Chapter XIV (1)

[13] Up to the year 1840, or thereabout, acids were not distinguished
by their basicity. Graham, while studying phosphoric acid,
H_{3}PO_{4}, and Liebig, while studying many organic acids,
distinguished mono-, bi-, and tribasic acids. Gerhardt and Laurent
generalised these relations, showing that this distinction extends
over many reactions (for instance, to the faculty of bibasic acids
of forming acid salts with alkalis, KHO or NaHO, or with alcohols,
RHO, &c.); but now, since a definite conception as to atoms and
molecules has been arrived at, _the basicity of an acid is
determined by the number of hydrogen atoms,_ contained in a
molecule of the acid, which can be exchanged for metals. If
carbonic acid forms acid salts, NaHCO_{3}, and normal salts,
Na_{2}CO_{3}, it is evident that the hydrate is H_{2}CO_{3}, a
bibasic acid. Otherwise it is at present impossible to account for
the composition of these salts. But when C = 6 and O = 8 were
taken, then the formula CO_{2} expressed the composition, but not
the molecular weight, of carbonic anhydride; and the composition
of the normal salt would be Na_{2}C_{2}O_{6} or NaCO_{3},
therefore carbonic acid might have been considered as a monobasic
acid. Then the acid salt would have been represented by
NaCO_{3},HCO_{3}. Such questions were the cause of much argument
and difference of opinion among chemists about forty years ago. At
present there cannot be two opinions on the subject if the law of
Avogadro-Gerhardt and its consequences be strictly adhered to. It
may, however, be observed here that the monobasic acids R(OH) were
for a long time considered to be incapable of being decomposed
into water and anhydride, and this property was ascribed to the
bibasic acids R(OH)_{2} as containing the elements necessary for
the separation of the molecule of water, H_{2}O. Thus H_{2}SO_{4}
or SO_{2}(OH)_{2}, H_{2}CO_{3}, or CO(OH)_{2}, and other bibasic
acids decompose into an anhydride, RO, and water, H_{2}O. But as
nitrous, HNO_{2}, iodic, HIO_{3}, hypochlorous, HClO, and other
monobasic acids easily give their anhydrides N_{2}O_{3},
I_{2}O_{5}, Cl_{2}O, &c., that method of distinguishing the
basicity of acids, although it fairly well satisfies the
requirements of organic chemistry, cannot be considered correct.
It may also be remarked that up to the present time not one of the
bibasic acids has been found to have the faculty of being
distilled without being decomposed into anhydride and water (even
H_{2}SO_{4}, on being evaporated and distilled, gives SO_{3} +
H_{2}O), and the decomposition of acids into water and anhydride
proceeds particularly easily in dealing with feebly energetic
acids, such as carbonic, nitrous, boric, and hypochlorous. Let us
add that carbonic acid, as a hydrate corresponding to marsh gas,
C(HO)_{4} = CO_{2} + 2H_{2}O, ought to be tetrabasic. But in
general it does not form such salts. Basic salts, however, such as
CuCO_{3}CuO, may be regarded in this sense, for CCu_{2}O_{4}
corresponds with CH_{4}O_{4}, as Cu corresponds with H_{2}.
Amongst the ethereal salts (alcoholic derivatives) of carbonic
acid corresponding cases are, however, observed; for instance,
ethylic orthocarbonate, C(C_{2}H_{5}O)_{4} (obtained by the action
of chloropicrin, C(NO_{2})Cl_{3}, on sodium ethoxide,
C_{2}H_{5}ONa; boiling point 158°; specific gravity, 0·92). The
name _orthocarbonic acid_ for CH_{4}O_{4} is taken from
_orthophosphoric acid_, PH_{3}O_{4}, which corresponds with PH_{3}
(_see_ Chapter on Phosphorus).

[14] Long ago endeavours were made to find a _measure of affinity_ of
acids and bases, because some of the acids, such as sulphuric or
nitric, form comparatively stable salts, decomposed with
difficulty by heat and water, whilst others, like carbonic and
hypochlorous acids, do not combine with feeble bases, and with
most of the other bases form salts which are easily decomposed.
The same may be said with regard to bases, among which those of
potassium, K_{2}O, sodium, Na_{2}O, and barium, BaO, may serve as
examples of the most powerful, because they combine with the most
feeble acids and form a mass of salts of great stability, whilst
as examples of the feeblest bases alumina, Al_{2}O_{3}, or bismuth
oxide, Bi_{2}O_{3}, may be taken, because they form salts easily
decomposed by water and by heat if the acid be volatile. Such a
division of acids and bases into the feeblest and most powerful is
justified by all evidence concerning them, and is quoted in this
work. But the teaching of this subject in certain circles has
acquired quite a new tone, which, in my opinion, cannot be
accepted without certain reservations and criticisms, although it
comprises many interesting features. The fact is that Thomsen,
Ostwald, and others proposed to express the measure of affinity of
acids to bases by figures drawn from data of the measure of
displacement of acids in aqueous solutions, judging (1) from the
amount of heat developed by mixing a solution of the salt with a
solution of another acid (the avidity of acids, according to
Thomsen); (2) from the change of the volumes accompanying such a
mutual action of solutions (Ostwald); (3) from the change of the
index of refraction of solutions (Ostwald), &c. Besides this there
are many other methods which allow us to form an opinion about the
distribution of bases among various acids in aqueous solutions.
Some of these methods will be described hereafter. It ought,
however, to be remarked that in making investigations in aqueous
solutions the affinity to water is generally left out of sight. If
a base N, combining with acids X and Y in presence of them both,
divides in such a way that one-third of it combines with X and
two-thirds with Y, a conclusion is formed that the affinity, or
power of forming salts, of the acid Y is twice as great as that of
X. But the presence of the water is not taken into account. If the
acid X has an affinity for water and for N it will be distributed
between them; and if X has a greater affinity for water than Y,
then less of X will combine with N than of Y. If, in addition to
this, the acid X is capable of forming an acid salt NX_{2}, and Y
is not, the conclusion of the relative strength of X and Y will be
still more erroneous, because the X set free will form such a salt
on the addition of Y to NX. We shall see in Chapter X. that when
sulphuric and nitric acids in weak aqueous solution act on sodium,
they are distributed exactly in this way: namely, one-third of the
sodium combines with the sulphuric and two-thirds with the nitric
acid; but, in my opinion, this does not show that sulphuric acid,
compared with nitric acid, possesses but half the degree of
affinity for bases like soda, and only demonstrates the greater
affinity of sulphuric acid for water compared with that of nitric
acid. In this way the methods of studying the distribution in
aqueous solutions probably only shows the difference of the
relation of the acid to a base and to water.

In view of these considerations, although the teaching of the
distribution of salt-forming elements in _aqueous solutions_ is an
object of great and independent interest, it can hardly serve to
determine the measure of affinity between bases and acids. Similar
considerations ought to be kept in view when determining the
energy of acids by means of the _electrical conductivity of their
weak solutions_. This method, proposed by Arrhenius (1884), and
applied on an extensive scale by Ostwald (who developed it in
great detail in his _Lehrbuch d. allgemeinen Chemie_, v. ii.,
1887), is founded on the fact that the relation of the so-called
molecular electrical-conductivity of weak solutions of various
acids (I) coincides with the relation in which the same acids
stand according to the distribution, (II) found by one of the
above-mentioned methods, and with the relation deduced for them
from observations upon the velocity of reaction, (III) for
instance, according to the rate of the splitting up of an ethereal
salt (into alcohol and acid), or from the rate of the so-called
inversion of sugar--that is, its transformation into glucose--as
is seen by comparing the annexed figures, in which the energy of
hydrochloric acid is taken as equal to 100:--

I II III

Hydrochloric acid, HCl 100 100 100
Hydrobromic acid, HBr 101 98 105
Nitric acid, HNO_{3} 100 100 96
Sulphuric acid, H_{2}SO_{4} 65 49 74
Formic acid, CH_{2}O_{2} 2 4 1
Acetic acid, C_{2}H_{4}O_{2} 1 2 1
Oxalic acid, C_{2}H_{2}O_{4} 20 24 18
Phosphoric acid, PH_{3}O_{4} 7 -- 6

The coincidence of these figures, obtained by so many various
methods, presents a most important and instructive relation
between phenomena of different kinds, but in my opinion it does
not permit us to assert that the degree of affinity existing
between bases and various acids is determined by all these various
methods, because the influence of the water must be taken into
consideration. On this account, until the theory of solution is
more thoroughly worked out, this subject (which for the present
ought to be treated of in special treatises on chemical mechanics)
must be treated with great caution. But now we may hope to decide
this question guided by a study of the rate of reaction, the
influence of acids and bases upon indicators, &c., all of which
are treated fully in works on physical and theoretical chemistry.

[15] Thus, for instance, in the washing of fabrics the caustic alkalis,
such as sodium hydroxide, in weak solutions, act in removing the
fatty matter just in the same way as carbonate solutions; for
instance, a solution of soda crystals, Na_{2}CO_{3}. Soap acts in
the same way, being composed of feeble acids, either fatty or
resinous, combined with alkali. On this account all such
substances are applied in manufacturing processes, and answer
equally well in practice for bleaching and washing fabrics. Soda
crystals or soap are preferred to caustic alkali, because an
excess of the latter may have a destructive effect on the fabrics.
It may be supposed that in aqueous solutions of soap or soda
crystals, part of the base will form caustic alkali; that is to
say, the water will compete with the weak acids, and the alkali
will be distributed between them and the water.

[16] Although carbonic acid is reckoned among the feeble acids, yet
there are evidently many others still feebler--for instance,
prussic acid, hypochlorous acid, many organic acids, &c. Bases
like alumina, or such feeble acids as silica, when in combination
with alkalis, are decomposed in aqueous solutions by carbonic
acid, but on fusion--that is, without the presence of water--they
displace it, which clearly shows in phenomena of this kind how
much depends upon the conditions of reaction and the properties of
the substances formed. These relations, which at first sight
appear complex, may be best understood if we represent that two
salts, MX and NY, in general always give more or less of two other
salts, MY and NX, and then examine the properties of the derived
substances. Thus, in solution, sodium silicate, Na_{2}SiO_{3},
with carbonic anhydride will to some extent form sodium carbonate
and silica, SiO_{2}; but the latter, being colloid, separates, and
the remaining mass of sodium silicate is again decomposed by
carbonic anhydride, so that finally silica separates and sodium
carbonate is formed. In a fused state the case is different;
sodium carbonate will react with silica to form carbonic anhydride
and sodium silicate, but the carbonic anhydride will be separated
as a gas, and therefore in the residue the same reaction will
again take place, and ultimately the carbonic anhydride is
entirely eliminated and sodium silicate remains. If, on the other
hand, nothing is removed from the sphere of the reaction,
distribution takes place. Therefore, although carbonic anhydride
is a feeble acid, still not for this reason, but only in virtue of
its gaseous form, do all soluble acids displace it in saline
solutions (_see_ Chapter X.)

Carbonic anhydride--which, like water, is formed with the development of a large amount of heat--is very stable. Only very few substances are capable of depriving it of its oxygen. However, certain metals, such as magnesium, potassium and the like, on being heated, burn in it, depositing carbon and forming oxides. If a mixture of carbonic anhydride and hydrogen be passed through a heated tube, the formation of water and carbonic oxide will be observed; CO_{2} + H_{2} = CO + H_{2}O. But only a portion of the carbonic acid gas undergoes this change, and therefore the result will be a mixture of carbonic anhydride, carbonic oxide, hydrogen, and water, which does not suffer further change under the action of heat.[17] Although, like water, carbonic anhydride is exceedingly stable, still on being heated it partially decomposes into carbonic oxide and oxygen. Deville showed that such is the case if carbonic anhydride be passed through a long tube containing pieces of porcelain and heated to 1,300°. If the products of decomposition--namely, the carbonic oxide and oxygen--be suddenly cooled, they can be collected separately, although they partly reunite together. A similar decomposition of carbonic anhydride into carbonic oxide and oxygen takes place on passing a series of electric sparks through it (for instance, in the eudiometer). Under these conditions an increase of volume occurs, because two volumes of CO_{2} give two volumes of CO and one volume of O. The decomposition reaches a certain limit (less than one-third) and does not proceed further, so that the result is a mixture of carbonic anhydride, carbonic oxide, and oxygen, which is not altered in composition by the continued action of the sparks. This is readily understood, as it is a reversible reaction. If the carbonic anhydride be removed, then the mixture explodes when a spark is passed and forms carbonic anhydride.[17 bis] If from an identical mixture the oxygen (and not the carbonic anhydride) be removed, and a series of sparks be again passed, the decomposition is renewed, and terminates with the complete dissociation of the carbonic anhydride. Phosphorus is used in order to effect the complete absorption of the oxygen. In these examples we see that a definite mixture of changeable substances is capable of arriving at a state of stable equilibrium, destroyed, however, by the removal of one of the substances composing the mixture. This is one of the instances of the influence of mass.

[17] Hydrogen and carbon are near akin to oxygen as regards affinity,
but it ought to be considered that the affinity of hydrogen is
slightly greater than that of carbon, because during the
combustion of hydrocarbons the hydrogen burns first. Some idea of
this similarity of affinity may be formed by the quantity of heat
evolved. Gaseous hydrogen, H_{2}, on combining with an atom of
oxygen, O = 16, develops 69,000 heat-units if the water formed be
condensed to a liquid state. If the water remains in the form of a
gas (steam) the latent heat of evaporation must be subtracted, and
then 58,000 calories will be developed. Carbon, C, as a solid, on
combining with O_{2} = 32 develops about 97,000 calories, forming
gaseous CO_{2}. If it were gaseous like hydrogen, and only
contained C_{2} in its molecule, much more heat would be
developed, and judging by other substances, whose molecules on
passing from the solid to the gaseous state absorb about 10,000 to
15,000 calories, it must be held that gaseous carbon on forming
gaseous carbonic anhydride would develop not less than 110,000
calories--that is, approximately twice as much as is developed in
the formation of water. And since there is twice as much oxygen in
a molecule of carbonic anhydride as in a molecule of water, the
oxygen develops approximately the same quantity of heat on
combining with hydrogen as with carbon. That is to say, that here
we find the same close affinity (_see_ Chapter II., Note 7)
determined by the quantity of heat as between hydrogen, zinc, and
iron. For this reason here also, as in the case of hydrogen and
iron, we ought to expect an equal distribution of oxygen between
hydrogen and carbon, if they are both in excess compared with the
amount of oxygen; but if there be an excess of carbon, it will
decompose water, whilst an excess of hydrogen will decompose
carbonic anhydride. Even if these phenomena and similar ones have
been explained in isolated cases, a complete theory of the whole
subject is still wanting in the present condition of chemical
knowledge.

[17 bis] The degree or relative magnitude of the dissociation of CO_{2}
varies with the temperature and pressure--that is, it increases
with the temperature and as the pressure decreases. Deville found
that at a pressure of 1 atmosphere in the flame of carbonic oxide
burning in oxygen, about 40 per cent. of the CO_{2}, is decomposed
when the temperature is about 3,000°, and at 1,500° less than 1
per cent. (Krafts); whilst under a pressure of 10 atmospheres
about 34 per cent. is decomposed at 3,300° (Mallard and Le
Chatelier). It follows therefore that, under very small pressures,
the dissociation of CO_{2} will be considerable even at
comparatively moderate temperatures, but at the temperature of
ordinary furnaces (about 1,000°) even under the small partial
pressure of the carbonic acid, there are only small traces of
decomposition which may be neglected in a practical estimation of
the combustion of fuels. We may here cite the molecular specific
heat of CO_{2} (_i.e._ the amount of heat required to raise 44
units of weight of CO_{2} 1°), according to the determinations and
calculations of Mallard and Le Chatelier, for a constant volume
C_{v} = 6·26 + 0·0037_t_; for a constant pressure C_{p} = C_{v} +
2 (_see_ Chapter XIV., Note 7), _i.e._ the specific heat of CO_{2}
increases rapidly with a rise of temperature: for example, at 0°
(per 1 part by weight), it is, at a constant pressure = 0·188, at
1,000° = 0·272, at 2,000°, about 0·356. A perfectly distinct rise
of the specific heat (for example, at 2,000°, 0·409), is given by
a comparison of observations made by the above-mentioned
investigators and by Berthelot and Vieille (Kournakoff). The cause
of this must be looked for in dissociation. T. M. Cheltzoff,
however, considers upon the basis of his researches upon
explosives that it must be admitted that a maximum is reached at a
certain temperature (about 2,500°), beyond which the specific heat
begins to fall.

Although carbonic anhydride is decomposed on heating, yielding oxygen, it is nevertheless, like water, an unchangeable substance at ordinary temperatures. Its decomposition, as effected by plants, is on this account all the more remarkable; in this case the whole of the oxygen of the carbonic anhydride is separated in the free state. The mechanism of this change is that the heat and light absorbed by the plants are expended in the decomposition of the carbonic anhydride. This accounts for the enormous influence of temperature and light on the growth of plants. But it is at present not clearly understood how this takes place, or by what separate intermediate reactions the whole process of decomposition of carbonic anhydride in plants into oxygen and the carbohydrates (Note 1) remaining in them, takes place. It is known that sulphurous anhydride (in many ways resembling carbonic anhydride) under the action of light (and also of heat) forms sulphur and sulphuric anhydride, SO_{3}, and in the presence of water, sulphuric acid. But no similar decomposition has been obtained directly with carbonic anhydride, although it forms an exceedingly easily decomposable higher oxide--percarbonic acid;[18] and perhaps that is the reason the oxygen separates. On the other hand, it is known that plants always form and contain _organic acids_, and these must be regarded as derivatives of carbonic acid, as is seen by all their reactions, of which we will shortly treat. For this reason it might be thought that the carbonic acid absorbed by the plants first forms (according to Baeyer) formic aldehyde, CH_{2}O, and from it organic acids, and that these latter in their final transformation form all the other complex organic substances of the plants. Many organic acids are found in plants in considerable quantity; for instance, tartaric acid, C_{4}H_{6}O_{6}, found in grape-juice and in the acid juice of many plants; malic acid, C_{4}H_{6}O_{5}, found not only in unripe apples but in still larger quantities in mountain ash berries; citric acid, C_{6}H_{8}O_{7}, found in the acid juice of lemons, in gooseberries, cranberries, &c.; oxalic acid, C_{2}H_{2}O_{4}, found in wood-sorrel and many other plants. Sometimes these acids exist in a free state in the plants, and sometimes in the form of salts; for instance, tartaric acid is met with in grapes as the salt known as cream of tartar, but in the impure state called argol, or tartar, C_{4}H_{5}KO_{6}. In sorrel we find the so-called salts of sorrel, or acid potassium oxalate, C_{2}HKO_{4}. There is a very clear connection between carbonic anhydride and the above-mentioned organic acids--namely, they all, under one condition or another, yield carbonic anhydride, and can all be formed by means of it from substances destitute of acid properties. The following examples afford the best demonstration of this fact: if acetic acid, C_{2}H_{4}O_{2}, the acid of vinegar, be passed in the form of vapour through a heated tube, it splits up into carbonic anhydride and marsh gas = CO_{2} + CH_{4}. But conversely it can also be obtained from those components into which it decomposes. If one equivalent of hydrogen in marsh gas be replaced (by indirect means) by sodium, and the compound CH_{3}Na is obtained, this directly absorbs carbonic anhydride, forming a salt of acetic acid, CH_{3}Na + CO_{2} = C_{2}H_{3}NaO_{2}; from this acetic acid itself may be easily obtained. Thus acetic acid decomposes into marsh gas and carbonic anhydride, and conversely is obtainable from them. The hydrogen of marsh gas does not, like that in acids, show the property of being directly replaced by metals; _i.e._ CH_{4} does not show any acid character whatever, but on combining with the elements of carbonic anhydride it acquires the properties of an acid. The investigation of all other organic acids shows similarly that their acid character depends on their containing the elements of carbonic anhydride. For this reason there is no organic acid containing less oxygen in its molecule than there is in carbonic anhydride; every organic acid contains in its molecule at least two atoms of oxygen. In order to express the relation between carbonic acid, H_{2}CO_{3}, and organic acids, and in order to understand the reason of the acidity of these latter, it is simplest to turn to that law of substitution which shows (Chapter VI.) the relation between the hydrogen and oxygen compounds of nitrogen, and permits us (Chapter VIII.) to regard all hydrocarbons as derived from methane. If we have a given organic compound, A, which has not the properties of an acid, but contains hydrogen connected to carbon, as in hydrocarbons, then ACO_{2} will be a monobasic organic acid, A2CO_{2} a bibasic, A3CO_{2} a tribasic, and so on--that is, each molecule of CO_{2} transforms one atom of hydrogen into that state in which it may be replaced by metals, as in acids. This furnishes a direct proof that in organic acids it is necessary to recognise the group HCO_{2}, or carboxyl. If the addition of CO_{2} raises the basicity, the removal of CO_{2} lowers it. Thus from the bibasic oxalic acid, C_{2}H_{2}O_{4}, or phthalic acid, C_{8}H_{6}O_{4}, by eliminating CO_{2} (easily effected experimentally) we obtain the monobasic formic acid, CH_{2}O_{2}, or benzoic acid, C_{7}H_{6}O_{2}, respectively. The nature of carboxyl is directly explained by the law of substitution. Judging from what has been stated in Chapters VI. and VIII. concerning this law, it is evident that CO_{2} is CH_{4} with the exchange of H_{4} for O_{2}, and that the hydrate of carbonic anhydride, H_{2}CO_{3}, is CO(OH)_{2}, that is, methane, in which two parts of hydrogen are replaced by two parts of the water radical (OH, hydroxyl) and the other two by oxygen. Therefore the group CO(OH), or carboxyl, HCO_{2}, is a part of carbonic acid, and is equivalent to (OH), and therefore also to H. That is, it is a univalent residue of carbonic acid capable of replacing one atom of hydrogen. Carbonic acid itself is a bibasic acid, both hydrogen atoms in it being replaceable by metals, therefore carboxyl, which contains one of the hydrogen atoms of carbonic acid, represents a group in which the hydrogen is exchangeable for metals. And therefore if 1, 2 ... _n_ atoms of non-metallic hydrogen are exchanged 1, 2 ... _n_ times for carboxyl, we ought to obtain 1, 2 ... _n_-basic acids. _Organic acids are the products of the carboxyl substitution in hydrocarbons._[18 bis] If in the saturated hydrocarbons, C_{n}H_{2n + 2}, one part of hydrogen is replaced by carboxyl, the monobasic saturated (or fatty) acids, C_{n}H_{2n + 1}(CO_{2}H), will be obtained, as, for instance, formic acid, HCO_{2}H, acetic acid, CH_{2}CO_{2}H, ... stearic acid, C_{17}H_{35}CO_{2}H, &c. The double substitution will give bibasic acids, C_{n}H_{2n}(CO_{2}H)(CO_{2}H); for instance, oxalic acid _n_ = 0, malonic acid _n_ = 1, succinic acid _n_ = 2, &c. To benzene, C_{6}H_{6} correspond benzoic acid, C_{6}H_{5}(CO_{2}H), phthalic acid (and its isomerides), C_{6}H_{4}(CO_{2}H)_{2}, up to mellitic acid, C_{6}(CO_{2}H)_{6}, in all of which the basicity is equal to the number of carboxyl groups. As many isomerides exist in hydrocarbons, it is readily understood not only that such can exist also in organic acids, but that their number and structure may be foreseen. This complex and most interesting branch of chemistry is treated separately in organic chemistry.

[18] Percarbonic acid, H_{2}CO_{4} (= H_{2}CO_{3} + O) is supposed
by A. Bach (1893) to be formed from carbonic acid in the action of
light upon plants, (in the same manner as, according to the above
scheme, sulphuric acid from sulphurous) with the formation of
carbon, which remains in the form of hydrates of carbon:
3H_{2}CO_{3} = 2H_{2}CO_{4} + CH_{2}O. This substance CH_{2}O
expresses the composition of formic aldehyde which, according to
Baeyer, by polymerisation and further changes, gives other
hydrates of carbon and forms the first product which is formed in
plants from CO_{2}. And Berthelot (1872) had already, at the time
of the discovery of persulphuric (Chapter XX.) and pernitric
(Chapter VI., Note 26) acids pointed out the formation of the
unstable percarbonic anhydride, CO_{3}. Thus, notwithstanding the
hypothetical nature of the above equation, it may be admitted all
the more as it explains the comparative abundance of peroxide of
hydrogen (Schöne, Chapter IV.) in the air, and this also at the
period of the most energetic growth of plants (in July), because
percarbonic acid should like all peroxides easily give H_{2}O_{2}.
Besides which Bach (1894) showed that, in the first place, traces
of formic aldehyde and oxidising agents (CO_{3} or H_{2}O_{2})
are formed under the simultaneous action of CO_{2} and sunlight
upon a solution containing a salt of uranium (which is oxidised),
and diethylaniline (which reacts with CH_{2}O), and secondly, that
by subjecting BaO_{2}, shaken up in water, to the action of a
stream of CO_{2} in the cold, extracting (also in the cold) with
ether, and then adding an alcoholic solution of NaHO, crystalline
plates of a sodium salt may be obtained, which with water evolve
oxygen and leave sodium carbonate; they are therefore probably the
per-salt. All these facts are of great interest and deserve
further verification and elaboration.

[18 bis] If CO_{2} is the anhydride of a bibasic acid, and carboxyl
corresponds with it, replacing the hydrogen of hydrocarbons, and
giving them the character of comparatively feeble acids, then
SO_{3} is the anhydride of an energetic bibasic acid, and
_sulphoxyl_, SO_{2}(OH), corresponds with it, being capable of
replacing the hydrogen of hydrocarbons, and forming comparatively
energetic _sulphur oxyacids_ (_sulphonic acids_); for instance,
C_{6}H_{5}(COOH), benzoic acid, and C_{6}H_{5}(SO_{2}OH),
benzenesulphonic acid, are derived from C_{6}H_{6}. As the
exchange of H for methyl, CH_{3}, is equivalent to the addition of
CH_{2}, the exchange of carboxyl, COOH, is equivalent to the
addition of CO_{2}; so the exchange of H for sulphoxyl is
equivalent to the addition of SO_{3}. The latter proceeds
directly, for instance: C_{6}H_{6} + SO_{3} =
C_{6}H_{5}(SO_{2}OH).

As accordingding to the determinations of Thomsen, the heat of
combustion of the _vapours_ of acids RCO_{2} is known where R is a
hydrocarbon, and the heat of combustion of the hydrocarbons R
themselves, it may be seen that the formation of acids, RCO_{2},
from R + CO_{2}, is always accompanied by a _small_ absorption or
development of heat. We give the heats of combustion in thousands
of calories, referred to the molecular weights of the
substances:--

R = H_{2} CH_{4} C_{2}H_{6} C_{6}H_{6}
68·4 212 370 777
RCO_{2} = 69·4 225 387 766

Thus H_{2}, corresponds with formic acid, CH_{2}O_{2}; benzene,
C_{6}H_{6}, with benzoic acid, C_{7}H_{6}O_{2}. The data for the
latter are taken from Stohmann, and refer to the solid condition.
For formic acid Stohmann gives the heat of combustion as 59,000
calories in a liquid state, but in a state of vapour, 64·6
thousand units, which is much less than according to Thomsen.

_Carbonic Oxide._--This gas is formed whenever the combustion of organic substances takes place in the presence of a large excess of incandescent charcoal; the air first burns the carbon into carbonic anhydride, but this in penetrating through the red-hot charcoal is transformed into carbonic oxide, CO_{2} + C = 2CO. By this reaction carbonic oxide is prepared by passing carbonic anhydride through charcoal at a red heat. It may be separated from the excess of carbonic anhydride by passing it through a solution of alkali, which does not absorb carbonic oxide. This reduction of carbonic anhydride explains why carbonic oxide is formed in ordinary clear fires, where the incoming air passes over a large surface of heated coal. A blue flame is then observed burning above the coal; this is the burning carbonic oxide. When charcoal is burnt in stacks, or when a thick layer of coal is burning in a brazier, and under many similar circumstances, carbonic oxide is also formed. In metallurgical processes, for instance when iron is smelted from the ore, very often the same process of conversion of carbonic anhydride into carbonic oxide occurs, especially if the combustion of the coal be effected in high, so-called blast, furnaces and ovens, where the air enters at the lower part and is compelled to pass through a thick layer of incandescent coal. In this way, also, combustion with flame may be obtained from those kinds of fuel which under ordinary conditions burn without flame: for instance, anthracite, coke, charcoal. Heating by means of a gas-producer--that is, an apparatus producing combustible carbonic oxide from fuel--is carried on in the same manner.[19] In transforming one part of charcoal into carbonic oxide 2,420 heat units are given out, and on burning to carbonic anhydride 8,080 heat units. It is evident that on transforming the charcoal first into carbonic oxide we obtain a gas which in burning is capable of giving out 5,660 heat units for one part of charcoal. This preparatory transformation of fuel into carbonic oxide, or producer gas containing a mixture of carbonic oxide (about 1/3 by volume) and nitrogen (2/3 volume), in many cases presents most important advantages, as it is easy to completely burn gaseous fuel without an excess of air, which would lower the temperature.[20] In stoves where solid fuel is burnt it is impossible to effect the complete combustion of the various kinds of fuel without admitting an excess of air. Gaseous fuel, such as carbonic oxide, is easily completely mixed with air and burnt without excess of it. If, in addition to this, the air and gas required for the combustion be previously heated by means of the heat which would otherwise be uselessly carried off in the products of combustion (smoke)[21] it is easy to reach a high temperature, so high (about 1,800°) that platinum may be melted. Such an arrangement is known as a _regenerative furnace_.[22] By means of this process not only may the high temperatures indispensable in many industries be obtained (for instance, glass-working, steel-melting, &c.), but great advantage also[23] is gained as regards the quantity of fuel, because the transmission of heat to the object to be heated, other conditions being equal, is determined by the difference of temperatures.

[19]

In gas-producers all carbonaceous fuels are transformed into
inflammable gas. In those which (on account of their slight
density and large amount of water, or incombustible admixtures
which absorb heat) are not as capable of giving a high temperature
in ordinary furnaces--for instance, fir cones, peat, the lower
kinds of coal, &c.--the same gas is obtained as with the best
kinds of coal, because the water condenses on cooling, and the
ashes and earthy matter remain in the gas-producer. The
construction of a gas-producer is seen from the accompanying
drawing. The fuel lies on the fire-bars O, the air enters through
them and the ash-hole (drawn by the draught of the chimney of the
stove where the gas burns, or else forced by a blowing apparatus),
the quantity of air being exactly regulated by means of valves.
The gases formed are then led by the tube V, provided with a
valve, into the gas main U. The addition of fuel ought to proceed
in such a way as to prevent the generated gas escaping; hence the
space A is kept filled with the combustible material and covered
with a lid.

[20] An excess of air lowers the temperature of combustion, because it
becomes heated itself, as explained in Chapter III. In ordinary
furnaces the excess of air is three or four times greater than the
quantity required for perfect combustion. In the best furnaces
(with fire-bars, regulated air supply, and corresponding chimney
draught) it is necessary to introduce twice as much air as is
necessary, otherwise the smoke contains much carbonic oxide.

[21] If in manufactories it is necessary, for instance, to maintain the
temperature in a furnace at 1,000°, the flame passes out at this
or a higher temperature, and therefore much fuel is lost in the
smoke. For the draught of the chimney a temperature of 100° to
150° is sufficient, and therefore the remaining heat ought to be
utilised. For this purpose the flues are carried under boilers or
other heating apparatus. The preparatory heating of the air is the
best means of utilisation when a high temperature is desired
(_see_ Note 22).

[22] Regenerative furnaces were introduced by the Brothers Siemens
about the year 1860 in many industries, and mark a most important
progress in the use of fuel, especially in obtaining high
temperatures. The principle is as follows: The products of
combustion from the furnace are led into a chamber, I, and heat up
the bricks in it, and then pass into the outlet flue; when the
bricks are at a red heat the products of combustion are passed (by
altering the valves) into another adjoining chamber, II, and air
requisite for the combustion of the generator gases is passed
through I. In passing round about the incandescent bricks the air
is heated, and the bricks are cooled--that is, the heat of the
smoke is returned into the furnace. The air is then passed through
II, and the smoke through I. The regenerative burners for
illuminating gas are founded on this same principle, the products
of combustion heat the incoming air and gas, the temperature is
higher, the light brighter, and an economy of gas is effected.
Absolute perfection in these appliances has, of course, not yet
been attained; further improvement is still possible, but
dissociation imposes a limit because at a certain high temperature
combinations do not ensue, possible temperatures being limited by
reverse reactions. Here, as in a number of other cases, the
further investigation of the matter must prove of direct value
from a practical point of view.

[23] At first sight it appears absurd, useless, and paradoxical to lose
nearly one-third of the heat which fuel can develop, by turning it
into gas. Actually the advantage is enormous, especially for
producing high temperatures, as is already seen from the fact that
fuels rich in oxygen (for instance, wood) when damp are unable,
with any kind of hearth whatever, to give the temperature required
for glass-melting or steel-casting, whilst in the gas-producer
they furnish exactly the same gas as the driest and most
carbonaceous fuel. In order to understand the principle which is
here involved, it is sufficient to remember that a large amount of
heat, but having a low temperature, is in many cases of no use
whatever. We are unable here to enter into all the details of the
complicated matter of the application of fuel, and further
particulars must be sought for in special technical treatises. The
following footnotes, however, contain certain fundamental figures
for calculations concerning combustion.

The transformation of carbonic anhydride, by means of charcoal, into carbonic oxide (C + CO_{2} = CO + CO) is considered a reversible reaction, because at a high temperature the carbonic oxide splits up into carbon and carbonic anhydride, as Sainte-Claire Deville showed by using the method of the 'cold and hot tube.' Inside a tube heated in a furnace another thin metallic (silvered copper) tube is fitted, through which a constant stream of cold water flows. The carbonic oxide coming into contact with the heated walls of the exterior tube forms charcoal, and its minute particles settle in the form of lampblack on the lower side of the cold tube, and, since they are cooled, do not act further on the oxygen or carbonic anhydride formed.[24] A series of electric sparks also decomposes carbonic oxide into carbonic anhydride and carbon, and if the carbonic anhydride be removed by alkali complete decomposition may be obtained (Deville).[24 bis] Aqueous vapour, which is so similar to carbonic anhydride in many respects, acts, at a high temperature, on charcoal in an exactly similar way, C + H_{2}O = H_{2} + CO. From 2 volumes of carbonic anhydride with charcoal 4 volumes of carbonic oxide (2 molecules) are obtained, and precisely the same from 2 volumes of water vapour with charcoal 4 volumes of a gas consisting of hydrogen and carbonic oxide (H_{2} + CO) are formed. This mixture of combustible gases is called _water gas_.[25] But aqueous vapour (and only when strongly superheated, otherwise it cools the charcoal) only acts on charcoal to form a large amount of carbonic oxide at a very high temperature (at which carbonic anhydride dissociates); it begins to react at about 500°, forming carbonic anhydride according to the equation C + 2H_{2}O = CO_{2} + 2H_{2}. Besides this, carbonic oxide on splitting up forms carbonic anhydride, and therefore water gas always contains a mixture[26] in which hydrogen predominates, the volume of carbonic oxide being comparatively less, whilst the amount of carbonic anhydride increases as the temperature of the reaction decreases (generally it is more than 3 per cent.)

[24] The first product of combustion of charcoal is always carbonic
anhydride, and not carbonic oxide. This is seen from the fact that
with a shallow layer of charcoal (less than a decimetre if the
charcoal be closely packed) carbonic oxide is not formed at all.
It is not even produced with a deep layer of charcoal if the
temperature is not above 500°, and the current of air or oxygen is
very slow. With a rapid current of air the charcoal becomes
red-hot, and the temperature rises, and then carbonic oxide
appears (Lang 1888). Ernst (1891) found that below 995° carbonic
oxide is always accompanied by CO_{2}, and that the formation of
CO_{2} begins about 400°. Naumann and Pistor determined that the
reaction of carbonic anhydride with carbon commences at about
550°, and that between water and carbon at about 500°. At the
latter temperature carbonic anhydride is formed, and only with a
rise of temperature is carbonic oxide formed (Lang) from the
action of the carbonic anhydride on the carbon, and from the
reaction CO_{2} + H_{2} = CO + H_{2}O. Rathke (1881) showed that
at no temperature whatever is the reaction as expressed by the
equation CO_{2} + C = 2CO_{2}, complete; a part of the carbonic
anhydride remains, and Lang determined that at about 1,000° not
less than 3 p.c. of the carbonic anhydride remains untransformed
into carbonic oxide, even after the action has been continued for
several hours. The endothermal reactions, C + 2H_{2}O = CO_{2}
+2H_{2}, and CO + H_{2}O = CO_{2} + H_{2}, are just as incomplete.
This is made clear if we note that on the one hand the
above-mentioned reactions are all reversible, and therefore
bounded by a limit; and, on the other hand, that at about 500°
oxygen begins to combine with hydrogen and carbon, and also that
the lower limits of dissociation of water, carbonic anhydride, and
carbonic oxide lie near one another between 500° and 1,200°. For
water and carbonic oxide the lower limit of the commencement of
dissociation is unknown, but judging from the published data
(according to Le Chatelier, 1888) that of carbonic anhydride may
be taken as about 1,050°. Even at about 200° half the carbonic
anhydride dissociates if the pressure be small, about 0·001
atmosphere. At the atmospheric pressure, not more than 0·05 p.c.
of the carbonic anhydride decomposes. The reason of the influence
of pressure is here evidently that the splitting up of carbonic
anhydride into carbonic oxide and oxygen is accompanied by an
increase in volume (as in the case of the dissociation of nitric
peroxide. _See_ Chapter VI., Note 46). As in stoves and lamps, and
also with explosive substances, the temperature is not higher than
2,000° to 2,500°, it is evident that although the partial pressure
of carbonic anhydride is small, still its dissociation cannot here
be considerable, and probably does not exceed 5 p.c.

[24 bis] Besides which L. Mond (1890) showed that the powder of freshly
reduced metallic nickel (obtained by heating the oxide to redness
in a stream of hydrogen) is able, when heated even to 350°, to
completely decompose carbonic oxide into CO_{2} and carbon, which
remains with the nickel and is easily removed from it by heating
in a stream of air. Here 2CO = CO_{2} + C. It should be remarked
that heat is evolved in this reaction (Note 25), and therefore
that the influence of 'contact' may here play a part. Indeed, this
reaction must be classed among the most remarkable instances of
the influence of contact, especially as metals analogous to Ni (Fe
and Co) do not effect this reaction (_see_ Chapter II., Note 17).

[25] A molecular weight of this gas, or 2 volumes CO (28 grams), on
combustion (forming CO_{2}) gives out 68,000 heat units (Thomsen
67,960 calories). A molecular weight of hydrogen, H_{2} (or 2
volumes), develops on burning into _liquid_ water 69,000 heat
units (according to Thomsen 68,300), but if it forms aqueous
vapour 58,000 heat units. Charcoal, resolving itself by combustion
into the molecular quantity of CO_{2} (2 volumes), develops 97,000
heat units. From the data furnished by these exothermal reactions
it follows: (1) that the oxidation of charcoal into carbonic oxide
develops 29,000 heat units; (2) that the reaction C + CO_{2} = 2CO
_absorbs_ 39,000 heat units; (3) C + H_{2}O = H_{2} + CO _absorbs_
(if the water be in a state of vapour) 29,000 calories, but if the
water be liquid 40,000 calories (almost as much as C + CO_{2});
(4) C + H_{2}O = CO_{2} + 2H_{2} _absorbs_ (if the water be in a
state of vapour) 19,000 heat units; (5) the reaction CO + H_{2}O =
CO_{2} + H_{2} _develops_ 10,000 heat units if the water be in the
state of vapour; and (6) the decomposition expressed by the
equation 2CO = C + CO_{2} (Note 24 bis) is accompanied by the
_evolution_ of 39,000 units of heat.

Hence it follows that 2 volumes of CO or H_{2} burning into CO_{2}
or H_{2}O develop almost the same amount of heat, just as also the
heat effects corresponding with the equations

C + H_{2}O = CO + H_{2}

C + CO_{2} = CO + CO

are nearly equal.

[26] _Water gas_, obtained from steam and charcoal at a white heat,
contains about 50 p.c. of hydrogen, about 40 p.c. of carbonic
oxide, about 5 p.c. of carbonic anhydride, the remainder being
nitrogen from the charcoal and air. Compared with producer gas,
which contains much nitrogen, this is a gas much richer in
combustible matter, and therefore capable of giving high
temperatures, and is for this reason of the greatest utility. If
carbonic anhydride could be as readily obtained in as pure a state
as water, then CO might be prepared directly from CO_{2} + C, and
in that case the utilisation of the heat of the carbon would be
the same as in water gas, because CO evolves as much heat as
H_{2}, and even more if the temperature of the smoke be over 100°,
and the water remains in the form of vapour (Note 25). But
producer gas contains a large proportion of nitrogen, so that its
effective temperature is below that given by water gas; therefore
in places where a particularly high temperature is required (for
instance, for lighting by means of incandescent lime or magnesia,
or for steel melting, &c.), and where the gas can be easily
distributed through pipes, water gas is at present held in high
estimation, but when (in ordinary furnaces, re-heating,
glass-melting, and other furnaces) a very high temperature is not
required, and there is no need to convey the gas in pipes,
producer gas is generally preferred on account of the simplicity
of its preparation, especially as for water gas such a high
temperature is required that the plant soon becomes damaged.

There are numerous systems for making water gas, but the American
patent of T. Lowe is generally used. The gas is prepared in a
cylindrical generator, into which hot air is introduced, in order
to raise the coke in it to a white heat. The products of
combustion containing carbonic oxide are utilised for superheating
steam, which is then passed over the white hot coke. Water gas, or
a mixture of hydrogen and carbonic oxide, is thus obtained.

Water gas is sometimes called '_the fuel of the future_,' because
it is applicable to all purposes, develops a high temperature, and
is therefore available, not only for domestic and industrial uses,
but also for gas-motors and for lighting. For the latter purpose
platinum, lime, magnesia, zirconia, and similar substances (as in
the Drummond light, Chapter III.), are rendered incandescent in
the flame, or else the gas is _carburetted_--that is, mixed with
the vapours of volatile hydrocarbons (generally benzene or
naphtha, naphthalene, or simply naphtha gas), which communicate to
the pale flame of carbonic oxide and hydrogen a great brilliancy,
owing to the high temperature developed by the combustion of the
non-luminous gases. As water gas, possessing these properties, may
be prepared at central works and conveyed in pipes to the
consumers, and as it may be produced from any kind of fuel, and
ought to be much cheaper than ordinary gas, it may as a matter of
fact be expected that in course of time (when experience shall
have determined the cheapest and best way to prepare it) it will
not only supplant ordinary gas, but will with advantage everywhere
replace the ordinary forms of fuel, which in many respects are
inconvenient. At present its consumption spreads principally for
lighting purposes, and for use in gas-engines instead of ordinary
illuminating gas. In some cases Dowson gas is prepared in
producers. This is a mixture of water and producer gases obtained
by passing steam into an ordinary producer (Note 19), when the
temperature of the carbon has become sufficiently high for the
reaction C + H_{2}O = CO + H_{2}.

Metals like iron and zinc which at a red heat are capable of decomposing water with the formation of hydrogen, also decompose carbonic anhydride with the formation of carbonic oxide; so both the ordinary products of complete combustion, water and carbonic anhydride, are very similar in their reactions, and we shall therefore presently compare hydrogen and carbonic oxide. The metallic oxides of the above-mentioned metals, when reduced by charcoal, also give carbonic oxide. Priestley obtained it by heating charcoal with zinc oxide. As free carbonic anhydride may be transformed into carbonic oxide, so, in precisely the same way, may that carbonic acid which is in a state of combination; hence, if magnesium or barium carbonates (MgCO_{3} or BaCO_{3}) be heated to redness with charcoal, or iron or zinc, carbonic oxide will be produced--for instance, it is obtained by heating an intimate mixture of 9 parts of chalk and 1 part of charcoal in a clay retort.

Many organic substances[27] on being heated, or under the action of various agents, yield carbonic oxide; amongst these are many organic or carboxylic acids. The simplest are formic and oxalic acids. Formic acid, CH_{2}O_{2}, on being heated to 200°, easily decomposes into carbonic oxide and water, CH_{2}O_{2} = CO + H_{2}O.[27 bis] Usually, however, carbonic oxide is prepared in laboratories, not from formic but from oxalic acid, C_{2}H_{2}O_{4}, the more so as formic acid is itself prepared from oxalic acid. The latter acid is easily obtained by the action of nitric acid on starch, sugar, &c.; it is also found in nature. Oxalic acid is easily decomposed by heat; its crystals first lose water, then partly volatilise, but the greater part is decomposed. The decomposition is of the following nature: it splits up into water, carbonic oxide, and carbonic anhydride,[28] C_{2}H_{2}O_{4} = H_{2}O + CO_{2} + CO. This decomposition is generally practically effected by mixing oxalic acid with strong sulphuric acid, because the latter assists the decomposition by taking up the water. On heating a mixture of oxalic and sulphuric acids a mixture of carbonic oxide and carbonic anhydride is evolved. This mixture is passed through a solution of an alkali in order to absorb the carbonic anhydride, whilst the carbonic oxide passes on.[28 bis]

[27] The so-called yellow prussiate, K_{4}FeC_{6}N_{6}, on being heated
with ten parts of strong sulphuric acid forms a considerable
quantity of very pure carbonic oxide quite free from carbonic
anhydride.

[27 bis] To perform this reaction, the formic acid is mixed with
glycerine, because when heated alone it volatilises much below its
temperature of decomposition. When heated with sulphuric acid the
salts of formic acid yield carbonic oxide.

[28] The decomposition of formic and oxalic acids, with the formation
of carbonic oxide, considering these acids as carboxyl
derivatives, may be explained as follows:--The first is H(COOH)
and the second (COOH)_{2}, or H_{2} in which one or both halves of
the hydrogen are exchanged for carboxyl; therefore they are equal
to H_{2} + CO_{2} and H_{2} + 2CO_{2}; but H_{2} reacts with
CO_{2}, as has been stated above, forming CO and H_{2}O. From this
it is also evident that oxalic acid on losing CO_{2} forms formic
acid, and also that the latter may proceed from CO + H_{2}O, as we
shall see further on.

[28 bis] Greshoff (1888) showed that with a solution of nitrate of
silver, iodoform, CHI_{3}, forms CO according to the equation
CHI_{3} + 3AgNO_{3} + H_{2}O = 3AgI + 3HNO_{3} + CO. The reaction
is immediate and is complete.