True elements — Lavoisier and Phlogiston

2019 International Year of the Periodic Table

timeline of elements based on their discovery by decades with oxygen, hydrogen and nitrogen[This series is a companion to our International Year of the Periodic Table project — Timeline of Elements and Mendeleev Mosaic. Time periods from our timeline project will be featured highlighting the historical and scientific advancements resulting in the discoveries of the elements of that period. Jim Marshall researches the history of the discovery of the elements. His studies have carried him to over 20 countries where he has photographed and documented specific sites which he describes in his publications on chemical history.  Explore the artwork for the Timeline of Elements for "True Elements" with individual discovery stories of each element with acknowledgement to artist(s), teacher, school and countries on our Timeline of Elements website. ]

The stage was now set for a giant leap in the understanding of chemistry. A vast amount of knowledge had been accumulated during the previous several thousand years. Man understood how to process ceramics, glass and metals. In the previous several hundred years, new compounds and acids had been discovered; the technology of ceramics, glass and metals had advanced greatly; gunpowder had been discovered and utilized in peace and in warfare; modern physics was born with the laws of motion; electricity had been discovered; and the center of the universe was no longer the earth, but the sun. However, the four basic elements were still considered to be fire, earth, water, and air — this belief held through virtually all of the 18th century and was emphasized in chemical dictionaries and treatises.

Since chemistry involves invisible atoms (not yet hypothesized, except briefly by the Greek Democritus), the basic understanding of chemical processes would be difficult. It would take a great genius to take that great leap from archaic thinking to a new theory of chemical processes. Ironically, this great leap involved not traditional substances — metals, sulphur, charcoal, salts and acids — but instead relied upon the investigation of the three gases nitrogen, oxygen and hydrogen. Antoine Laurent Lavoisier (1743-1794), a French tax collector and part-time chemist, was able to piece together all the fragmentary information to arrive at a new view of the universe that allowed the birth of a new view of chemistry. Indeed, Lavoisier is sometimes called the “Father of Modern Chemistry”.

Before Lavoisier, the theory of chemistry was based on a theory proposed a hundred years before his time. In the 1600s Becher founded, and Stahl elaborated, the so-called phlogiston theory, suggested to explain the overall observations of combustion (burning), calxing (rusting), and respiration (animal breathing). Although this theory is now considered crude and far off the mark, it was one of the most important advances in chemistry because it allowed for the first time a theory that allowed predictions and therefore could be tested. As any modern scientist knows, the progress of science rests upon the proposal of theories that predict and can therefore be tested, validated, modified or discarded. Stahl proposed that the three processes of combustion, calxing and respiration together reflected Nature’s management of a universal principle called “phlogiston” (Greek “inflammable” principle). According to this theory, when a material burns (combusts), phlogiston is emanated and is released into the atmosphere. One can actually “see” and “hear” the release of phlogiston during, for example, the burning of wood, as heat waves are observed and the hissing of escaping phlogiston is detected. Whereas the ancients believed that fire ascended into the empyreum (the “highest heaven,” the source of light and creation), Stahl believed that the phlogiston combined with the atmosphere. Plants then could absorb the phlogiston into their organic material to produce “phlogisticated” substances which animals could then eat. As the animals respired, the foods released their phlogiston back into the atmosphere. Thus was observed a continuous cycle, with phlogiston flowing from fires and animals into the atmosphere, where plants could produce “rephlogisticated” material. 

Meanwhile, metals were observed to calcine to produce a calx (red-brown in the case of iron, but different colors for different metals, such as white for antimony or black for manganese). This calcining phenomenon was explained by the same process: metals would release phlogiston to the atmosphere (today we know this process as “oxidation,” but “oxidize” was not known as a word yet, because “oxygen” had not been identified). The calx could be “rephlogisticated” by reaction with charcoal to regenerate the metal. Obviously, Stahl said, charcoal was very rich in phlogiston, which made sense since charcoal comes from plants (wood). Although wood and other combustible materials lost weight during burning, metals when calcined mysteriously gained weight. The former observation — that wood disappeared upon burning — was easily explained by simply assuming the products of burning disappeared into the atmosphere. The gain of weight of metals, however, was more difficult to explain. Since metals gained weight when they lost phlogiston, then it followed that phlogiston must have negative weight! That is, the prediction was made that phlogiston had “anti-gravity”!

a portrait of Daniel RutherfordDaniel Rutherford

 “Phlogisticated” air was first characterized by Daniel Rutherford in Edinburgh, Scotland (not to be confused with Ernest Rutherford, who investigated the structure of the atom more than a century later). Rutherford prepared “phlogisticated air” in three separate experiments: by burning a candle, calcining a metal, and by collecting the respiration gases of a mouse. He showed that all of these gases behaved similarly. For example, after the mouse died in a closed container (it usually took about fifteen minutes), Rutherford showed that this “dead mouse gas” would not support the combustion of a candle nor the calcining of metal. Likewise, the air produced from a calcined metal or a burned candle would not support a mouse for the full fifteen minutes. Hence, Rutherford had shown that “Stahl was correct”: The air from the three processes of combustion, calcining, and respiration were indeed the same, and hence this air must be “phlogisticated air”.

A statue of PriestleyPriestley

a statue of Scheele

 The next step was to prepare “dephlogisticated air”. Priestley (England) and Scheele (Sweden) were independently able to produce this air. Although Priestley is generally credited with the discovery (because his publications appeared first), Scheele performed more precise experiments and actually made the discovery earlier. Scheele heated a calx (mercuric oxide) and collected the resulting gas in a bladder (rubber balloons had not yet been invented). He called his product “Feuerluft” (fire air). Priestley called his gas, produced in a similar fashion, “dephlogisticated air”.

a drawing of CavendishCavendish

 It only remained for phlogiston itself to be found. This “discovery” was made by Cavendish, an eccentric English millionaire who performed experiments in laboratories he set up in his mansions. Shy and retiring, Cavendish rarely showed himself in public; however, his lectures were brilliant and reflected extremely careful and meticulous work. His retiring ways were so extreme that he fired a servant who ventured to speak with him rather than communicate by his preferred method of writing notes and shoving them under the door. He never posed for a portrait and only one quick sketch of him exists. Cavendish’s experimentation involved the addition of “Mars” (iron) to “oil of vitriol” (sulfuric acid) to produce a gas which he collected in a bladder. He observed that the bladder floated in the air, and thus he must have phlogiston, which was predicted to have negative weight! This trapped air was shown to be quite combustible and accordingly was termed “inflammable air."

a statue of LavoisierLavoisier

 It made perfect sense that phlogiston itself should be so reactive; and the discovery of dephlogisticated air, phlogisticated air and phlogiston itself would appear to fully vindicate Stahl’s theory. However, something more subtle and different was actually occurring, and it took a powerful genius to figure this out. This person was Lavoisier, who boldly made the prediction: If Stahl were correct, then dephlogisticated air (oxygen) would react with phlogiston (hydrogen) to produce phlogisticated air (nitrogen). To test this prediction, Lavoisier prepared a glass globe and filled it with dephlogisticated air (oxygen) and phlogiston (hydrogen). He produced a spark inside the enclosed globe and witnessed a mighty explosion. (He had fashioned strong glass walls to withstand the force of the reaction.) Upon carefully inspecting the contents, he observed not phlogisticated air (nitrogen) as the product as predicted by Stahl but instead water!

In every science, there has been an event that signaled the evolution from a primitive faith to a true modern science. In biology this event was Darwin’s theory that all species were created by natural mutation and selection; ever since, nothing in biology makes sense without evolution. In physics perhaps this milestone was laid by Newton, who formulated the laws of gravity, and Galileo, who observed that all objects fell at the same rate and showed that all physical phenomena obeyed mathematical laws. In astronomy this turning point was founded by Copernicus, who postulated the heliocentric theory. In geology it was the understanding that earthquakes were caused by natural events in Terra instead of by the rage of God, and that the present geology of the earth reflects deep time with billions of years of evolution of the earth; in bacteriology it was that microscopic living beings caused diseases instead of humors and evil spirits. And in chemistry it was Lavoisier’s insight that water was a compound, and therefore the “inflammable air” (which had been supposed by others to be phlogiston) and the “dephlogisticated air” or “vital force” were actually elements. In fact, Lavoisier gave names to these two elements, calling the first “hydrogen” (“water-former”) and the latter “oxygen” (“acid-former”). With this keen perception, Lavoisier went on to identify thirty-one substances as elements. Incredibly, he recognized that chlorine (“muriatic radical”) and fluorine (“fluoric radical”) were elemental, even though the elemental substances themselves had never been isolated (only salts)! He continued to realize that other elements must exist from their compounds: boron (from borax and similar compounds), silicon (from quartz and glass), magnesium (from Epsom salt), calcium (from limestone), and aluminum (from clays) — when none of these elements had ever been isolated in their elemental form! This was insight of a staggering degree. These are the elements, called "simple substances" by Lavoisier, which he recognized in his famous treatise of 1789; he organized them into four groups:

Universal simple substances: oxygen, nitrogen (azote), hydrogen
Nonmetals: sulfur, phosphorus, carbon, chlorine, fluorine, boron
Metals: antimony, silver, arsenic, bismuth, cobalt, copper, tin, iron, manganese, mercury, molybdenum, nickel, gold, platinum, lead, tungsten, zinc
Earthy simple substances: calcium, magnesium, barium, aluminum, silicon.

(He also recognized light and heat as elements, but this idea lay dormant for decades until the thermodynamic nature of chemical energy was understood.)

Lavoisier’s laboratory was located in Le Petit Arsenal, near the Bastille in Paris, France. Neither the Bastille, the Arsenal or Lavoisier’s laboratory still exist. Only a plaque presently exists on the wall of a modern building. A wonderful exhibit of Lavoisier’s apparatus exists in the Musée des arts et métiers (Museum of arts and measurement) on the Right Bank of the Seine. The exhibit includes some of his delicate equipment with which he carefully weighed specific amounts of hydrogen and oxygen and the spark flask which produced the water. This equipment was state-of-the-art apparatus of the day, without which Lavoisier could not have quantitatively studied the basic reaction of hydrogen + oxygen → water. His vocation as a tax collector trained him to quantify and expect conservation of mass: “The ledger must balance; what goes in must come out; there must be no loss in either an accountant’s books or in a chemist’s laboratory.” His avocation as a lover of science and dreamer allowed him to make the leap of genius to recognize the true elements.

Lavoisier, being a tax collector for King Louis XVI, was very unpopular. He was caught up in political events during the Revolution, and he was guillotined by the masses. The execution of Lavoisier was indeed tragic. “It required but a moment to cut off his head and perhaps a hundred years will not suffice to produce another like it.” — Comte de Joseph-Louis Lagrange.    


Photos taken from J. L. Marshall, Discovery of the Elements, Peterson Custom Publishing, 2002.