic character, like the hydrates of oxide of copper,
lead, &c. If the acid does not act on the given insoluble hydrate
(at any temperature), then it has not a basic character, and it
should be tested as to whether it has an acid character. This is
done by taking an alkali, instead of the acid, and by observing
whether the unknown hydrate then dissolves, or whether the
alkaline reaction disappears. Thus it may he proved that hydrate
of silica is acid, because it dissolves in alkalis and not in
acids. If it be a case of an insoluble intermediate hydrate, then
it will be observed to react on both the acid and alkali. Hydrate
of alumina is an instance in question, which is soluble both in
caustic potash and in sulphuric acid.

The _degree of affinity_ or chemical _energy_ proper to oxides
and their hydrates is very dissimilar; some extreme members
of the series possess it to a great extent. When acting on
each other they evolve a large quantity of heat, and when
acting on intermediate hydrates they also evolve heat to a
considerable degree, as we saw in the combination of lime and
sulphuric anhydride with water. When extreme oxides combine
they form stable salts, which are decomposed with difficulty,
and often show characteristic properties. The compounds of the
intermediate oxides with each other, or even with basic and acid
oxides, present a very different case. However much alumina we
may dissolve in sulphuric acid, we cannot saturate the acid
properties of the sulphuric acid, the resulting solution will
always have an acid reaction. So also, whatever quantity of
alumina is dissolved in an alkali, the resulting solution will
always present an alkaline reaction.

[53] In order to give an idea of the quantity of heat evolved in the
formation of salts I append a table of data for _very dilute
aqueous solutions_ of acids and alkalis, according to the
determinations of Berthelot and Thomsen. The figures are given
in major calories--that is, in thousands of units of heat. For
example, 49 grams of sulphuric acid, H_{2}SO_{4}, taken in a
dilute aqueous solution, when mixed with such an amount of a weak
solution of caustic soda, NaHO, that a neutral salt is formed
(when all the hydrogen of the acid is replaced by the sodium),
evolves 15,800 units of heat.

49 parts of 63 parts of
H_{2}SO_{4} HNO_{3}

NaHO 15·8 13·7
KHO 15·7 13·8
NH_{3} 14·5 12·5
CaO 15·6 13·9
BaO 18·4 13·9
MgO 15·6 13·8
FeO 12·5 10·7 (?)
ZnO 11·7 9·8
Fe_{2}O_{3} 5·7 5·9

These figures cannot be considered as the heat of neutralisation,
because the water here plays an important part. Thus, for
instance, sulphuric acid and caustic soda in dissolving in water
evolve very much heat, and the resultant sodium sulphate very
little; consequently, the amount of heat evolved in an anhydrous
combination will be different from that evolved in a hydrated
combination. Those acids which are not energetic in combining
with the same quantity of alkalis required for the formation of
normal salts of sulphuric or nitric acids always, however, give
less heat. For instance, with caustic soda: carbonic acid gives
10·2, hydrocyanic, 2·9, hydrogen sulphide, 3·9 major calories.
And as feeble bases (for example, Fe_{2}O_{3}) also evolve less
heat than those which are more powerful, so a certain general
correlation between thermochemical data and the degree of
affinity shows itself here, as in other cases (_see_ Chapter II.,
Note 7); this does not, however, give any reason for measuring
the affinity which binds the elements of salts by the heat
of their formation in dilute solutions. This is very clearly
demonstrated by the fact that water is able to decompose many
salts, and is separated in their formation.

[54] Carbonic anhydride evolves heat in dissolving in water. The
solution easily dissociates and evolves carbonic anhydride,
according to the law of Henry and Dalton (_see_ Chapter I.)
In dissolving in caustic soda, it either gives a normal salt,
Na_{2}CO_{3}, which does not evolve carbonic anhydride, or an
acid salt, NaHCO_{3} which easily evolves carbonic anhydride when
heated. The same gas, when dissolved in solutions of salts, acts
in one or the other manner (_see_ Chapter II., Note 38). Here
it is seen what a successive series of relations exists between
compounds of a different order, between substances of different
degrees of stability. By making a distinction between the
phenomena of solutions and chemical compounds, we overlook those
natural transitions which in reality exist.

Judging from the above, a salt is a compound of basic and acid oxides, or the result of the action of hydrates of these classes on each other with separation of water. But salts may be obtained by other methods. It must not be forgotten that basic oxides are formed by metals, and acid oxides usually by non-metals. But metals and non-metals are capable of combining together, and a salt is frequently formed by the oxidation of such a compound. For example, iron very easily combines with sulphur, forming iron sulphide FeS (as we saw in the Introduction); this in air, and especially moist air, absorbs oxygen, with the formation of the same salt FeSO_{4}, that may be obtained by the combination of the oxides of iron and sulphur, or of the hydrates of these oxides. Hence, it cannot be said or supposed that a salt has the properties of the oxides, or must necessarily contain two kinds of oxides in itself. The derivation of salts from oxides is merely one of the methods of their preparation. We saw, for instance, that in sulphuric acid it was possible to replace the hydrogen by zinc, and that by this means zinc sulphate was formed; so likewise the hydrogen in many other acids may be replaced by zinc, iron, potassium, sodium, and a whole series of similar metals, corresponding salts being obtained. The hydrogen of the acid, in all these cases, is exchanged for a metal, and a salt is obtained from the hydrate. Regarding a salt from this point of view, it may be said that _a salt is an acid in which hydrogen is replaced by a metal_. This definition shows that a salt and an acid are essentially compounds of the same series, with the difference that the latter contains hydrogen and the former a metal. Such a definition is more exact than the first definition of salts, inasmuch as it likewise includes those acids which do not contain oxygen, and, as we shall afterwards learn, there is a series of such acids. Such elements as chlorine and bromine form compounds with hydrogen in which the hydrogen may be replaced by a metal, forming substances which, in their reactions and external characters, resemble the salts formed from oxides. Table salt, NaCl, is an example of this. It may be obtained by the replacement of hydrogen in hydrochloric acid, HCl, by the metal sodium, just as sulphate of sodium, Na_{2}SO_{4}, may be obtained by the replacement of hydrogen in sulphuric acid, H_{2}SO_{4}, by sodium. The exterior appearance of the resulting products, their neutral reaction, and even their saline taste, show their resemblance to one another.

To the fundamental properties of salts yet another must be added--namely, that they are more or less _decomposed by the action of a galvanic current_. The results of this decomposition are very different according to whether the salt be taken in a fused or dissolved state. But the decomposition may generally be so represented, that the metal appears at the electro-negative pole or cathode (like hydrogen in the decomposition of water, or its mixture with sulphuric acid), and the remaining parts of the salt appear at the electro-positive pole or anode (where the oxygen of water appears). If, for instance, an electric current acts on an aqueous solution of sodium sulphate, then the sodium appears at the negative pole, and oxygen and the anhydride of sulphuric acid at the positive pole. But in the solution itself the result is different, for sodium, as we know, decomposes water with evolution of hydrogen, forming caustic soda; consequently hydrogen will be evolved, and caustic soda appear at the negative pole: while at the positive pole the sulphuric anhydride immediately combines with water and forms sulphuric acid, and therefore oxygen will be evolved and sulphuric acid formed round this pole.[55] In other cases, when the metal separated is not able to decompose water, it will be deposited in a free state. Thus, for example, in the decomposition of copper sulphate, copper separates out at the cathode, and oxygen and sulphuric acid appear at the anode, and if a copper plate be attached to the positive pole, then the oxygen evolved will oxidise the copper, and the oxide of copper will dissolve and be deposited at the negative pole--that is, a transfer of copper from the positive to the negative pole ensues. The galvanoplastic art (electro-typing) is based on this principle.[56] Therefore the most radical and general properties of salts (including also such salts as table salt, which contain no oxygen) may be expressed by representing the salt as composed of a metal M and a haloid X--that is, by expressing the salt by MX. In common table salt the metal is sodium, and the haloid an elementary body, chlorine. In sodium sulphate, Na_{2}SO_{4}, sodium is again the metal, but the complex group, SO_{4}, is the haloid. In sulphate of copper, CuSO_{4}, the metal is copper and the haloid the same as in the preceding salt. Such a representation of salts expresses with great simplicity the _capacity of every salt to enter into saline double decompositions with other salts_; consisting in the mutual replacement of the metals in the salts. This exchange of their metals is the fundamental property of salts. In the case of two salts with different metals and haloids, which are in solution or fusion, or in any other manner brought into contact, the metals of these salts will always partially or wholly exchange places. If we designate one salt by MX, and the other by NY, then we either partially or wholly obtain from them new salts, MY and NX. Thus we saw in the Introduction, that on mixing solutions of table salt, NaCl, and silver nitrate, AgNO_{3}, a white insoluble precipitate of silver chloride, AgCl, is formed and a new salt, sodium nitrate, NaNO_{3}, is obtained in solution. If the metals of salts exchange places in reactions of double decomposition, it is clear that metals themselves, taken in a separate state, are able to act on salts, as zinc evolves hydrogen from acids, and as iron separates copper from copper sulphate. When, to what extent, and which metals displace each other, and how the metals are distributed between the haloids, will be discussed in Chapter X., where we shall be guided by those reflections and deductions which Berthollet introduced into the science at the beginning of this century.

[55] This kind of decomposition may be easily observed by pouring
a solution of sodium sulphate into a U-shaped tube and
inserting electrodes in the two branches. If the solution be
coloured with an infusion of litmus, it will easily be seen
that it turns blue at the cathode, owing to the formation of
sodium hydroxide, and red at the electro-positive pole, from the
formation of sulphuric acid.

[56] In other cases the decomposition of salts by the electric current
may be accompanied by much more complex results. Thus, when the
metal of the salt is capable of a higher degree of oxidation,
such a higher oxide may be formed at the positive pole by the
oxygen which is evolved there. This takes place, for instance,
in the decomposition of salts of silver and manganese by the
galvanic current, peroxides of these metals being formed. Thus in
the electrolysis of a solution of KCl, KClO_{3} is formed, and
of sulphuric acid (corresponding to SO_{3}) persulphuric acid,
corresponding to S_{2}O_{7}. But all the phenomena as yet known
may be expressed by the above law--that the current decomposes
salts into metals, which appear at the negative pole, and into
the remaining component parts, which appear at the positive pole.

According to the above observations, an acid is nothing more than a salt of hydrogen. Water itself may be looked on as a salt in which the hydrogen is combined with either oxygen or the aqueous radicle, OH; water will then be HOH, and alkalis or basic hydrates, MOH. The group OH, or the _aqueous radicle_, otherwise called _hydroxyl_, may be looked on as a haloid like the chlorine in table salt, not only because the element Cl and the group OH very often change places, and combine with one and the same element, but also because free chlorine is very similar in many properties and reactions to peroxide of hydrogen, which is the same in composition as the aqueous radicle, as we shall afterwards see in Chapter IV. Alkalis and basic hydrates are also salts consisting of a metal and hydroxyl--for instance, caustic soda, NaOH; this is therefore termed _sodium hydroxide_. According to this view, _acid salts_ are those in which a portion only of the hydrogen is replaced by a metal, and a portion of the hydrogen of the acid remains. Thus sulphuric acid (H_{2}SO_{4}) not only gives the normal salt Na_{2}SO_{4}, with sodium, but also an acid salt, NaHSO_{4}. A _basic salt_ is one in which the metal is combined not only with the haloids of acids, but also with the aqueous radicale of basic hydrates--for example, bismuth gives not only a normal salt of nitric acid, Bi(NO_{3})_{3}, but also basic salts like Bi(OH)_{2}(NO_{3}).

As basic and acid salts of the oxygen acids contain hydrogen and oxygen, they are able to part with these as water and to give anhydro-salts, which it is evident will be compounds of normal salts with anhydrides of the acids or with bases. Thus the above-mentioned acid sodium sulphate corresponds with the anhydro-salt, Na_{2}S_{2}O_{7}, equal to 2NaHSO_{4}, less H_{2}O. The loss of water is here, and frequently in other cases, brought about by heat alone, and therefore such salts are frequently termed _pyro-salts_--for instance, the preceding is sodium pyrosulphate (Na_{2}S_{2}O_{7}), or it may be regarded as the normal salt Na_{2}SO_{4} + sulphuric anhydride, SO_{3}. _Double_ salts are those which contain either two metals, KAl(SO_{4})_{2}, or two haloids.[57]

[57] The above-enunciated generalisation of the conception of salts as
compounds of the metals (simple, or compound like ammonium,
NH_{4}), with the haloids (simple, like chlorine, or compound,
like cyanogen, CN, or the radical of sulphuric acid, SO_{4}),
capable of entering into double saline decomposition, which
is in accordance with the general data respecting salts, was
only formed little by little after a succession of most varied
propositions as to the chemical structure of salts.

Salts belong to the class of substances which have been known
since very early times, and have long been investigated in many
directions. At first, however, no distinction was made between
salts, acids, and bases. Glauber prepared many artificial salts
during the latter half of the seventeenth century. Up to that
time the majority of salts were obtained from natural sources,
and that salt which we have referred to several times--namely,
sodium sulphate--was named Glauber's salt after this chemist.
Rouelle distinguished normal, acid, and basic salts, and showed
their action on vegetable dyes, still he confounded many salts
with acids (even now every acid salt ought to be regarded as an
acid, because it contains hydrogen, which may be replaced by
metals--that is, it is the hydrogen of an acid). Baumé disputed
Rouelle's opinion concerning the subdivision of salts, contending
that normal salts only are true salts, and that basic salts are
simple mixtures of normal salts with bases and acid salts with
acids, considering that washing alone could remove the base or
acid from them. Rouelle, in the middle of the last century,
however, rendered a great service to the study of salts and the
diffusion of knowledge respecting this class of compounds in
his attractive lectures. He, like the majority of the chemists
of that period, did not employ the balance in his researches,
but satisfied himself with purely qualitative data. The first
quantitative researches on salts were carried on about this
time by Wenzel, who was the director of the Freiburg mines, in
Saxony. Wenzel studied the double decomposition of salts, and
observed that in the double decomposition of neutral salts a
neutral salt was always obtained. He proved, by a method of
weighing, that this is due to the fact that the saturation of a
given quantity of a base requires such relative quantities of
different acids as are capable of saturating every other base.
Having taken two neutral salts--for example, sodium sulphate and
calcium nitrate--let us mix their solutions together. Double
decomposition takes place, because calcium sulphate is formed,
which is almost insoluble. However much we might add of each
of the salts, the neutral reaction will still be preserved,
consequently the neutral character of the salts is not destroyed
by the interchange of metals; that is to say, that quantity of
sulphuric acid which saturated the sodium is sufficient for the
saturation of the calcium, and that amount of nitric acid which
saturated the calcium is enough to saturate the sodium contained
in combination with sulphuric acid in sodium sulphate. Wenzel
was even convinced that matter does not disappear in nature, and
on this principle he corrects, in his _Doctrine of Affinity_,
the results of his experiments when he found that he obtained
less than he had originally taken. Although Wenzel deduced the
law of the double decomposition of salts quite correctly, he did
not determine those quantities in which acids and bases act on
each other. This was carried out at the end of the last century
by Richter. He determined the quantities by weight of the bases
which saturate acids and of the acids which saturate bases, and
obtained comparatively correct results, although his conclusions
were not correct, for he states that the quantity of a base
saturating a given acid varies in arithmetical progression, and
the quantity of an acid saturating a given base in geometrical
progression. Richter studied the deposition of metals from their
salts by other metals, and observed that the neutral reaction
of the solution is not destroyed by this exchange. He also
determined the quantities by weight of the metals replacing one
another in salts. He showed that copper displaces silver from
its salt, and that zinc displaces copper and a whole series of
other metals. Those quantities of metals which were capable of
replacing one another were termed equivalents.

Richter's teaching found no followers, because, although he
fully believed in the discoveries of Lavoisier, yet he still
held to the phlogistic reasonings which rendered his expositions
very obscure. The works of the Swedish savant Berzelius freed
the facts discovered by Wenzel and Richter from the obscurity
of former conceptions, and led to their being explained in
accordance with Lavoisier's views, and in the sense of the law of
multiple proportions which had already been discovered by Dalton.
On applying to salts those conclusions which Berzelius arrived at
by a whole series of researches of remarkable accuracy, we arrive
at the following law of equivalents--_one part by weight of
hydrogen in an acid is replaced by the corresponding equivalent
weight of any metal_; and, therefore, when metals replace each
other their weights are in the same ratio as their equivalents.
Thus, for instance, one part by weight of hydrogen is replaced by
23 parts of sodium, 39 parts of potassium, 12 parts of magnesium,
20 parts of calcium, 28 parts of iron, 108 parts of silver, 33
parts of zinc, &c.; and, therefore, if zinc replaces silver, then
33 parts of zinc will take the place of 108 parts of silver, or
33 parts of zinc will he substituted by 23 parts of sodium, &c.

The doctrine of equivalents would be precise and simple did
every metal only give one oxide or one salt. It is rendered
complicated from the fact that many metals form several oxides,
and consequently offer different equivalents in their different
degrees of oxidation. For example, there are oxides containing
iron in which its equivalent is 28--this is in the salts formed
by the suboxide; and there is another series of salts in which
the equivalent of iron equals 18-2/3--which contain less iron,
and consequently more oxygen, and correspond with a higher degree
of oxidation--ferric oxide. It is true that the former salts are
easily formed by the direct action of metallic iron on acids, and
the latter only by a further oxidation of the compound formed
already; but this is not always so. In the case of copper,
mercury, and tin, under different circumstances, salts are formed
which correspond with different degrees of oxidation of these
metals, and many metals have two equivalents in their different
salts--that is, in salts corresponding with the different degrees
of oxidation. Thus it is impossible to endow every metal with one
definite equivalent weight. Hence the conception of equivalents,
while playing an important part from an historical point of view,
appears, with a fuller study of chemistry, to be but subordinate
to a higher conception, with which we shall afterwards become
acquainted.

The fate of the theoretical views of chemistry was for a
long time bound up with the history of salts. The clearest
representation of this subject dates back to Lavoisier, and was
systematically developed by Berzelius. This representation is
called the _binary_ theory. All compounds, and especially salts,
are represented as consisting of two parts. Salts are represented
as compounds of a basic oxide (a base) and an acid (that is, an
anhydride of an acid, then termed an acid), whilst hydrates are
represented as compounds of anhydrous oxides with water. Such
an expression was employed not only to denote the most usual
method of formation of these substances (where it would be quite
true), but also to express that internal distribution of the
elements by which it was proposed to explain all the properties
of these substances. Copper sulphate was supposed to contain
two most intimate component parts--copper oxide and sulphuric
anhydride. This is an hypothesis. It arose from the so-called
_electro-chemical hypothesis_, which supposed the two component
parts to be held in mutual union, because one component (the
anhydride of the acid) has electro-negative properties, and the
other (the base in salts) electro-positive. The two parts are
attracted together, like substances having opposite electrical
charges. But as the decomposition of salts in a state of fusion
by an electric current always gives a metal, that representation
of the constitution and decomposition of salts called the
_hydrogen theory_ of acids is nearer the truth than that which
considers salts as made up of a base and an anhydride of an acid.
But the hydrogen theory of acids is also a binary hypothesis,
and does not contradict the electro-chemical hypothesis, but
is rather a modification of it. The binary theory dates from
Rouelle and Lavoisier, the electro-chemical aspect was zealously
developed by Berzelius, and the hydrogen theory of acids is due
to Davy and Liebig.

These hypothetical views simplified and generalised the study of
a complicated subject, and served to support further arguments,
but when salts were in question it was equally convenient to
follow one or the other of these hypotheses. But these theories
were brought to bear on all other substances, on all compound
substances. Those holding the binary and electro-chemical
hypotheses searched for two anti-polar component parts, and
endeavoured to express the process of chemical reactions by
electro-chemical and similar differences. If zinc replaces
hydrogen, they concluded that it is more electro-positive
than hydrogen, whilst they forgot that hydrogen may, under
different circumstances, displace zinc--for instance, at a
red heat. Chlorine and oxygen were considered as being of
opposite polarity to hydrogen because they easily combine with
it, nevertheless both are capable of replacing hydrogen, and,
what is very characteristic, in the replacement of hydrogen
by chlorine in carbon compounds not only does the chemical
character often remain unaltered, but even the external form
may remain unchanged, as Laurent and Dumas demonstrated. These
considerations undermine the binary, and more especially the
electro-chemical theory. An explanation of known reactions then
began to be sought for not in the difference of the polarity of
the different substances, but in the joint influences of all the
elements on the properties of the compound formed. This is the
reverse of the preceding hypothesis.

This reversal was not, however, limited to the destruction of
the tottering foundations of the preceding theory; it proposed a
new doctrine, and laid the foundation for the modern course of
our science. This doctrine may be termed the unitary theory--that
is, it strictly acknowledges the joint influences of the elements
in a compound substance, denies the existence of separate and
contrary components in them, regards copper sulphate, for
instance, as a strictly definite compound of copper, sulphur,
and oxygen; then seeks for compounds which are analogous in
their properties, and, placing them side by side, endeavours
to express the influence of each element in determining the
united properties of its compound. In the majority of cases it
arrives at conclusions similar to those which are obtained by
the above-mentioned hypotheses, but in certain special cases the
conclusions of the unitary theory are in entire opposition to
those of the binary theory and its corollaries. Cases of this
kind are most often met with in the consideration of compounds of
a more complex nature than salts, especially organic compounds
containing hydrogen. But it is not in this change from an
artificial to a natural system, important as it is, that the
chief service and strength of the unitary doctrine lies. By a
simple review of the vast store of data regarding the reactions
of typical substances, it succeeded from its first appearance
in establishing a new and important law, it introduced a new
conception into science--namely, the conception of molecules,
with which we shall soon become acquainted. The deduction of
the law and of the conception of molecules has been verified by
facts in a number of cases, and was the cause of the majority of
chemists of our times deserting the binary theory and accepting
the unitary theory, which forms the basis of the present work.
Laurent and Gerhardt must be considered as the founders of this
doctrine.

Inasmuch as oxygen compounds predominate in nature, it should be expected from what has been said above, that salts, rather than acids or bases, would occur most frequently in nature, for these latter would always tend to combine forming salts, especially through the medium of the all-pervading water. And, as a matter of fact, salts are found everywhere in nature. They occur in animals and plants, although in but small quantity, because, as forming the last stage of chemical reaction, they are capable of only a few chemical transformations. And organisms are bodies in which a series of uninterrupted, varied, and active chemical transformations proceed, whilst salts, which only enter into double decompositions between each other, are little prone to such changes. But organisms always contain salts. Thus, for instance, bones contain calcium phosphate, the juice of grapes potassium tartrate (cream of tartar), certain lichens calcium oxalate, and the shells of mollusca calcium carbonate, &c. As regards water and soil, portions of the earth in which the chemical processes are less active, they are full of salts. Thus the waters of the oceans, and all others (Chap. I.), abound in salts, and in the soil, in the rocks of the earth's crust, in the upheaved lavas, and in the falling meteorites the salts of silicic acid, and especially its double salts, predominate. Saline substances also make up the composition of those limestones which often form mountain chains and whole thicknesses of the earth's strata, these consisting of calcium carbonate, CaCO_{3}.

Thus we have seen oxygen in a free state and in various compounds of different degrees of stability, from the unstable salts, like Berthollet's salt and nitre, to the most stable silicon compounds, such as exist in granite. We saw an entirely similar gradation of stability in the compounds of water and of hydrogen. In all its aspects oxygen, as an element, or single substance, remains the same however varied its chemical states, just as a substance may appear in many different physical states of aggregation. But our notion of the immense variety of the chemical states in which oxygen can occur would not be completely understood if we did not make ourselves acquainted with it in the form in which it occurs in ozone and peroxide of hydrogen. In these it is most active, its energy seems to have increased. They illustrate fresh aspects of chemical correlations, and the variety of the forms in which matter can appear stand out clearly. We will therefore consider these two substances somewhat in detail.