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Robert Boyle
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The development of the Phlogiston Theory
Antoine Lavoisier and the demise of the Phlogiston Theory

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The Phlogiston Theory

The Natural Philosophers of Lavoisier's day adhered to the Phlogiston Theory. This theory that dated from 1667 stated that anything that could be combusted (burnt) contains an ethereal-like substance called phlogiston. When combustion was complete it meant that all the phlogiston had been released into the air. Phlogiston had no substance - as one proponent described it, it could not be 'put in a bottle'. However, the phlogiston theory seemed at odds with the fact that when a metal is heated in the air its calx (its oxide) is formed. This increase in weight did not seem to fit the theory but it was explained by assuming that phlogiston could have negative weight.

N.B. In the 18th century the term 'weight' was used as a measurement that is now termed 'mass'.

 

 

Lavoisier

Antoine-Laurent Lavoisier. Line engraving by Louis Jean Desire Delaistre, after a design by Julien Leopold Boilly

 

A Summary of Lavoisier's 1774 Experiments and Observations

It should be remembered that the composition of the air was not know in 1774. Gases produced by experiment that had different properties from atmospheric air were referred to as "airs". A summary of Lavoisier's observations is shown below. The modern day chemical names are shown in blue:

  • Heating mercury calx (mercury oxide) with carbon in a vacuum in a sealed flask produced "fixed air" (carbon dioxide). The sealed flask showed no change in weight after the reaction.

  • Heating mercury calx (mercury oxide) in a vacuum in a sealed flask produces an "air" - Priestley's "dephlogisticated air" (oxygen) - that supports respiration and combustion. The sealed flask showed no change in weight after the reaction.

  • Heating mercury in atmospheric air in a closed jar for two days produces a red solid (mercury II oxide). The gas left in the jar had decreased in volume by around 21% and no longer supported combustion (nitrogen). There was no change in the weight of the jar before and after the reaction.

  • Heating the red solid (mercury II oxide) in the "air" remaining in the flask (nitrogen) caused the red solid to decompose and the gas in the flask returned to its original volume.

  • When a candle was burnt in this gas (now atmospheric air) it used up the same volume that had been added by the red solid (it used up the oxygen).

 

aparatus01

Antoine Lavoisier apparatus used to heat mercury and mercury II Oxide.
Engraving by Mme Lavoisier in the 1780s taken from Traité élémentaire de chimie.

 

1787 Méthode de nomenclature chimique (Method of Chemical Nomenclature)

Lavoisier, having made the distinction between elements and compounds, then proceeded to tackle chemical nomenclature.

He also set rules for the naming of compounds as follows:

  • They should have fixed names

  • The names should reflect their composition

  • That the name assigned should have roots in Greek or Latin

N.B. Lavoisier did not get things completely right. He considered 'light' and 'caloric' as elements. In other words light and heat, although they had no weight, he still classed as 'elements' that he could measure quantitatively.

Lists elements

 

 

HISTORY OF SCIENCE AND TECHNOLOGY

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Antoine Lavoisier (1743 - 1794)

 

Lavoisier

Antoine Lavoisier and his wife Marie-Anne Pierrette Paulze 1788 by Jacques-Louis David

Antoine-Laurent Lavoisier was born into a well-off, upper class (bourgeois) Parisian family. His father, Jean-Antoine Lavoisier, and his paternal grandfather were lawyers and both worked as government officials. His mother's family was in the meat trade and, on his mother's death in 1748 Antoine Lavoisier went to live with his maternal grandfather. When his grandfather passed away it was Antoine's unmarried aunt who took care of his upbringing. Antoine Lavoisier never wanted for money since he received a considerable inheritance from his mother.

Lavoisier's secondary school was Collège Mazarin (also known as the Collège des Quatre-Nations) which was linked to the University of Paris (the Sorbonne). Lavoisier excelled in Literature (Lettres) and, when he was seventeen, he was awarded the 'Prix de Discours Français' as the top candidate in the 'concours générale' (an exam open to all students in the same age group in France). When Lavoisier moved from secondary school into university in 1761 he studied law influenced, no doubt, by his father and grandfather. He went on to qualify for the bar in 1764.

Lavoisier's secondary education would have included Natural Philosophy and Collège Mazarin was renowned for its excellence in science education. He would have been well-grounded in mathematics, physics and astronomy. One of his teachers, Abbé Nicolas Louis de Lacaille, was influential in stimulating Lavoisier's interest in meteorology. Even while studying law at university, Lavoisier attended Natural Philosophy lectures and these would have included lectures on the Phlogiston Theory (see left) given by Francois Rouelle. Only two years after qualifying to be a practicing lawyer, Lavoisier was awarded a 'Médaille d'Or' (Gold Medal) for his essay on how to best light the streets of a large city.

With his interest torn in different directions, Lavoisier decided not to make his living as a lawyer but, when an old friend from school offered him the opportunity to work on a geological survey, he took it. Etienne Guettard had studied geology at university and had been employed to conduct a geological survey of France. Lavoisier worked with him on the Alsace-Lorraine area of the country. It was during this time that Lavoisier developed his practice of making precise, quantitative measurements and from geology it was an easy step into chemistry.

Lavoisier started his inroads into chemistry with his work on gypsum (calcium sulfate). He was only twenty one years old when he read his paper on making Plaster of Paris to the Académie des Sciences. He had discovered that heating gypsum produces a powder and that, adding water to this powder when it has cooled down, forms a hard plaster. Lavoisier had been scrupulous in recording accurate measurements of the quantities of the mineral and water needed at each stage of the process.

By 1768 Lavoisier was under the employ of the Académie des Sciences as a geological map-maker and he was admitted to the Académie as a full member in 1769, at only twenty six years old. Also in 1768 he made a decision that would proof to have disastrous consequences - he invested on organisation called the Ferme Générale. This group of investors oversaw the collection of taxes on behalf of the king. They were, in fact, middle-men in the tax process and they made money themselves by keeping a percentage of the taxes they collected as fees for their work. In the case of Lavoisier there are some sources that suggest he went into the business to make the tax system fairer for the poor. This may have been true but he did use the money he made from his involvement with the Ferme Générale to pay for his laboratory and help to finance his research.

In 1771, aged twenty eight, Lavoisier married Marie-Anne Pierrette Paulze, the thirteen year old daughter of Jacques Paulze, another member of the Ferme Générale. In fact, his daughter's marriage to Lavoisier was a way to get out of a proposal Marie-Anne had received from a sixty year old Count. Jacques Paulze asked Lavoisier to marry his daughter instead and Lavoisier agreed. This arranged marriage worked out well. Marie-Anne Lavoisier was an intelligent young lady who provided invaluable help with her husband's work. She learnt English so that she could translate scientific papers coming from Lavoisier's Anglophone contemporaries. Marie-Anne Lavoisier also wrote up her husband's experiments and she took lessons in art and engraving so that she could illustrate his laboratory apparatus.

In 1772, Lavoisier and his co-workers experimented on a small diamond. The diamond was placed in a closed flask and heated by directing the sun's rays onto it using a very powerful magnifying glass until the diamond had 'disappeared'. This experiment was repeated with an equal mass (the term weight was used in the 18th century) of charcoal. Lavoisier, with his insistence on accurate, qualitative measurements, weighed the flask before and after the experiment and recorded that the weight had not changed despite the disappearance of the original solid. The air in the closed flask had changed - "fixed air" (that we now know as carbon dioxide) had been added. Lavoisier came to the conclusion that matter had not been lost during these reactions. The quantitative results also showed him that diamond and charcoal were made from the same element that he called carbon. Lavoisier went on to carry out similar experiments with other chemicals, such as sulfur and phosphorus.

Lavoisier began to question the Phlogiston Theory since during these reactions there had been no loss or gain in mass which would have been the case if phlogiston had been lost or gained. In 1774 Lavoisier met with Joseph Priestley in Paris and Priestley described the "dephlogisticated air" that had been produced when he heated mercury calx (mercury oxide). Lavoisier repeated Priestley's experiment with mercury oxide and other oxides. He went further by heating mercury oxide with carbon and heating mercury in atmospheric air. A summary of his observations is shown in the box to the left.

Lavoisier came to the conclusion that that atmospheric air was made of two different "airs" - one at 21% that supported combustion and respiration and combined with metals. The other, at around 79%, that did not support either combustion or respiration. In 1775 Lavoisier presented a paper to the Académie des Sciences that is known as the 'Easter memoire'. Its longer, translated title is 'On the Nature of the Principle which combines with metals during their calcination and increases their weight'. This marked the beginning of his attack on the Phlogiston Theory.

The Lavoisier's moved into an apartment in the Paris Arsenal in 1775. Lavoisier had been engaged by the Royal Gunpowder Commission (l'Administration Royale des Poudres) to work on improving the quality of gunpowder. He built a laboratory next to his apartment and his notoriety as a chemist meant that he received visits from other European researchers. Lavoisier's work for the Royal Gunpowder Commission gave France superior gunpowder by improving the quality of the ingredients (saltpeter, sulfur and charcoal) as well as adjusting the properties of its granulation so making the gunpowder more effective.

Between 1777 and 1779 Lavoisier continued to investigate his new theory that disproved the Phlogiston Theory. During this time he indentified sulfur as an element since his experiments showed that sulfur could not be broken down into smaller parts. In 1778 he presented an updated version of the 'Easter Memoire' to the Academie des Sciences naming the element oxygen (meaning "acid maker") as Priestley's "dephlogisticated air". He was able to give an accurate percentage for the amount of oxygen in the air and he had realized its significance in combustion and respiration. Lavoisier had shown that atmospheric air is not one gas but a mixture of gases in precise proportions.

Together with another scientist, Pierre-Simon Laplace, Lavoisier used a calorimeter to measure the heat given out and the amount of carbon dioxide produced by a respiring guinea pig in a given amount of time. He compared this to the time it took for ignited carbon to produce the same amount of heat and carbon dioxide. The two scientists concluded that respiration was a 'slow combustion'.

Respiration expts

Madame Lavoisier assisting her husband in his scientific research of human respiration measuring basal metabolic rate.

 

Lavoisier's investigations during this period served to validate the conservation of matter - that the weight of the products of a reaction is equal to the weight of the reactants. He was not the first scientist to make this observation. More than a century earlier, in 1630, Jean Rey had proposed a similar theory, as had Joseph Black in 1755 and Mikhail Lomonsov in 1760.

In 1766 Henry Cavendish had discovered an "inflammable air" that had been produced when he added acid to certain metals. He had burnt this "inflammable air" in Priestley's "dephlogisticated air" and observed that water was formed as a product of the reaction. However, Cavendish explained this reaction using the Phlogiston Theory. For him, the water was already present before the two gases had been ignited.

In 1783 Lavoisier repeated Cavendish's experiment and came to a very different conclusion. He realized that the very pure sample of water that was formed was a product of the reaction between the two gases. This led him to the conclusion that water was a compound and he proved this by reversing the reaction and producing the two original gases. Lavoisier named Cavendish's "inflammable air" hydrogen. Lavoisier's 1783 paper, Réflexions sur le phlogistique (Reflections on Phlogiston) was a victory for quantitative chemistry over hypotheses that had existed since the Greek philosophers wrote about the four "elements" - earth, fire, air and water.

Treatise

In 1789 Lavoisier published his Traité élémentaire de chimie (Elementary Treatise of Chemistry) that detailed his experiments, observations and conclusions and rejected the Phlogiston Theory. He was well aware that many of his contemporaries would not accept his conclusions but his hope was that the new generation of scientists would use them to explain and develop their own investigations. This publication is considered as the first "modern" chemistry textbook and has earnt Antoine Lavoisier the title 'Father of Chemistry'.

In 1791 Lavoisier sat on a committee set up by the Academie des Sciences to develop a system that would unify weights and measures. This gave rise to the metric system. The committee set the definition of the unit of mass (one grave - the modern day gramme) to be the mass as a cubic decimetre of distilled water at 0°C. Lavoisier was one of the scientists responsible for finding the mass of one grave by experiment.

This was the last duty Lavoisier performed for the Académie. The year 1789 had marked the beginning of the French Revolution and all government organisations and officials were being persecuted. The Académie des Science was dissolved in 1793 and Antoine Lavoisier was arrested in 1794. He was accused of being a traitor to the revolution because of his involvement in the Ferme Générale and his so-called collusion with foreign scientists. Pleas to spare his life by the mathematician Joseph Louis Lagrange went unheeded and a certain Jean-Paul Marat, who had a grievance against Lavoisier because he had been dismissive of one of Marat's inventions in the past, led the call for Lavoisier's guilty sentence claiming that he had robbed the poor as a tax collector.

Lavoisier was guillotined the same day that he was sentenced, along with his father-in-law and twenty six other members of the Ferme Générale. Lagrange lamented Lavoisier's execution with the sentence:

 "Il ne leur a fallu qu'un moment pour faire tomber cette tête, et cent années peut-être ne suffiront pas pour en reproduire une semblable."
("It only took a moment to cut off this head but one hundred years will not be enough time to produce another like it").

Lavoisier was buried in a mass grave for guillotined victims in Picpus Cemetery close to Place de la Nation where the guillotine had been erected.

Marie Anne Lavoisier fought tirelessly for her husband's name to be cleared of all charges and this happened eighteen months after his death when he was declared innocent. She went on to marry a scientist named Benjamin Thompson who continued the work of Lavoisier that helped to finally put an end to the Phlogiston Theory.

 

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