Greek philosophy.
Although chemical actions must have been observed by man in the most
remote times, and also utilized in such processes as the extraction of
metals from their ores and in the arts of tanning and dyeing, there is
no evidence to show that, beyond an unordered accumulation of facts, the
early developments of these industries were attended by any real
knowledge of the nature of the processes involved. All observations were
the result of accident or chance, or possibly in some cases of
experimental trial, but there is no record of a theory or even a general
classification of the phenomena involved, although there is no doubt
that the ancients had a fair knowledge of the properties and uses of the
commoner substances. The origin of chemistry is intimately bound up with
the arts which we have indicated; in this respect it is essentially an
experimental science. A unifying principle of chemical and physical
changes was provided by metaphysical conceptions of the structure of
matter. We find the notion of "elements," or primary qualities, which
confer upon all species of matter their distinctive qualities by
appropriate combination, and also the doctrine that matter is composed
of minute discrete particles, prevailing in the Greek schools. These
"elements," however, had not the significance of the elements of to-day;
they connoted physical appearances or qualities rather than chemical
relations; and the atomic theory of the ancients is a speculation based
upon metaphysical considerations, having, in its origin, nothing in
common with the modern molecular theory, which was based upon
experimentally observed properties of gases (see ELEMENT; MOLECULE).
Alchemy.
Although such hypotheses could contribute nothing directly to the
development of a science which laid especial claim to experimental
investigations, yet indirectly they stimulated inquiry into the nature
of the "essence" with which the four "elements" were associated. This
_quinta essentia_ had been speculated upon by the Greeks, some regarding
it as immaterial or aethereal, and others as material; and a school of
philosophers termed alchemists arose who attempted the isolation of this
essence. The existence of a fundamental principle, unalterable and
indestructible, prevailing alike through physical and chemical changes,
was generally accepted. Any change which a substance may chance to
undergo was simply due to the discarding or taking up of some proportion
of the primary "elements" or qualities: of these coverings "water,"
"air," "earth" and "fire" were regarded as clinging most tenaciously to
the essence, while "cold," "heat," "moistness" and "dryness" were more
easily cast aside or assumed. Several origins have been suggested for
the word alchemy, and there seems to have been some doubt as to the
exact nature and import of the alchemical doctrines. According to M.P.E.
Berthelot, "alchemy rested partly on the industrial processes of the
ancient Egyptians, partly on the speculative theories of the Greek
philosophers, and partly on the mystical reveries of the Gnostics and
Alexandrians." The search for this essence subsequently resolved itself
into the desire to effect the transmutation of metals, more especially
the base metals, into silver and gold. It seems that this secondary
principle became the dominant idea in alchemy, and in this sense the
word is used in Byzantine literature of the 4th century; Suidas, writing
in the 11th century, defines chemistry as the "preparation of silver
and gold" (see ALCHEMY).
From the Alexandrians the science passed to the Arabs, who made
discoveries and improved various methods of separating substances, and
afterwards, from the 11th century, became seated in Europe, where the
alchemical doctrines were assiduously studied until the 15th and 16th
centuries. It is readily understood why men imbued with the authority of
tradition should prosecute the search for a substance which would confer
unlimited wealth upon the fortunate discoverer. Some alchemists honestly
laboured to effect the transmutation and to discover the "philosopher's
stone," and in many cases believed that they had achieved success, if we
may rely upon writings assigned to them. The period, however, is one of
literary forgeries; most of the MSS. are of uncertain date and
authorship, and moreover are often so vague and mystical that they are
of doubtful scientific value, beyond reflecting the tendencies of the
age. The retaining of alchemists at various courts shows the high
opinion which the doctrines had gained. It is really not extraordinary
that Isaac Hollandus was able to indicate the method of the preparation
of the "philosopher's stone" from "adamic" or "virgin" earth, and its
action when medicinally employed; that in the writings assigned to Roger
Bacon, Raimon Lull, Basil Valentine and others are to be found the exact
quantities of it to be used in transmutation; and that George Ripley, in
the 15th century, had grounds for regarding its action as similar to
that of a ferment.
In the view of some alchemists, the ultimate principles of matter were
Aristotle's four elements; the proximate constituents were a "sulphur"
and a "mercury," the father and mother of the metals; gold was supposed
to have attained to the perfection of its nature by passing in
succession through the forms of lead, brass and silver; gold and silver
were held to contain very pure red sulphur and white quicksilver,
whereas in the other metals these materials were coarser and of a
different colour. From an analogy instituted between the healthy human
being and gold, the most perfect of the metals, silver, mercury, copper,
iron, lead and tin, were regarded in the light of lepers that required
to be healed.
Iatrochemistry.
Notwithstanding the false idea which prompted the researches of the
alchemists, many advances were made in descriptive chemistry, the metals
and their salts receiving much attention, and several of our important
acids being discovered. Towards the 16th century the failure of the
alchemists to achieve their cherished purpose, and the general increase
of medical knowledge, caused attention to be given to the utilization of
chemical preparations as medicines. As early as the 15th century the
alchemist Basil Valentine had suggested this application, but the great
exponent of this doctrine was Paracelsus, who set up a new definition:
"The true use of chemistry is not to make gold but to prepare
medicines." This relation of chemistry to medicine prevailed until the
17th century, and what in the history of chemistry is termed the
iatrochemical period (see MEDICINE) was mainly fruitful in increasing
the knowledge of compounds; the contributions to chemical theory are of
little value, the most important controversies ranging over the nature
of the "elements," which were generally akin to those of Aristotle,
modified so as to be more in accord with current observations. At the
same time, however, there were many who, opposed to the Paracelsian
definition of chemistry, still laboured at the problem of the
alchemists, while others gave much attention to the chemical industries.
Metallurgical operations, such as smelting, roasting and refining, were
scientifically investigated, and in some degree explained, by Georg
Agricola and Carlo Biringuiccio; ceramics was studied by Bernard
Palissy, who is also to be remembered as an early worker in agricultural
chemistry, having made experiments on the effect of manures on soils and
crops; while general technical chemistry was enriched by Johann Rudolf
Glauber.[1]
Boyle.
The second half of the 17th century witnessed remarkable transitions and
developments in all branches of natural science, and the facts
accumulated by preceding generations during their generally unordered
researches were replaced by a co-ordination of experiment and deduction.
From the mazy and incoherent alchemical and iatrochemical doctrines, the
former based on false conceptions of matter, the latter on erroneous
views of life processes and physiology, a new science arose--the study
of the composition of substances. The formulation of this definition of
chemistry was due to Robert Boyle. In his _Sceptical Chemist_ (1662) he
freely criticized the prevailing scientific views and methods, with the
object of showing that true knowledge could only be gained by the
logical application of the principles of experiment and deduction.
Boyle's masterly exposition of this method is his most important
contribution to scientific progress. At the same time he clarified the
conception of elements and compounds, rejecting the older notions, the
four elements of the "vulgar Peripateticks" and the three principles of
the "vulgar Stagyrists," and defining an element as a substance
incapable of decomposition, and a compound as composed of two or more
elements. He explained chemical combination on the hypotheses that
matter consisted of minute corpuscles, that by the coalescence of
corpuscles of different substances distinctly new corpuscles of a
compound were formed, and that each corpuscle had a certain affinity for
other corpuscles.
Phlogistic theory.
Although Boyle practised the methods which he expounded, he was unable
to gain general acceptance of his doctrine of elements; and, strangely
enough, the theory which next dominated chemical thought was an
alchemical invention, and lacked the lucidity and perspicuity of Boyle's
views. This theory, named the phlogistic theory, was primarily based
upon certain experiments on combustion and calcination, and in effect
reduced the number of the alchemical principles, while setting up a new
one, a principle of combustibility, named phlogiston (from [Greek:
phlogistos], burnt). Much discussion had centred about fire or the
"igneous principle." On the one hand, it had been held that when a
substance was burned or calcined, it combined with an "air"; on the
other hand, the operation was supposed to be attended by the destruction
or loss of the igneous principle. Georg Ernst Stahl, following in some
measure the views held by Johann Joachim Becher, as, for instance, that
all combustibles contain a "sulphur" (which notion is itself of older
date than Becher's _terra pinguis_), regarded all substances as capable
of resolution into two components, the inflammable principle phlogiston,
and another element--"water," "acid" or "earth." The violence or
completeness of combustion was proportional to the amount of phlogiston
present. Combustion meant the liberation of phlogiston. Metals on
calcination gave calces from which the metals could be recovered by
adding phlogiston, and experiment showed that this could generally be
effected by the action of coal or carbon, which was therefore regarded
as practically pure phlogiston; the other constituent being regarded as
an acid. At the hands of Stahl and his school, the phlogistic theory, by
exhibiting a fundamental similarity between all processes of combustion
and by its remarkable flexibility, came to be a general theory of
chemical action. The objections of the antiphlogistonists, such as the
fact that calces weigh more than the original metals instead of less as
the theory suggests, were answered by postulating that phlogiston was a
principle of levity, or even completely ignored as an accident, the
change of _qualities_ being regarded as the only matter of importance.
It is remarkable that this theory should have gained the esteem of the
notable chemists who flourished in the 18th century. Henry Cavendish, a
careful and accurate experimenter, was a phlogistonist, as were J.
Black, K. W. Scheele, A. S. Marggraf, J. Priestley and many others who
might be mentioned.
Lavoisier.
Descriptive chemistry was now assuming considerable proportions; the
experimental inquiries suggested by Boyle were being assiduously
developed; and a wealth of observations was being accumulated, for the
explanation of which the resources of the dominant theory were sorely
taxed. To quote Antoine Laurent Lavoisier, "... chemists have turned
phlogiston into a vague principle, ... which consequently adapts itself
to all the explanations for which it may be required. Sometimes this
principle has weight, and sometimes it has not; sometimes it is free
fire and sometimes it is fire combined with the earthy element;
sometimes it passes through the pores of vessels, sometimes these are
impervious to it; it explains both causticity and non-causticity,
transparency and opacity, colours and their absence; it is a veritable
Proteus changing in form at each instant." Lavoisier may be justly
regarded as the founder of modern or quantitative chemistry. First and
foremost, he demanded that the balance must be used in all
investigations into chemical changes. He established as fundamental that
combustion and calcination were attended by an increase of weight, and
concluded, as did Jean Rey and John Mayow in the 17th century, that the
increase was due to the combination of the metal with the air. The
problem could obviously be completely solved only when the composition
of the air, and the parts played by its components, had been determined.
At all times the air had received attention, especially since van
Helmont made his far-reaching investigations on gases. Mayow had
suggested the existence of two components, a _spiritus nitroaerus_ which
supported combustion, and a _spiritus nitri acidi_ which extinguished
fire; J. Priestley and K. W. Scheele, although they isolated oxygen,
were fogged by the phlogistic tenets; and H. Cavendish, who had isolated
the nitrogen of the atmosphere, had failed to decide conclusively what
had really happened to the air which disappeared during combustion.
Lavoisier adequately recognized and acknowledged how much he owed to the
researches of others; to himself is due the co-ordination of these
researches, and the welding of his results into a doctrine to which the
phlogistic theory ultimately succumbed. He burned phosphorus in air
standing over mercury, and showed that (1) there was a limit to the
amount of phosphorus which could be burned in the confined air, (2) that
when no more phosphorus could be burned, one-fifth of the air had
disappeared, (3) that the weight of the air lost was nearly equal to the
difference in the weights of the white solid produced and the phosphorus
burned, (4) that the density of the residual air was less than that of
ordinary air. The same results were obtained with lead and tin; and a
more elaborate repetition indubitably established their correctness. He
also showed that on heating mercury calx alone an "air" was liberated
which differed from other "airs," and was slightly heavier than ordinary
air; moreover, the weight of the "air" set free from a given weight of
the calx was equal to the weight taken up in forming the calx from
mercury, and if the calx be heated with charcoal, the metal was
recovered and a gas named "fixed air," the modern carbon dioxide, was
formed. The former experiment had been performed by Scheele and
Priestley, who had named the gas "phlogisticated air"; Lavoisier
subsequently named it oxygen, regarding it as the "acid producer"
([Greek: oxys], sour). The theory advocated by Lavoisier came to
displace the phlogistic conception; but at first its acceptance was
slow. Chemical literature was full of the phlogistic modes of
expression--oxygen was "dephlogisticated air," nitrogen "phlogisticated
air," &c.--and this tended to retard its promotion. Yet really the
transition from the one theory to the other was simple, it being only
necessary to change the "addition or loss of phlogiston" into the "loss
or addition of oxygen." By his insistence upon the use of the balance as
a quantitative check upon the masses involved in all chemical reactions,
Lavoisier was enabled to establish by his own investigations and the
results achieved by others the principle now known as the "conservation
of mass." Matter can neither be created nor destroyed; however a
chemical system be changed, the weights before and after are equal.[2]
To him is also due a rigorous examination of the nature of elements and
compounds; he held the same views that were laid down by Boyle, and with
the same prophetic foresight predicted that some of the elements which
he himself accepted might be eventually found to be compounds.
Chemical Affinity.
It is unnecessary in this place to recapitulate the many results which
had accumulated by the end of the 18th century, or to discuss the
labours and theories of individual workers since these receive attention
under biographical headings; in this article only the salient features
in the history of our science can be treated. The beginning of the 19th
century was attended by far-reaching discoveries in the nature of the
composition of compounds. Investigations proceeded in two
directions:--(1) the nature of chemical affinity, (2) the laws of
chemical combination. The first question has not yet been solved,
although it has been speculated upon from the earliest times. The
alchemists explained chemical action by means of such phrases as "like
attracts like," substances being said to combine when one "loved" the
other, and the reverse when it "hated" it. Boyle rejected this
terminology, which was only strictly applicable to intelligent beings;
and he used the word "affinity" as had been previously done by Stahl and
others. The modern sense of the word, viz. the force which holds
chemically dissimilar substances together (and also _similar_ substances
as is seen in di-, tri-, and poly-atomic molecules), was introduced by
Hermann Boerhaave, and made more precise by Sir Isaac Newton. The laws
of chemical combination were solved, in a measure, by John Dalton, and
the solution expressed as Dalton's "atomic theory." Lavoisier appears to
have assumed that the composition of every chemical compound was
constant, and the same opinion was the basis of much experimental
inquiry at the hands of Joseph Louis Proust during 1801 to 1809, who
vigorously combated the doctrine of Claude Louis Berthollet (_Essai de
statique chimique_, 1803), viz. that fixed proportions of elements and
compounds combine only under exceptional conditions, the general rule
being that the composition of a compound may vary continuously between
certain limits.[3]
Dalton.
This controversy was unfinished when Dalton published the first part of
his _New System of Chemical Philosophy_ in 1808, although the _per
saltum_ theory was the most popular. Led thereto by speculations on
gases, Dalton assumed that matter was composed of atoms, that in the
elements the atoms were simple, and in compounds complex, being composed
of elementary atoms. Dalton furthermore perceived that the same two
elements or substances may combine in different proportions, and showed
that these proportions had always a simple ratio to one another. This is
the "law of multiple proportions." He laid down the following arbitrary
rules for determining the number of atoms in a compound:--if only one
compound of two elements exists, it is a binary compound and its atom is
composed of one atom of each element; if two compounds exist one is
binary (say A + B) and the other ternary (say A + 2B); if three, then
one is binary and the others may be ternary (A + 2B, and 2A + B), and so
on. More important is his deduction of equivalent weights, i.e. the
relative weights of atoms. He took hydrogen, the lightest substance
known, to be the standard. From analyses of water, which he regarded as
composed of one atom of hydrogen and one of oxygen, he deduced the
relative weight of the oxygen atom to be 6.5; from marsh gas and
olefiant gas he deduced carbon = 5, there being one atom of carbon and
two of hydrogen in the former and one atom of hydrogen to one of carbon
in the latter; nitrogen had an equivalent of 5, and so on.[4]
The value of Dalton's generalizations can hardly be overestimated,
notwithstanding the fact that in several cases they needed correction.
The first step in this direction was effected by the co-ordination of
Gay Lussac's observations on the combining volumes of gases. He
discovered that gases always combined in volumes having simple ratios,
and that the volume of the product had a simple ratio to the volumes of
the reacting gases. For example, one volume of oxygen combined with two
of hydrogen to form two volumes of steam, three volumes of hydrogen
combined with one of nitrogen to give two volumes of ammonia, one volume
of hydrogen combined with one of chlorine to give two volumes of
hydrochloric acid. An immediate inference was that the Daltonian "atom"
must have parts which enter into combination with parts of other atoms;
in other words, there must exist two orders of particles, viz. (1)
particles derived by limiting _mechanical_ subdivision, the modern
_molecule_, and (2) particles derived from the first class by _chemical_
subdivision, i.e. particles which are incapable of existing alone, but
may exist in combination. Additional evidence as to the structure of the
molecule was discussed by Avogadro in 1811, and by Ampere in 1814. From
the gas-laws of Boyle and J.A.C. Charles--viz. equal changes in
temperature and pressure occasion equal changes in equal volumes of all
gases and vapours--Avogadro deduced the law:--Under the same conditions
of temperature and pressure, equal volumes of gases contain equal
numbers of molecules; and he showed that the relative weights of the
molecules are determined as the ratios of the weights of equal volumes,
or densities. He established the existence of molecules and atoms as we
have defined above, and stated that the number of atoms in the molecule
is generally 2, but may be 4, 8, &c. We cannot tell whether his choice
of the powers of 2 is accident or design.
Berzelius.
Notwithstanding Avogadro's perspicuous investigation, and a similar
exposition of the atom and molecule by A. M. Ampere, the views therein
expressed were ignored both by their own and the succeeding generation.
In place of the relative molecular weights, attention was concentrated
on relative atomic or equivalent weights. This may be due in some
measure to the small number of gaseous and easily volatile substances
then known, to the attention which the study of the organic compounds
received, and especially to the energetic investigations of J. J.
Berzelius, who, fired with enthusiasm by the original theory of Dalton
and the law of multiple proportions, determined the equivalents of
combining ratios of many elements in an enormous number of compounds.[5]
He prosecuted his labours in this field for thirty years; as proof of
his industry it may be mentioned that as early as 1818 he had determined
the combining ratios of about two thousand simple and compound
substances.
Chemical notation.
We may here notice the important chemical symbolism or notation
introduced by Berzelius, which greatly contributed to the definite and
convenient representation of chemical composition and the tracing of
chemical reactions. The denotation of elements by symbols had been
practised by the alchemists, and it is interesting to note that the
symbols allotted to the well-known elements are identical with the
astrological symbols of the sun and the other members of the solar
system. Gold, the most perfect metal, had the symbol of the Sun, [*];
silver, the semiperfect metal, had the symbol of the Moon, [*];
copper, iron and antimony, the imperfect metals of the gold class, had
the symbols of Venus [*], Mars [*], and the Earth [*]; tin and lead,
the imperfect metals of the silver class, had the symbols of Jupiter
[*], and Saturn [*]; while mercury, the imperfect metal of both the
gold and silver class, had the symbol of the planet, [*]. Torbern Olof
Bergman used an elaborate system in his _Opuscula physica et chemica_
(1783); the elements received symbols composed of circles, arcs of
circles, and lines, while certain class symbols, such as [*] for
metals, [*] for acids, [*] for alkalies, [*] for salts, [*] for
calces, &c., were used. Compounds were represented by copulating
simpler symbols, e.g. mercury calx was [*][*].[6] Bergman's symbolism
was obviously cumbrous, and the system used in 1782 by Lavoisier was
equally abstruse, since the forms gave no clue as to composition; for
instance water, oxygen, and nitric acid were [*], [*], and [*].
A partial clarification was suggested in 1787 by J.H. Hassenfratz and
Adet, who assigned to each element a symbol, and to each compound a
sign which should record the elements present and their relative
quantities. Straight lines and semicircles were utilized for the
non-metallic elements, carbon, nitrogen, phosphorus and sulphur (the
"simple acidifiable bases" of Lavoisier), and circles enclosing the
initial letters of their names for the metals. The "compound
acidifiable bases," i.e. the hypothetical radicals of acids, were
denoted by squares enclosing the initial letter of the base; an alkali
was denoted by a triangle, and the particular alkali by inserting the
initial letter. Compounds were denoted by joining the symbols of the
components, and by varying the manner of joining compounds of the same
elements were distinguished. The symbol \/ was used to denote a
liquid, and a vertical line to denote a gas. As an example of the
complexity of this system we may note the five oxides of nitrogen,
which were symbolized as
[*], [*], [*], [*], and [*],
the first three representing the gaseous oxides, and the last two the
liquid oxides.
A great advance was made by Dalton, who, besides introducing simpler
symbols, regarded the symbol as representing not only the element or
compound but also one atom of that element or compound; in other
words, his symbol denoted equivalent weights.[7] This system, which
permitted the correct representation of molecular composition, was
adopted by Berzelius in 1814, who, having replaced the geometric signs
of Dalton by the initial letter (or letters) of the Latin names of the
elements, represented a compound by placing a _plus_ sign between the
symbols of its components, and the number of atoms of each component
(except in the case of only one atom) by placing Arabic numerals
before the symbols; for example, copper oxide was Cu+O, sulphur
trioxide S+3O. If two compounds combined, the + signs of the free
compounds were discarded, and the number of atoms denoted by an Arabic
index placed after the elements, and from these modified symbols the
symbol of the new compound was derived in the same manner as simple
compounds were built up from their elements. Thus copper sulphate was
CuO + SO^3, potassium sulphate 2SO^3 + PoO^2 (the symbol Po for
potassium was subsequently discarded in favour of K from _kalium_). At
a later date Berzelius denoted an oxide by dots, equal in number to
the number of oxygen atoms present, placed over the element; this
notation survived longest in mineralogy. He also introduced barred
symbols, i.e. letters traversed by a horizontal bar, to denote the
double atom (or molecule). Although the system of Berzelius has been
modified and extended, its principles survive in the modern notation.
Extension of the atomic theory.
In the development of the atomic theory and the deduction of the atomic
weights of elements and the formulae of compounds, Dalton's arbitrary
rules failed to find complete acceptance. Berzelius objected to the
hypothesis that if two elements form only one compound, then the atoms
combine one and one; and although he agreed with the adoption of simple
rules as a first attempt at representing a compound, he availed himself
of other data in order to gain further information as to the structure
of compounds. For example, at first he represented ferrous and ferric
oxides by the formulae FeO2, FeO3, and by the analogy of zinc and other
basic oxides he regarded these substances as constituted similarly to
FeO2, and the acidic oxides alumina and chromium oxide as similar to
FeO3. He found, however, that chromic acid, which he had represented as
CrO6, neutralized a base containing 1/3 the quantity of oxygen. He
inferred that chromic acid must contain only three atoms of oxygen, as
did sulphuric acid SO3; consequently chromic oxide, which contains half
the amount of oxygen, must be Cr2O3, and hence ferric oxide must be
Fe2O3. The basic oxides must have the general formula MO. To these
results he was aided by the law of isomorphism formulated by E.
Mitscherlich in 1820; and he confirmed his conclusions by showing the
agreement with the law of atomic heat formulated by Dulong and Petit in
1819.
While successfully investigating the solid elements and their compounds
gravimetrically, Berzelius was guilty of several inconsistencies in his
views on gases. He denied that gaseous atoms could have parts, although
compound gases could. This attitude was due to his adherence to the
"dualistic theory" of the structure of substances, which he deduced from
electrochemical researches. From the behaviour of substances on
electrolysis (q.v.) he assumed that all substances had two components,
one bearing a negative charge, the other a positive charge. Combination
was associated with the coalescence of these charges, and the nature of
the resulting compound showed the nature of the residual electricity.
For example, positive iron combined with negative oxygen to form
positive ferrous oxide; positive sulphur combined with negative oxygen
to form negative sulphuric acid; positive ferrous oxide combined with
negative sulphuric acid to form neutral ferrous sulphate. Berzelius
elevated this theory to an important position in the history of our
science. He recognized that if an elementary atom had parts, his theory
demanded that these parts should carry different electric charges when
they entered into reaction, and the products of the reaction should vary
according as a positive or negative atom entered into combination. For
instance if the reaction 2H2 + O2 = H2O + H2O be true, the molecules of
water should be different, for a negative oxygen atom would combine in
one case, and a positive oxygen atom in the other. Hence the gaseous
atoms of hydrogen and oxygen could not have parts. A second
inconsistency was presented when he was compelled by the researches of
Dumas to admit Avogadro's hypothesis; but here he would only accept it
for the elementary gases, and denied it for other substances. It is to
be noticed that J.B. Dumas did not adopt the best methods for
emphasizing his discoveries. His terminology was vague and provoked
caustic criticism from Berzelius; he assumed that all molecules
contained two atoms, and consequently the atomic weights deduced from
vapour density determinations of sulphur, mercury, arsenic, and
phosphorus were quite different from those established by gravimetric
and other methods.
Chemists gradually tired of the notion of atomic weights on account of
the uncertainty which surrounded them; and the suggestion made by W.H.
Wollaston as early as 1814 to deal only with "equivalents," i.e. the
amount of an element which can combine with or replace unit weight of
hydrogen, came into favour, being adopted by L. Gmelin in his famous
text-book.
Atomic and molecular weights.
Simultaneously with this discussion of the atom and molecule, great
controversy was ranging over the constitution of compounds, more
particularly over the carbon or organic compounds. This subject is
discussed in section IV., _Organic Chemistry_.The gradual accumulation
of data referring to organic compounds brought in its train a revival of
the discussion of atoms and molecules. A. Laurent and C.F. Gerhardt
attempted a solution by investigating chemical reactions. They assumed
the atom to be the smallest part of matter which can exist in
combination, and the molecule to be the smallest part which can enter
into a chemical reaction. Gerhardt found that reactions could be best
followed if one assumed the molecular weight of an element or compound
to be that weight which occupied the same volume as two unit weights of
hydrogen, and this assumption led him to double the equivalents accepted
by Gmelin, making H = 1, O = 16, and C = 12, thereby agreeing with
Berzelius, and also to halve the values given by Berzelius to many
metals. Laurent generally agreed, except when the theory compelled the
adoption of formulae containing fractions of atoms; in such cases he
regarded the molecular weight as the weight occupying a volume equal to
four unit weights of hydrogen. The bases upon which Gerhardt and Laurent
founded their views were not sufficiently well grounded to lead to the
acceptance of their results; Gerhardt himself returned to Gmelin's
equivalents in his _Lehrbuch der Chemie_ (1853) as they were in such
general use.
In 1860 there prevailed such a confusion of hypotheses as to the atom
and molecule that a conference was held at Karlsruhe to discuss the
situation. At the conclusion of the sitting, Lothar Meyer obtained a
paper written by Stanislas Cannizzaro in 1858 wherein was found the
final link required for the determination of atomic weights. This link
was the full extension of Avogadro's theory to all substances,
Cannizzaro showing that chemical reactions in themselves would not
suffice. He chose as his unit of reference the weight of an atom of
hydrogen, i.e. the weight contained in a molecule of hydrochloric acid,
thus differing from Avogadro who chose the weight of a hydrogen
molecule. From a study of the free elements Cannizzaro showed that an
element may have more than one molecular weight; for example, the
molecular weight of sulphur varied with the temperature. And from the
study of compounds he showed that each element occurred in a definite
weight or in some multiple of this weight. He called this proportion the
"atom," since it invariably enters compounds without division, and the
weight of this atom is the atomic weight. This generalization was of
great value inasmuch as it permitted the deduction of the atomic weight
of a non-gasifiable element from a study of the densities of its
gasifiable compounds.
Valency.
From the results obtained by Laurent and Gerhardt and their predecessors
it immediately followed that, while an element could have but one atomic
weight, it could have several equivalent weights. From a detailed study
of organic compounds Gerhardt had promulgated a "theory of types" which
represented a fusion of the older radical and type theories. This theory
brought together, as it were, the most varied compounds, and stimulated
inquiry into many fields. According to this theory, an element in a
compound had a definite saturation capacity, an idea very old in itself,
being framed in the law of multiple proportions. These saturation
capacities were assiduously studied by Sir Edward Frankland, who from
the investigation, not of simple inorganic compounds, but of the
organo-metallic derivatives, determined the kernel of the theory of
valency. Frankland showed that any particular element preferentially
combined with a definite number (which might vary between certain
limits) of other atoms; for example, some atoms always combined with one
atom of oxygen, some with two, while with others two atoms entered into
combination with one of oxygen. If an element or radical combined with
one atom of hydrogen, it was termed monovalent; if with two (or with one
atom of oxygen, which is equivalent to two atoms of hydrogen) it was
divalent, and so on. The same views were expressed by Cannizzaro, and
also by A.W. von Hofmann, who materially helped the acceptance of the
doctrine by the lucid exposition in his _Introduction to Modern
Chemistry_, 1865.
The recognition of the quadrivalency of carbon by A. Kekule was the
forerunner of his celebrated benzene theory in particular, and of the
universal application of structural formulae to the representation of
the most complex organic compounds equally lucidly as the representation
of the simplest salts. Alexander Butlerow named the "structure theory,"
and contributed much to the development of the subject. He defined
structure "as the manner of the mutual linking of the atoms in the
molecule," but denied that any such structure could give information as
to the orientation of the atoms in space. He regarded the chemical
properties of a substance as due to (1) the chemical atoms composing it,
and (2) the structure, and he asserted that while different compounds
might have the same components (isomerism), yet only one compound could
have a particular structure. Identity in properties necessitated
identity in structure.
While the principle of varying valency laid down by Frankland is still
retained, Butlerow's view that structure had no spatial significance has
been modified. The researches of L. Pasteur, J.A. Le Bel, J.
Wislicenus, van't Hoff and others showed that substances having the same
graphic formulae vary in properties and reactions, and consequently the
formulae need modification in order to exhibit these differences. Such
isomerism, named stereoisomerism (q.v.), has been assiduously developed
during recent years; it prevails among many different classes of organic
compounds and many examples have been found in inorganic chemistry.
Periodic law.
The theory of valency as a means of showing similarity of properties and
relative composition became a dominant feature of chemical theory, the
older hypotheses of types, radicals, &c. being more or less discarded.
We have seen how its utilization in the "structure theory" permitted
great clarification, and attempts were not wanting for the deduction of
analogies or a periodicity between elements. Frankland had recognized
the analogies existing between the chemical properties of nitrogen,
phosphorus, arsenic and antimony, noting that they act as tri- or
penta-valent. Carbon was joined with silicon, zirconium and titanium,
while boron, being trivalent, was relegated to another group. A general
classification of elements, however, was not realized by Frankland, nor
even by Odling, who had also investigated the question from the valency
standpoint. The solution came about by arranging the elements in the
order of their atomic weights, tempering the arrangement with the
results deduced from the theory of valencies and experimental
observations. Many chemists contributed to the establishment of such a
periodicity, the greatest advances being made by John Newlands in
England, Lothar Meyer in Germany, and D.J. Mendeleeff in St Petersburg.
For the development of this classification see ELEMENT.
Summary.
In the above sketch we have briefly treated the history of the main
tendencies of our science from the earliest times to the establishment
of the modern laws and principles. We have seen that the science took
its origin in the arts practised by the Egyptians, and, having come
under the influence of philosophers, it chose for its purpose the
isolation of the _quinta essentia_, and subsequently the "art of making
gold and silver." This spirit gave way to the physicians, who regarded
"chemistry as the art of preparing medicines," a denotation which in
turn succumbed to the arguments of Boyle, who regarded it as the
"science of the composition of substances," a definition which
adequately fits the science to-day. We have seen how his classification
of substances into elements and compounds, and the definitions which he
assigned to these species, have similarly been retained; and how
Lavoisier established the law of the "conservation of mass," overthrew
the prevailing phlogistic theory, and became the founder of modern
chemistry by the overwhelming importance which he gave to the use of the
balance. The development of the atomic theory and its concomitants--the
laws of chemical combination and the notion of atoms and equivalents--at
the hands of Dalton and Berzelius, the extension to the modern theory of
the atom and molecule, and to atomic and molecular weights by Avogadro,
Ampere, Dumas, Laurent, Gerhardt, Cannizzaro and others, have been
noted. The structure of the molecule, which mainly followed
investigations in organic compounds, Frankland's conception of valency,
and finally the periodic law, have also been shown in their
chronological order. The principles outlined above constitute the
foundations of our science; and although it may happen that experiments
may be made with which they appear to be not in complete agreement, yet
in general they constitute a body of working hypotheses of inestimable
value.
_Chemical Education._--It is remarkable that systematic instruction in
the theory and practice of chemistry only received earnest attention in
our academic institutions during the opening decades of the 19th
century. Although for a long time lecturers and professors had been
attached to universities, generally their duties had also included the
study of physics, mineralogy and other subjects, with the result that
chemistry received scanty encouragement. Of practical instruction there
was none other than that to be gained in a few private laboratories and
in the shops of apothecaries. The necessity for experimental
demonstration and practical instruction, in addition to academic
lectures, appears to have been urged by the French chemists L.N.
Vauquelin, Gay Lussac, Thenard, and more especially by A.F. Fourcroy and
G.F. Rouelle, while in England Humphry Davy expounded the same idea in
the experimental demonstrations which gave his lectures their brilliant
charm. But the real founder of systematic instruction in our science was
Justus von Liebig, who, having accepted the professorship at Giessen in
1824, made his chemical laboratory and course of instruction the model
of all others. He emphasized that the practical training should include
(1) the qualitative and quantitative analysis of mixtures, (2) the
preparation of substances according to established methods, (3) original
research--a course which has been generally adopted. The pattern set by
Liebig at Giessen was adopted by F. Wohler at Gottingen in 1836, by R.W.
Bunsen at Marburg in 1840, and by O.L. Erdmann at Leipzig in 1843; and
during the 'fifties and 'sixties many other laboratories were founded. A
new era followed the erection of the laboratories at Bonn and Berlin
according to the plans of A.W. von Hofmann in 1867, and of that at
Leipzig, designed by Kolbe in 1868. We may also mention the famous
laboratory at Munich designed by A. von Baeyer in 1875.
In Great Britain the first public laboratory appears to have been opened
in 1817 by Thomas Thomson at Glasgow. But the first important step in
providing means whereby students could systematically study chemistry
was the foundation of the College of Chemistry in 1845. This institution
was taken over by the Government in 1853, becoming the Royal College of
Chemistry, and incorporated with the Royal School of Mines; in 1881 the
names were changed to the Normal School of Science and Royal School of
Mines, and again in 1890 to the Royal College of Science. In 1907 it was
incorporated in the Imperial College of Science and Technology. Under
A.W. von Hofmann, who designed the laboratories and accepted the
professorship in 1845 at the instigation of Prince Albert, and under his
successor (in 1864) Sir Edward Frankland, this institution became one of
the most important centres of chemical instruction. Oxford and Cambridge
sadly neglected the erection of convenient laboratories for many years,
and consequently we find technical schools and other universities having
a far better equipment and offering greater facilities. In the provinces
Victoria University at Manchester exercised the greater impetus,
numbering among its professors Sir W.H. Perkin and Sir Henry Roscoe.
In America public laboratory instruction was first instituted at Yale
College during the professorship of Benjamin Silliman. To the great
progress made in recent years F.W. Clarke, W. Gibbs, E.W. Morley, Ira
Remsen, and T.W. Richards have especially contributed.
In France the subject was almost entirely neglected until late in the
19th century. The few laboratories existing in the opening decades were
ill-fitted, and the exorbitant fees constituted a serious bar to general
instruction, for these institutions received little government support.
In 1869 A. Wurtz reported the existence of only _one_ efficient
laboratory in France, namely the Ecole Normale Superieure, under the
direction of H. Sainte Claire Deville. During recent years chemistry has
become one of the most important subjects in the curriculum of technical
schools and universities, and at the present time no general educational
institution is complete until it has its full equipment of laboratories
and lecture theatres.
_Chemical Literature_.--The growth of chemical literature since the
publication of Lavoisier's famous _Traite de chimie_ in 1789, and of
Berzelius' _Lehrbuch der Chemie_ in 1808-1818, has been enormous.
These two works, and especially the latter, were the models followed
by Thenard, Liebig, Strecker, Wohler and many others, including Thomas
Graham, upon whose _Elements of Chemistry_ was founded Otto's famous
_Lehrbuch der Chemie_, to which H. Kopp contributed the general
theoretical part, Kolbe the organic, and Buff and Zamminer the
physico-chemical. Organic chemistry was especially developed by the
publication of Gerhardt's _Traite de chimie organique_ in 1853-1856,
and of Kekule's _Lehrbuch der organischen Chemie_ in 1861-1882.
General theoretical and physical chemistry was treated with
conspicuous acumen by Lothar Meyer in his _Moderne Theorien_, by W.
Ostwald in his _Lehrbuch der allgem. Chemie_ (1884-1887), and by
Nernst in his _Theoretische Chemie_. In English, Roscoe and
Schorlemmer's _Treatise on Chemistry_ is a standard work; it records
a successful attempt to state the theories and facts of chemistry,
not in condensed epitomes, but in an easily read form. The _Traite de
chimie minerale_, edited by H. Moissan, and the _Handbuch der
anorganischen Chemie_, edited by Abegg, are of the same type. O.
Dammer's _Handbuch der anorganischen Chemie_ and F. Beilstein's
_Handbuch der organischen Chemie_ are invaluable works of reference.
Of the earlier encyclopaedias we may notice the famous _Handworterbuch
der reinen und angewandten Chemie_, edited by Liebig; Fremy's
_Encyclopedie de chimie_, Wurtz's _Dictionnaire de chimie pure et
appliquee_, Watts' _Dictionary of Chemistry_, and Ladenburg's
_Handworterbuch der Chemie_.
The number of periodicals devoted to chemistry has steadily increased
since the early part of the 19th century. In England the most
important is the _Journal of the Chemical Society of London_, first
published in 1848. Since 1871 abstracts of papers appearing in the
other journals have been printed. In 1904 a new departure was made in
issuing _Annual Reports_, containing resumes of the most important
researches of the year. The _Chemical News_, founded by Sir W. Crookes
in 1860, may also be noted. In America the chief periodical is the
_American Chemical Journal_, founded in 1879. Germany is provided with
a great number of magazines. The _Berichte der deutschen chemischen
Gesellschaft_, published by the Berlin Chemical Society, the
_Chemisches Centralblatt_, which is confined to abstracts of papers
appearing in other journals, the _Zeitschrift fur Chemie_, and
Liebig's _Annalen der Chemie_ are the most important of the general
magazines. Others devoted to special phases are the _Journal fur
praktische Chemie_, founded by Erdmann in 1834, the _Zeitschrift fur
anorganische Chemie_ and the _Zeitschrift fur physikalische Chemie_.
Mention may also be made of the invaluable _Jahresberichte_ and the
_Jahrbuch der Chemie_. In France, the most important journals are the
_Annales de chimie et de physique_, founded in 1789 with the title
_Annales de chimie_, and the _Comptes rendus_, published weekly by the
Academie francaise since 1835.