Het famille-en Kampongleven op Groot Atjeh (Leyden,
1894); C. Snouck Hurgronje, De Atjehers (Batavia, 1894).
ACHOLI, a negro people of the upper Nile valley, dwelling on
the east bank of the Bahr-el-Jebel, about a hundred miles north
of Albert Nyanza. They are akin to the Shilluks of the White
Nile. They frequently decorate the temples or cheeks with
wavy or zigzag scars, and also the thighs with scrolls; some
pierce the ears. Their dwelling-places are circular huts
with a high peak, furnished with a mud sleeping-platform,
jars of grain and a sunk fireplace. The interior walls are
daubed with mud and decorated with geometrical or conventional
designs in red, white or grey. The Acholi are good hunters,
using nets and spears, and keep goats, sheep and cattle. In
war they use spears and long, narrow shields of giraffe or ox
hide. Their dialect is closely allied to those of the Alur,
Lango and ja-Luo tribes, all four being practically pure
Nilotic. Their religion is a vague fetishism. By early
explorers the Acholi were called Shuli, a name now obsolete.
ACHROMATISM (Gr. a-, privative, chroma, colour),
in optics, the property of transmitting white light,
without decomposing it into the colours of the spectrum;
``achromatic lenses'' are lenses which possess this
property. (See LENS, ABERRATION and PHOTOGRAPHY.)
ACID (from the Lat. root ac-, sharp; acere, to be
sour), the name loosely applied to any sour substance;
in chemistry it has a more precise meaning, denoting a
substance containing hydrogen which may be replaced by
metals with the formation of salts. An acid may therefore be
regarded as a salt of hydrogen. Of the general characters
of acids we may here notice that they dissolve alkaline
substances, certain metals, &c., neutralize alkalies and
redden many blue and violet vegetable colouring matters.
The ancients probably possessed little knowledge indeed of
acids. Vinegar (or impure acetic acid), which is produced
when wine is allowed to stand, was known to both the
Greeks and Romans, who considered it to be typical of acid
substances; this is philologically illustrated by the words
oxus, acidus, sour, and oxos, acetus, vinegar.
Other acids became known during the alchemistic period;
and the first attempt at a generalized conception of these
substances was made by Paracelsus, who supposed them to contain
a principle which conferred the properties of sourness and
solubility. Somewhat similar views were promoted by Becher,
who named the principle acidum primogenium, and held
that it was composed of the Paracelsian elements ``earth''
and ``water.'' At about the same time Boyle investigated
several acids; he established their general reddening of
litmus, their solvent power of metals and basic substances,
and the production of neutral bodies, or salts, with alkalies.
Theoretical conceptions were revived by Stahl, who held that
acids were the fundamentals of all salts, and the erroneous
idea that sulphuric acid was the principle of all acids.
The phlogistic theory of the processes of calcination and
combustion necessitated the view that many acids, such as
those produced by combustion, e.g. sulphurous, phosphoric,
carbonic, &c., should be regarded as elementary substances.
This principle more or less prevailed until it was overthrown
by Lavoisier's doctrine that oxygen was the acid-producing
element; Lavoisier being led to this conclusion by the almost
general observation that acids were produced when non-metallic
elements were burnt. The existence of acids not containing
oxygen was, in itself, sufficient to overthrow this idea,
but, although Berthollet had shown, in 1789, that sulphuretted
hydrogen (or hydrosulphuric acid) contained no oxygen,
Lavoisier's theory held its own until the researches of Davy,
Gay-Lussac and Thenard on hydrochloric acid and chlorine,
and of Gay-Lussac on hydrocyanic acid, established beyond all
cavil that oxygen was not essential to acidic properties.
In the Lavoisierian nomenclature acids were regarded as
binary oxygenated compounds, the associated water being
relegated to the position of a mere solvent. Somewhat
similar views were held by Berzelius, when developing his
dualistic conception of the composition of substances. In
later years Berzelius renounced the ``oxygen acid'' theory,
but not before Davy, and, almost simultaneously, Dulong, had
submitted that hydrogen and not oxygen was the acidifying
principle. Opposition to the ``hydrogen-acid'' theory centred
mainly about the hypothetical radicals which it postulated;
moreover, the electrochemical theory of Berzelius exerted
a stultifying influence on the correct views of Davy and
Dulong. In Berzelius' system potassium sulphate is to be
regarded as K2O+.SO3-; electrolysis should simply effect
the disruption of the positive and negative components, potash
passing with the current, and sulphuric acid against the
current. Experiment showed, however, that instead of only
potash appearing at the negative electrode, hydrogen is also
liberated; this is inexplicable by Berzelius's theory, but
readily explained by the ``hydrogen-acid'' theory. By this
theory potassium is liberated at the negative electrode and
combines immediately with water to form potash and hydrogen.
Further and stronger support was given when J. Liebig promoted
his doctrine of polybasic acids. Dalton's idea that elements
preferentially combined in equiatomic proportions had as an
immediate inference that metallic oxides contained one atom
of the metal to one atom of oxygen, and a simple expansion
of this conception was that one atom of oxide combined with
one atom of acid to form one atom of a neutral salt. This
view, which was specially supported by Gay-Lussac and Leopold
Gmelin and accepted by Berzelius, necessitated that all
acids were monobasic. The untenability of this theory was
proved by Thomas Graham's investigation of the phosphoric
acids; for he then showed that the ortho- (ordinary), pyro-
and metaphosphoric acids contained respectively 3, 2 and
1 molecules of ``basic water'' (which were replaceable by
metallic oxides) and one molecule of phosphoric oxide, P2
O5. Graham's work was developed by Liebig, who called into
service many organic acids---citric, tartaric, cyanuric,
comenic and meconic---and showed that these resembled
phosphoric acid; and he established as the criterion of
polybasicity the existence of compound salts with different
metallic oxides. In formulating these facts Liebig at
first retained the dualistic conception of the structure of
acids; but he shortly afterwards perceived that this view
lacked generality since the halogen acids, which contained
no oxygen but yet formed salts exactly similar in properties
to those containing oxygen, could not be so regarded. This
and other reasons led to his rejection of the dualistic
hypothesis and the adoption, on the ground of probability,
and much more from convenience, of the tenet that ``acids
are particular compounds of hydrogen, in which the latter
can be replaced by metals''; while, on the constitution of
salts, he held that ``neutral salts are those compounds
of the same class in which the hydrogen is replaced by its
equivalent in metal. The substances which we at present
term anhydrous acids (acid oxides) only become, for the
most part, capable of forming salts with metallic oxides
after the addition of water, or they are compounds which
decompose these oxides at somewhat high temperatures.''
The hydrogen theory and the doctrine of polybasicity as
enunciated by Liebig is the fundamental characteristic of
the modern theory. A polybasic acid contains more than one
atom of hydrogen which is replaceable by metals; moreover, in
such an acid the replacement may be entire with the formation
of normal salts, partial with the formation of acid salts,
or by two or more different metals with the formation of
compound salts (see SALTS). These facts may be illustrated
with the aid of orthophosphoric acid, which is tribasic:--
Acid. Normal salt. Acid salts.
H3PO4 Ag3PO4 Na2HPO4; NaH2PO4
Phosphoric Silver phosphate. Acid sodium
acid. phosphates.
Compound salts.
Mg(NH4)PO4; Na(NH4)HPO4.
Magnesium ammonium Microcosmic
phosphate; salt.
Reference should be made to the articles CHEMICAL
ACTION, THERMOCHEMISTRY and SOLUTIONS, for
the theory of the strength or avidity of acids.
Organic Acids.---Organic acids are characterized by the
presence of the monovalent group--CO.OH, termed the carboxyl
group, in which the hydrogen atom is replaceable by metals
with the formation of salts, and by alkyl radicals with the
formation of esters. The basicity of an organic acid, as
above defined, is determined by the number of carboxyl groups
present. Oxy-acids are carboxyllc acids which also contain
a hydroxyl group; similarly we may have aldehyde-acids,
ketone-acids, &c. Since the more important acids are treated
under their own headings, or under substances closely allied to
them, we shall here confine ourselves to general relations.
Classification.--It is convenient to distinguish between
aliphatic and aromatic acids; the first named being derived from
open-chain hydrocarbons, the second from ringed hydrocarbon
nuclei. Aliphatic monobasic acids are further divided
according to the nature of the parent hydrocarbon. Methane
and its homologues give origin to the ``paraffin'' or ``fatty
series'' of the general formula Cn H2n+1COOH, ethylene
gives origin to the acrylic acid series, CnH2n-1COOH, and
soon. Dibasic acids of the paraffin series of hydrocarbons
have the general formula CnH2(COOH)2n; malonic and succinic
acids are important members. The isomerism which occurs as
soon as the molecule contains a few carbon atoms renders any
classification based on empirical molecular formulae somewhat
ineffective; on the other hand, a scheme based on molecular
structure would involve more detail than it is here possible to
give. For further information, the reader is referred to
any standard work on organic chemistry. A list of the acids
present in fats and oils is given in the article OILS.
Syntheses of Organic Acids.---The simplest syntheses are
undoubtedly those in which a carboxyl group is obtained
directly from the oxides of carbon, carbon dioxide and carbon
monoxide. The simplest of all include: (1) the synthesis
of sodium oxalate by passing carbon dioxide over metallic
sodium heated to 350 deg. -360 deg. ; (2) the synthesis of potassium
formate from moist carbon dioxide and potassium, potassium
carbonate being obtained simultaneously; (3) the synthesis
of potassium acetate and propionate from carbon dioxide
and sodium methide and sodium ethide; (4) the synthesis of
aromatic acids by the interaction of carbon dioxide, sodium
and a bromine substitution derivative; and (5) the synthesis
of aromatic oxy-acids by the interaction of carbon dioxide
and sodium phenolates (see SALICYLIC ACID). (Carbon monoxide
takes part in the syntheses of sodium formate from sodium
hydrate, or soda lime (at 200 deg. -220 deg. ), and of sodium acetate
and propionate from sodium methylate and sodium ethylate at
160 deg. --200 deg. . Other reactions which introduce carboxyl groups
into aromatic groups ave: the action of carbonyl chloride on
aromatic hydrocarbons in the presence of aluminium chloride,
acid-chlorides being formed which are readily decomposed
by water to give the acid; the action of urea chloride
