CHEMISTRY WORLD (history,proposed law, scientist that propose and modify,particles’ )

Van Helmont: 1648
A book is published in Amsterdam in 1648 which
can be seen as a definitive turning point between
alchemy and chemistry. Entitled Ortus Medicinae
(Origin of Medicine), it is the collected papers of
Jan Baptista van Helmont, an aristocrat who has
lived quietly on his estate near Brussels
conducting scientific experiments.
Van Helmont is inclined to mysticism. He
believes in alchemy and in the philosopher’s
stone which, if found, could turn base metals into
gold. But he also conducts experiments on
entirely scientific principles. Some, like his
famous five-year project with a willow tree, lead
him to the wrong conclusion. But the method is
valid.
Van Helmont weighs out 200 lbs of dried earth,
places it in an earthenware container and plants
a willow tree weighing 5 lbs. For five years he
waters the plant daily. At the end of the
experiment the willow tree weighs 169lbs and
the earth, when dried, not much less than 200
lbs. Van Helmont concludes, reasonably that the
wood, bark and leaves of the tree must be
composed of water, which he therefore considers
to be the chief constituent of all matter.
He is half right – any willow tree is about 50%
water. What van Helmont is unaware of is that
the tree has also absorbed carbon and oxygen,
as carbon dioxide or CO2, from the air.
Ironically, Van Helmont himself becomes the first
scientist to postulate the existence of carbon
dioxide. He burns 62 lbs of charcoal and finds
that he is left with only 1 lb of ash. What has
happened to the rest? Van Helmont is convinced,
ahead of his time, of the indestructibility of
matter. Indeed he is able to demonstrate that
metal dissolved in acid can be recovered without
loss of weight.
So he now reasons that the missing 61 lbs have
escaped in the form of an airy substance to
which he gives the name gas sylvestre (wood
gas).
The identity of this wood gas is not discovered
until a century later (by Joseph Black ), but van
Helmont is the first to have suggested the
existence of gaseous substances other than air.
It is he who coins the word ‘gas’ – deriving it
from Chaos (sounding similar in Flemish), which
is used in Greek mythology to mean the original
emptiness before creation.
The principles of experiment enter chemistry in
the work of van Helmont, and are developed by
another aristocrat fascinated by the puzzles of
science – Robert Boyle.
Robert Boyle: 1661-1666
The experimental methods of modern science
are considerably advanced by the work of Robert
Boyle during the 1660s. He is skilful at devising
experiments to test theories, though an early
success is merely a matter of using von
Guericke ‘s air pump to create a vacuum in which
he can observe the behaviour of falling bodies.
He is able to demonstrate the truth of Galileo ‘s
proposition that all objects will fall at the same
speed in a vacuum.
But Boyle also uses the air pump to make
significant discoveries of his own – most notably
that reduction in pressure reduces the boiling
temperature of a liquid (water boils at 100° at
normal air pressure, but at only 46°C if the
pressure is reduced to one tenth).
Boyle’s best-known experiment involves a U-
shaped glass tube open at one end. Air is
trapped in the closed end by a column of
mercury. Boyle can show that if the weight of
mercury is doubled, the volume of air is halved.
The conclusion is the principle known still in
Britain and the USA as Boyle’s Law – that
pressure and volume are inversely proportional
for a fixed mass of gas at a constant
temperature.
Boyle’s most famous work has a title perfectly
expressing a correct scientific attitude. The
Sceptical Chymist appears in 1661. Boyle is
properly sceptical about contemporary theories
on the nature of matter, which still derive mainly
from the Greek theory of four elements .
His own notions are much closer to the truth.
Indeed it is he who introduces the concept of the
element in its modern sense, suggesting that
such entities are ‘primitive and simple, or
perfectly unmingled bodies’. Elements, as he
imagines them, are ‘corpuscles’ of different sorts
and sizes which arrange themselves into
compounds – the chemical substances familiar to
our senses. Compounds, he argues, can be
broken down into their constituent elements.
Boyle’s ideas in this field are further developed
in his Origin of Forms and Qualities (1666).
Chemistry is Boyle’s prime interest, but he also
makes intelligent contributions in the field of pure
physics.
In an important work of 1663, Experiments and
Considerations Touching Colours, Boyle argues
that colours have no intrinsic identity but are
modifications in light reflected from different
surfaces. (This is demonstrated within a few
years by Newton in his work on the spectrum.)
As a man of his time, Boyle is as much
interested in theology as science. It comes as a
shock to read his requirements for the annual
Boyle lecture which he founds in his will. Instead
of discussing science, the lecturers are to prove
the truth of Christianity against ‘notorious
infidels, viz., atheists, theists, pagans, Jews and
Mahommedans’. The rules specifically forbid any
mention of disagreement among Christian sects.
The phlogiston theory: 18th century
Two natural processes, burning and rusting,
particularly intrigue the chemists of the 17th and
18th centuries. A concept is put forward in 1667
in Germany in a book by Johann Joachim
Becher. explaining such changes as the release
of a particular substance, present in all materials
which are capable of changes of this kind. The
theory was developed by George Ernst Stahl,
who in a 1702 edition of Becher’s work gave the
mystery substance the name phlogiston – from
the Greek phlogizein, to set alight.
Stahl is correct in his link between burning and
rusting, for each depends on oxidization (a
chemical reaction with oxygen). But experimental
evidence immediately provides a stumbling block
for the phlogiston theory.
If phlogiston is a substance released both in
burning and rusting, then the resulting ash and
rust should weigh either the same as or less than
the weight lost by the original object (there is
much debate as to the weight or weightlessness
of phlogiston). But experiments reveal that
oxygen-rich rust is heavier than unrusted iron,
while ash is much lighter than the burnt organic
material. Yet this difficulty is not enough to
prevent most scientists believing in the existence
of phlogiston, until Lavoisier and the discovery of
oxygen finally disprove the case.
Demons in the ore: 1742-1751
From the mid-18th century there is rapid
acceleration in the discovery of new elements,
as chemists improve their analytical methods in
the laboratory. These substances are not at first
recognized as elements (a concept only firmly
established in the 19th century), but in each case
it is evident that a previously unidentified
material has been isolated.
Two of the earliest in this series of discoveries
take place in Sweden. Both involve the analysis
of familiar metallic ores, and both acquire their
lasting names from the superstitions of German
miners.
Miners in the Harz mountains have often been
frustrated by a substance which appears to be
copper ore but which, when heated, yields none
of the expected metal. Even worse, it emits
noxious fumes. The miners blame this on the
influence of a spirit, the mischievous kobold, and
the name becomes attached to this kind of ore.
The only use found for the residue of such ore
after roasting is in the making of glass, to which
it adds a beautiful blue colour. In about 1735
Georg Brandt is able to show in his Swedish
laboratory that the blue derives from a previously
unknown metal. The mischievous spirit has been
identified, and Brandt gives its name to the new
substance – as cobalt.
A similar demon is blamed by miners in Saxony
for another ore which yields a brittle substance
instead of copper. In German a Nickel is a
dwarfish troublemaker, and the miners call the
disappointing ore Kupfernickel (copper scamp).
The impurity in ore of this type is analyzed in
Sweden in 1751 by Axel Cronstedt. He identifies
its components as arsenic and a previously
unknown hard white metal, quite distinct from
copper. Following the example of Brandt, he
honours the offending demon in the naming of
the new substance and calls it nickel.
Several other new metallic elements are isolated
in the following decades. But the main focus of
research moves now to the gases.
Joseph Black and fixed air: 1754-1756
Joseph Black presents his doctoral thesis to the
university of Edinburgh in 1754 and publishes it in
expanded form two years later as Experiments
upon Magnesia Alba, Quicklime, and Some Other
Alcaline Substances . The experiments which he
describes are a classically complete series of
compound transformations of calcium, carbon
and oxygen – though it is not as yet possible to
express his results in these terms.
Black has observed that if he heats chalk
(calcium carbonate), he gets quicklime (calcium
oxide) and a gas, the presence of which he can
identify by its weight. Unwilling as yet to
speculate on its identity, he calls it fixed air –
because it exists in solid form until released.
As a next stage, Black demonstrates that he can
reverse the process. Mixing water with the
quicklime, he gets a substance (slaked lime)
whch will take up the fixed air again – leaving
him with his original amount of chalk and the
water.
In other experiments Black is able to show that
this same unknown gas, his fixed air, is
produced as a result of burning charcoal, of
fermentation and of breathing. He demonstrates
this last point to his students by breathing
through a tube into a jar of limewater (a clear
solution of slaked lime). The liquid turns cloudy
as grains of chalk form in it.
Black’s fixed air is the gas sylvestre of which
the existence has been postulated by van
Helmont a century earlier. Its composition as
carbon dioxide is not discovered until the 1780s,
when Lavoisier achieves it by burning carbon in
oxygen.
Black’s proof that such a gas exists prompts an
energetic search for others. Hydrogen is
identified by Cavendish in 1766, and oxygen
almost simultaneously by Scheele and Priestley
in the 1770s. Meanwhile Black has observed
another important scientific principle, latent heat.
Cavendish and hydrogen:1766
In 1766 Henry Cavendish presents his first paper
to the Royal Society. Under the title Factitious
Airs he describes his experiments with two
gases. One is the ‘fixed air’ identified by Joseph
Black . The other is a gas which Cavendish calls
‘inflammable air’, soon to be given the name
hydrogen by Lavoisier .
Hydrogen has been observed as a phenomenon
for at least two centuries. The 16th-century
alchemist and charlatan Paracelsus finds that
the dissolving of a metal in acid releases a form
of air which will burn. But Cavendish is the first to
identify it as specific substance. He believes that
he has found the inflammable essence,
phlogiston.
The study of gases in the laboratory is by now a
standard chemical process thanks to the
pneumatic trough developed in the early part of
the 18th century by Stephen Hales. An upturned
vessel, full of water, stands in a shallow trough of
water. Gas is collected in the top of the vessel,
displacing water and being sealed in by it. With
this device Cavendish is able to calculate the
specific gravity of hydrogen.
He finds that it is one fourteenth that of common
air (it is the lightest substance known). Within
less than two decades of his observation, a
dramatic use is found for this very light new gas
– in ballooning .
Priestley and oxygen: 1774
Joseph Priestley, a nonconformist minister, is
employed as librarian from about 1773 in an
English nobleman’s house, Bowood in Wiltshire.
He is provided with a laboratory to carry out his
chemical researches. And he has recently
acquired a large 12-inch lens, with which he can
focus intense heat on chemical substances.
In August 1774 he directs his lens at some
mercury oxide. He discovers that it gives off a
colourless gas in which a candle burns with an
unusually brilliant light. Experimenting further
with this gas, he records a few months later that
‘two mice and myself have had the privilege of
breathing it’. The mice were presumably offered
the privilege first.
Priestley has isolated oxygen. He foresees a
medical use for it (‘it may be peculiarly salutary
for the lungs in certain cases’), but he does not
fully appreciate its chemical significance –
largely because he believes in the phlogiston
theory. He calls the new gas ‘dephlogisticated
air’, on the assumption that the phlogiston has
been removed from it.
In October 1774, visiting Paris with his noble
patron, he describes his discovery to a gathering
of French scientists. Among them is Lavoisier ,
who develops Priestley’s experiments in his own
laboratory and realizes that he has the evidence
to disprove the phlogiston theory.
Priestley meanwhile isolates a great many other
gases. Though he is the first to publish his
discovery of oxygen, he has in fact been
preceded in the identification of both oxygen and
nitrogen by the Swedish chemist Carl Wilhelm
Scheele.
Scheele separates air in 1773 into two gases
which he calls ‘fire air’ (oxygen) and ‘foul air
(nitrogen). His findings only become known with
the publication of his book Air and Fire in 1777,
but it is established that the experiments date
from four years earlier. Like Priestley, Scheele is
handicapped by his belief in the phlogiston
theory. When isolating hydrogen, he concludes –
as has Cavendish – that it is pure phlogiston.
Cavendish and water: 1784
During the last three decades of the 18th
century, with more and more chemical
substances becoming identified, there is great
interest in which of them may be elements – in
Boyle ‘s sense of being pure substances unmixed
with anything else. Of the four ancient Greek
elements , earth is clearly no longer a candidate.
Air is separated in 1773 by Scheele into oxygen
and nitrogen. Water receives its dismissal from
the club at Cavendish’s hands in a paper entitled
Experiments in Air (1784).
Cavendish mixes hydrogen and oxygen, in the
proportion 2:1, in a glass globe through which he
passes an electric spark. The resulting chemical
reaction leaves him with water, which stands
revealed as a compound (H2O).
Lavoisier: 1777-1794
Although Antoine Laurent Lavoisier has no single
glamorous discovery to add lustre to his name
(such as postulating the first gas , or identifying
oxygen), he is regarded as the father of modern
chemistry. The reason is that during the last two
decades of the 18th century he interprets the
findings of his colleagues with more scientific
clarity than they have mustered, and creates the
rational framework within which chemistry can
develop.
He gives evidence of this in his response to
Priestley’s discovery of ‘dephlogisticated air’. He
undertakes a series of experiments which reveal
the involvement of this new gas in the processes
where phlogiston has been assumed to play a
key role.
He is able to show that Priestley’s gas is
involved in chemical reactions in the processes
of burning and rusting, and that it is transformed
in both burning and breathing into the ‘fixed air ‘
discovered by Joseph Black. His researches
with phosphorus and sulphur cause him to
believe that the new gas is invariably a
component of acids. He therefore gives it in 1777
the name oxygen (from the Greek for ‘acid
maker’). On a similar principle Lavoisier coins
the word hydrogen (‘water maker’) for the very
light gas isolated by Cavendish.
With these two names chemistry takes a clear
and decisive step into the modern era. It is an
advance which Lavoisier soon consolidates.
With three other French colleagues Lavoisier
publishes in 1787 Méthode de nomenclature
chimique (Method of Chemical Nomenclature).
Their scheme, soon universally accepted,
sweeps away the muddled naming of substances
which has descended from alchemy and
replaces it with a logical system of classification.
This is an achievement of French rationalism
comparable to the metric system, in the planning
of which Lavoisier is also involved.
In 1789 Lavoisier follows this book on chemical
methodology with the related fruits of his own
researches – Traité élémentaire de chimie
(Elementary Treatise of Chemistry). In this he
attempts a list of the known elements.
Lavoisier names more than thirty elements,
which he defines – in the tradition begun by Boyle
a century earlier – as substances which can be
broken down no further by any known method of
analysis. The majority are metals, but there are
by now three gases which Lavoisier identifies as
elements – oxygen, hydrogen and nitrogen
(which he calls azote, ‘without life’).
Lavoisier is immensely active in public affairs, in
addition to his scientific work. Unfortunately his
tasks have included, under the ancien régime ,
membership of the ferme générale or tax
authority.
By the time of the Terror, in 1794, it makes little
difference that Lavoisier has been on the liberal
and reformist side on contemporary issues. An
order is given for the arrest of all the former
members of the ferme générale . In May 1794, in
a trial lasting only part of a day, twenty-eight of
them including Lavoisier appear before a
revolutionary tribunal. Condemned to death, they
are guillotined that same afternoon.
A colleague of Lavoisier, who has worked with
him on the commission to introduce the metric
system, comments: ‘It took only a moment to cut
off that head; a century may not be enough to
produce another like it.’
Stay tune. Am yet to coplete it