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 different materials.

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:

Magnesia Alba + heatMagnesia Usta + Fixed Air
Magnesia Usta + acidMagnesia salt + water
Magnesia Alba + acidMagnesia salt + Fixed Air + water
Limestone chalk, shells + heatQuicklime + Fixed Air
Quicklime + acida salt + water
Limestone chalk, shells + acida salt + Fixed Air + water
In modern terms, these reactions are
MgCO3 + heatMgO + CO2
MgO + 2HClMgCl2 + H2O
MgCO3 + 2HClMgCl2 + CO2+ H2O
CaCO3 + heatCaO + CO2
CaO + 2HClCaCl2 + H2O
CaCO3 + 2HClCaCl2 + CO2 + H2O
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.

British 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 + heat2PbO
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 suppport burning
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 physics."

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.