The Experimental Chemistry of Airs
© William C. Kimler
The development of chemical knowledge during the 1700s is a story of
increasing ability to handle materials in the laboratory, defining unique,
pure substances by their action in an experiment. Such technical facility
depended upon developments in technique and equipment, such as furnaces
and burning glasses for heat control in reactions, glassware, and especially
precision balances for quantifying weights.
Burning glasses were one of the clever innovations in the 18th
century for controlling chemical reactions -- allowing combustion without
the open flame of a furnace, or the need to capture gases from the flue.
This contemporary French plate shows Joseph Priestley working with a giant
lens -- an early example of science depending upon big, expensive, rare
equipment. It also reveals the French interest in Priestley's work on
the chemistry of fixed airs.
The chemistry of a few particular
gases -- the so-called "airs" -- played a large role in refining ideas
of elements as separable pure materials that enter into reactions under
precise rules of quantitative combination. Ways of controlling reactions
and collecting their products are shown at left. At top left, the glass
vessel inverted in water isolates the test material from outside air;
a lens focuses light onto it to ignite a reaction inside the vessel.
The "pneumatic trough" below that collects a product of burning by conveying
the gas emitted by the burning substance into a water-filled vessel,
in which the collected gas displaces the water. Stephen Hales developed
this apparatus in the 1720s, to collect the products of burning and
of destructive distillation. Because the "air" came out of a solid material
during the reaction, he called it "fixed air".
The sophisticated equipment at bottom left is a later apparatus for
collecting "fixed air", by dripping acid into the bottle, which is filled
with water and a salt (e.g., calcium carbonate, in modern terms). The
"fixed air" is collected in the suspended cylinder; and to the left,
a candle is being immersed into the gas. By the mid-1700s, chemists'
precision in collecting such gases led them to identify the "airs" as
unique materials. The usual experimental "fixed air" is heavier than
"ordinary air" and does not to support combustion or breathing. The
new ability to capture the gaseous products of reactions revealed specific
traits of airs collected in different reactions, which suggested new
theories of separate substances and their combinations.
Laboratory success encouraged chemical philosophers to define themselves
as part of the Newtonian revolution. The ability to isolate and manipulate
metals, acids and alkalis, salts, oxides, and gases created practical
results, such as purified materials, better assays for metallurgy, and
new chemicals for industrial uses. Mineral acids and caustic alkalis,
for example, found use in new formulations of medicines, cements, ceramics,
and textile dyeing and bleaching. Laboratory success also led to the
development of new concepts of substance and reaction. "Newtonian" chemists
sought analysis, or the decomposition of materials into their
primary constituents (and new theories of pure substances) and synthesis,
or the ability to predictably recombine primary parts into known substances.
They sought a theory of mechanism to replace the ancient elements and
alchemical principles. The fact that "fire" and "air" could be measured,
fixed and released, and finally quantified in reactions brought a new
focus to chemical theorizing. A new chemistry would require new explanations
of the results of basic laboratory processes -- the meaning of distillation
and fermentation, acidity and alkalinity, metal calcination, and other
products of combustion. In particular, air chemistry results brought
attention to the relations between gases, chemical reactions, and burning.
Joseph Black (1728-99) first identified "fixed air" as a unique substance.
He knew there was a chemical controversy over the action of alkalis,
in the "limewater" used to dissolve kidney stones. Limewater is quicklime
dissolved in water; quicklime is the brittle, white, powdery product
of heating chalky limestone or shells in a lime-kiln. This was an old
industrial process, used by Roman builders. Quicklime mixed with sand
and water made a mortar that dried to a hard cement. Interested in limewater's
medical use, Black explored other caustic alkalis. In 1756, his treatise
"Experiments upon Magnesia Alba, Quicklime, and some other Alcaline
Substances" revealed precise quantities entering into reaction to make
Black heated the mild alkali Magnesia Alba (the chalk-like "white
magnesia", which in solution was the new antacid medicine "milk of magnesia")
and measured a constant loss of weight as "fixed air" left the substance.
What was left was Magnesia Usta, a calcined magnesia somewhat like quicklime.
Treated with acid, it produced Magnesia Salt. Magnesia Alba treated
with acid effervesced and left the same weight of the same salt. Black
determined that lime and quicklime acted in comparable reactions: the
caustic quicklime could be neutralized with a quantity of acid, and
that same quantity of acid added to chalk left the same salt after effervescing.
The effervesced gas was "fixed air", making up 40% of the weight of
the limestone chalk or shells:
In modern terms, these reactions are
+ heatMagnesia Usta + Fixed Air
Magnesia Usta + acidMagnesia salt
Magnesia Alba + acidMagnesia salt
+ Fixed Air + water
chalk, shells + heatQuicklime
+ Fixed Air
Quicklime + acida salt + water
Limestone chalk, shells + acida
salt + Fixed Air + water
Black deduced the "principle of causticity" to be the Fixed Air -- mild
alkalis (carbonates) contain Fixed Air, and caustic alkalis (hydroxides)
do not. Later, this Fixed Air was shown to be the same as that from fermentation
and combustion -- it turns limewater milky, it will not support combustion
or animals' respiration, and it is heavier than atmospheric air. Black
also improved lime-kiln designs -- producing pure quicklime more effectively
by better venting away of the water and carbon dioxide produced by heating.
+ heatMgO + CO2
MgO + 2HClMgCl2 + H2O
MgCO3 + 2HClMgCl2
+ CO2+ H2O
+ heatCaO + CO2
CaO + 2HClCaCl2 + H2O
CaCO3 + 2HClCaCl2
+ CO2 + H2O
laboratory experimenters began to isolate more of the different "airs".
Lord Cavendish in the 1760s increased precision with pneumatic troughs
using mercury instead of water, which eliminated the gas lost into aqueous
solution. He also isolated the "inflammable air" produced by adding
Vitriol (sulphuric acid) to metals (e.g. iron, tin, zinc). Unlike "fixed
air", this "air" was much less dense than "ordinary air" and burned
explosively. Joseph Priestley (1733-1804) made ingenious troughs, such
as the one shown to the left, and collected
the many "airs" reported in his "Observations on Different Kinds of
Air" (1772) -- fixed air (carbon dioxide), inflammable air
(hydrogen), nitrous air (nitric oxide), marine acid air
(hydrogen chloride), alkaline air (ammonia), vitriolic acid
air (sulphur dioxide), and phlogisticated nitrous air (nitrous
oxide, or laughing gas).
One place to collect fixed air was a site of fermentation, namely in the bubbling vats in the production of beer, as in this picture of a brewery in Leeds. Interest in the new airs was not simply theoretical. Black's work had medical and industrial applications, and he and Priestley helped develop the manufacture of carbonated water. Carbonated mineral water was a fashionable health fad. Priestley published a pamphlet on "Impregnating Water with Fixed Air" (1772), and sold the rights for his process to Jacob Schweppe in 1783, who developed a profitable business of "tonic water".
As the names of the airs imply, these gas chemists accepted
the theory of phlogiston as the substance of burning (embedded in fuels).
Most importantly, their quantitative methods allowed the measurement
of the amount of each air involved in reactions -- nitrous air, for
instance, put into common air would reduce its volume by 20%, leaving
an air that would no longer support burning or life (azote, or
nitrogen). If the common air had been "spoiled" by combustion or respiration,
the test volume would not reduce. That provides a test for the "goodness" of air -- in modern terms, the nitric oxide was combining with the 20%
oxygen of the atmosphere to make nitrogen dioxide, which dissolved in
the water of the pneumatic trough.
Also suggestive for a theory of combustion was the action of the inflammable
air. Priestley confined it to a chamber and ignited it with a spark
from a Leyden jar, and it left a dew on the glass. So did dephlogisticated
air. Cavendish identified the dew as water, and suggested that water
is the combination of the two airs, though still in the terminology
of phlogiston theory.
The existence of many airs raised new questions about reactions taking
place in "ordinary air" -- which part of the air is involved
in each reaction? It was well known that heating a metal produces the
calx of the metal, and that the reaction needs air. The calcined metal
also weighs more than the original metal. In terms of the phlogiston
theory, for example, they knew that:
Lead + heatLead
Calx (Red Lead, or Litharge) + Phlogiston
Georg Stahl (1660-1734), the originator of phlogiston theory, had noted
that dropping a bit of charcoal onto the calx in the furnace produces
a bit of lead -- the charcoal (a good fuel, full of phlogiston) obviously
put phlogiston back into the calx to make the lead. The dilemma was
that the calx is heavier -- did phlogiston have "negative weight" or
a "principle of levity"? Black's heated alkalis suggested similar reactions
without phlogiston's involvement.
Antoine Lavoisier (1743-94)
recast chemical theory with a new theory of combustion that redefined
the airs terms of the components of the reactions. He knew that metals
(lead) gained weight when calcined, and in his own experiments phosphorus
and sulfur gained weight when burned (unlike most things when burned),
also producing their acids. Lavoisier's idea in 1772 was that some part
of the air was absorbed in these reactions. By ingenious use of closed
containers, he was able to show that the lead turned into the calx at
the moment that air was allowed into the vessel:
Lead + common air
+ heatLead Calx
Lead Calx + CharcoalLead + Fixed Air
The dilemma was that Fixed Air was given off by the calx, but calcination
does not take place in Fixed Air. The isolation of other airs gave Lavoisier
the clues he needed. Priestley had heated Red Mercury, the calx of mercury,
with a large burning glass -- not being in a furnace, there was no charcoal
present. Surprisingly, he produced pure mercury and a volume of an air
that he thought might be "phlogisticated nitrous air", not Fixed Air --
it supported burning and life exceptionally well. In 1774, Lavoisier repeated
these experiments, showing that the air liberated from Red Mercury fit
Priestley's test for good air -- 1 part nitrous air in 2 parts of the
test air reduced the volume by 20%. Priestley then investigated this,
and found that adding even more nitrous air (5 times as much) continued
the reduction. The test air from the Red Mercury he called "dephlogisticated
air". For Lavoisier, this was confirmation that different airs were involved
in calcination and reduction. The "eminently respirable" part of air created
the calx, and some part of the charcoal added to the calx liberated Fixed
Air and lead. In modern terms,
2Pb + O2
2PbO + C2Pb + CO2
This was confirmed with precise measurements of mercury and its calx and
the air left in the test vessel:
2Hg + O2+
gentle heat2HgO + air that would not
2HgO+ higher heat2Hg + O2 (air that
would suppport burning)
In the good-air test, the same volume was lost in the calcination that
was regained in the reduction.
Combined with experiments on producing acids, Lavoisier concluded
that this eminently respirable part of air was a necessary material
for combustion and for acidity -- and thus in 1779 he renamed it "oxygen"
[="acid-former" ]. "Inflammable air" combined with it to make water,
and received the name "hydrogen" [="water-maker"] in 1783. Air had become
a mixture of "gases," and Water a "compound", and phlogiston theory
was rejected as needless for the explanations of calcination and combustion
reactions. The new names for chemicals introduced in 1787 by Lavoisier
in his Méthode de nomenclature chimique [Method of Chemical
Nomenclature] were a part of a new system of explanation of reactions
and compounds, a system which he saw as a complete revolution, organized
and explained in 1789 in the textbook Traité élémentaire
de chimie [Elementary Treatise of Chemistry]. It was a "revolution"
meant to "follow as much as possible the torch of observation and experiment."
which would bring chemistry newly "closer than heretofore to experimental
As an end-note, Priestley's
and Lavoisier's lives both dramatically intersected with the French
Revolution. A Unitarian preacher and radical supporter of the French
Revolution as an anti-monarchist, Priestley was run out of Birmingham,
England. In 1791, during political celebrations of the anniversary of
the storming of the Bastille, an opposing mob destroyed the dissenters'
chapel and then assailed Priestley's home. They smashed and then burned
his house and belongings -- including his laboratory. Priestley and
his family escaped to London, where he lived under an assumed name until
emigrating to Pennsylvania in 1794. In that same year, the French Revolution
caught up with Lavoisier. Although his work in his laboratory in the
Arsenal had provided the French government with the best gunpowder in
Europe, Lavoisier was wealthy and his wealth derived in part from his
work as a "tax farmer", or contracted collector of state taxes. He was
charged and executed by guillotine.