The 18th Century: Scientific Revolution and the Birth of Modern Chemistry

The Age of Enlightenment

The eighteenth century, known as the Age of Enlightenment, saw a wave of new ideas sweeping across Europe. This intellectual movement promoted modernization and a rejection of the Old Regime. Monarchies, influenced by these new ideas, implemented financial and educational reforms. This system of government was characterized as enlightened despotism, aiming to maintain the status quo of class domination and the perpetuation of their economic privileges. Meanwhile, the bourgeoisie, aligned with the social progress these changes represented, continued to undermine the foundations of the monarchy. They championed political and economic liberalism and embraced the rational, empiricist model.

Revolutionary Movements

This social atmosphere, combined with the unfolding crisis in the second half of the century, led to a wave of revolutionary movements. The French Revolution was the most prominent example. Colonial rule was shaken by the explosion of the Haitian Revolution, the War of Independence of the 13 Colonies, and the revolt of Tupac Amaru in Peru. These events marked the beginning of the modern era.

The Industrial Revolution and its Impact on Science

In the field of technology, the Industrial Revolution began in the United Kingdom. Driven by a favorable economic context and innovations like Watt’s steam engine (1769) and Cartwright’s power loom (1783), it created significant changes in the steel and textile industries. The growth of the textile industry spurred the development of paints and finishes, paving the way for industrial chemistry. A growing relationship between technology and science was established. While the previous century was dominated by the mechanics revolution, the eighteenth century witnessed a paradigm shift in the field of encyclopedic thought regarding chemistry.

Early Theories of Heat

The first works on heat and energy developed in this century laid the groundwork for understanding the structure of matter and its forms of movement in the nineteenth century. Skirting the border of physical and chemical interest, initial ideas about heat introduced a range of different agents, including ether, caloric, and phlogiston. These positions were based on the principle of not introducing action at a distance to explain physical phenomena in the absence of core concepts and theories about fields, multiple forms of energy, and their transformations. The idea that heat was a form of movement of a substance had already been expressed by Robert Boyle and Robert Hooke (1635-1701), among others, but it was not fully developed until the mid-nineteenth century. From around 1787, Lavoisier’s conception of heat as a substantial substance called caloric prevailed.

In the eighteenth century, heat was thought to have the following properties:

  • It is a subtle substance that cannot be created or destroyed but flows from one body to another when they are in contact.
  • Heat behaves as an elastic fluid, and its parts repel each other but are attracted by the particles that make up bodies, and this attraction depends on the nature of each body.
  • Heat can be supplied in a “sensitive” or “dormant” state. In the first state, heat surrounds the particles like an atmosphere. In a dormant state, it combines with particulate materials in ways similar to chemical combinations. For many, heat was a chemical element.

Joseph Black and the Development of the Modern Theory of Heat

One of the pioneers in the construction of the modern theory of heat was the Scottish physicist and chemist Joseph Black (1728-1799). He introduced the concepts of specific heat and latent heat of vaporization of substances. He also found that different substances have different heat capacities. Black also influenced his pupil and assistant, James Watt, who used these findings to improve the first steam engine. It would not be until the mid-nineteenth century that new experimental results allowed the construction of a comprehensive theory of heat as energy in transit. However, experiments carried out by Benjamin Thompson (Count Rumford) at the end of the eighteenth century showed that mechanical work could produce heat. This led to the identification of heat as a form of energy and the development of the law of conservation of energy.

The Birth of Chemistry as an Experimental Science

The beginning of chemistry as an experimental science is marked by the work of the school headed by the eminent French chemist Antoine Laurent Lavoisier (1743-1794). From this point on, the history of chemistry is split in two. The milestone was the systematic study of chemical reactions on a quantitative basis and the intention to explain them on an atomistic basis.

The Swedish School of Chemists

In Sweden, where the Royal Society of Sciences in Uppsala was founded in 1710, the development of mining and mineralogy led to the emergence of a school of chemists. Throughout the eighteenth century, they made numerous contributions to the analysis of minerals and understanding reduction processes, finally burying the alchemist’s ideal of transforming base metals into gold. Between 1730 and 1782, they discovered cobalt, nickel, manganese, tungsten, titanium, and molybdenum. In a little over fifty years, they exceeded the number of metals discovered in over six centuries of fruitless alchemical searching. Over time, these metals would be used in the manufacture of strategic materials for technological advancement.

The Phlogiston Theory

Given the practical importance of combustion processes, it is understandable that the first theoretical proposals were strung together to explain what happened during the burning of fuels. In Europe, during the second half of the seventeenth century, the metal industry experienced some expansion, and this development meant that energy costs were based on the logging of forests. Surprisingly, however, combustion reactions were related early to the rusting of metals. In the first decade of the eighteenth century, the phlogiston theory arose, defended by the German physician-chemist George E. Stahl (1660-1734). According to Stahl, phlogiston could be regarded as an elementary principle that is rapidly released by fuel when it burns and during the calcination of metals, or slowly during rusting.

Following the phlogiston theory, a metal was considered a compound substance, while the resulting slag or calx, resulting from the loss of phlogiston, was considered a more elemental substance. The third chemical process of utmost importance at the time, the release of metals by reducing minerals under the action of charcoal and heat, was interpreted as a transfer of phlogiston from the charcoal to the mineral, resulting in a metal rich in phlogiston.

The influence of the phlogiston theory on the early development of chemistry is debated. However, Stahl’s professorial activity at the University of Halle prepared numerous disciples who then spread his doctrines. Moreover, the theoretical effort to integrate two important chemical processes can be seen as positive. However, applying these concepts to interpret experimental results obtained by studying reactions involving gases led to errors and deviations from the objective explanation of the facts.

The Discovery of Hydrogen and the Nature of Air

Henry Cavendish (1731-1810), while investigating the properties of the gas released during the reaction of hydrochloric acid with some metals, speculated about the possibility of isolating phlogiston itself. He based this hypothesis on two properties of the gas: it was the lightest gas known, and it was highly flammable. Cavendish’s merit was in determining some physical constants that allowed for the objective differentiation of one gas from another.

In the mid-eighteenth century, Joseph Black studied the thermal decomposition of limestone. He observed that when limestone was heated, it formed lime and released a gas. He also noticed that the lime produced in this reaction, when exposed to air, regenerated limestone. This was the first clear evidence of the reversibility of a chemical process. It also showed that air must contain the gas that combines with lime to “return” to limestone. However, the conception of air as an inert element prevented Black from fully understanding the process.

The Relationship Between Combustion and Respiration

Further experiments involving the burning of a candle in a closed container were again misinterpreted. It was found that the same gas was released in the decomposition of limestone and that if this gas was collected in a container, the resulting atmosphere could not sustain the combustion process of a candle. In terms of the phlogiston theory, this was interpreted as obtaining air saturated with phlogiston, which prevented burning.

The first link between the burning of a substance and the respiration of an animal was established by Black’s disciple, Daniel Rutherford (1749-1819). He went further in experiments, showing that a mouse could not survive in air “saturated with phlogiston.” To clarify this relationship, it was necessary to break with the notion that air was an inert element that carried phlogiston.

Joseph Priestley and the Discovery of Oxygen

In the 1770s, the English chemist and physicist J. Priestley (1738-1804) conducted experiments that demonstrated a deep bond between combustion and respiration. He found that in an atmosphere composed of the gas released in the burning of a candle (saturated with phlogiston, in which a mouse died), a plant could live. More surprisingly, he showed that the residual air, after a plant had spent long hours in it, was particularly invigorating for a mouse. He also observed that air initially “saturated with phlogiston” and then modified by the action of plants allowed materials to burn more easily. Priestley’s results were the first indications that plants and animals formed a chemical balance that made up the Earth’s atmosphere. The enormous significance of this balance has been slowly realized by humanity. However, in the eighteenth century, the phlogiston theory again imposed a line of thought that led to the interpretation of the collected air as dephlogisticated air, the antithesis of the air isolated by Rutherford.

The Reversibility of Chemical Processes and the Work of Carl Scheele

The chemical reversibility of the process had already been pointed out by Black when studying the decomposition of limestone. In the summer of 1774, Priestley obtained strong evidence in favor of this trend. He found that the solid formed during the reaction of air with mercury, when heated, regenerated mercury and released a gas that could be collected over water and showed the qualities known for invigorating air (dephlogisticated air). This experiment is the cause of the historical controversy surrounding the discovery of oxygen.

Two years earlier in Stockholm, the Swedish chemist C. Scheele (1742-1786) had independently isolated dephlogisticated air, which he more properly named fire air, emphasizing that a candle burned brightly in it and a glowing splinter quickly caught fire. He did not publish his research until 1777, in the book entitled *”Chemical Treatise on Air and Fire.”* This book describes the procedures for determining the composition of air, demonstrating that it is composed of “two types of fluid light.” For the first time, the existence of two main components of air was pointed out: nitrogen and oxygen. This shattered the notion of air as something elementary and inert.

Lavoisier’s Contributions and the Overthrow of the Phlogiston Theory

The experiments of Cavendish, Black, and Priestley have a common denominator: they are intended to penetrate the qualitative understanding of the phenomena they study, and in doing so, they displayed immense imagination and creativity. When Priestley traveled to Paris in 1774 and revealed his discovery of dephlogisticated air to Lavoisier, the French researcher made it clear that air is not an inert element that receives or delivers a substantial principle known as phlogiston, but rather that dephlogisticated air is an element itself. He then conducted experiments with mercury oxide and, in 1775, isolated “pure” air. At this time, he developed the idea that in all combustion, “pure” air is consumed, and the weight of the burned body increases by exactly the same amount as the air absorbed. Lavoisier broke with the phlogiston vision of combustion, and his new understanding of chemical phenomena was based on the results of using a scale to measure the masses of the substances involved in reactions.

Summary of Experimental Findings

Let’s summarize the experimental facts known at the time: when metals such as tin and lead are heated in a closed container containing air, the weight of the “burned” metal increases, and the total weight of the system remains constant. A partial vacuum is created inside the container, and only about one-fifth of the volume of air is consumed. Lavoisier’s interpretation of these facts differed from his British counterparts. He proposed that metals, when calcined, did not release phlogiston but combined with a component of air that corresponds to “pure” air, hence the increase in weight. He named this new element oxygen gas. The residual component of the combustion gas, making up four-fifths of the volume of air and characterized by its relative chemical inertness (Black’s phlogisticated air), was called nitrogen. Finally, the enigmatic flammable gas discovered by Cavendish, which was found in 1783 to burn, producing vapors that condense into drops of water, was named hydrogen. Thus, the elementary substances that Stahl sought to associate with substances combined with phlogiston were recognized.

The Birth of Modern Chemistry

In 1789, almost coinciding with the French Revolution, Lavoisier published his *”Elementary Treatise of Chemistry,”* in which he outlined the quantitative method for performing chemical reactions and proposed the first system of nomenclature for chemical compounds, which still exists in a binomial form. This marked the birth of a new paradigm, crowning a revolutionary process in the field of ideas. Unfortunately, four years later, Lavoisier was executed by guillotine after being accused of being associated with a group of tax collectors that the French revolutionaries considered an instrument of corruption of the hated monarchy.

The Law of Definite Proportions

The century did not close its doors without a representative of the French School, Joseph L. Proust (1754-1826), who, through the systematic use of the scale, discovered that substances combine to form a compound in fixed mass proportions, the so-called law of definite proportions. He showed that compounds have identical chemical compositions regardless of their origin.

Conclusion

The chemistry of the eighteenth century represents a revolutionary process, debuting as an experimental science grounded in the quantitative treatment of results. The nineteenth century would bring a new paradigm for the physical universe, electromagnetism. Again, the most famous mathematicians would provide the instruments to operate on physical quantities and often contribute decisively to the construction of meanings. Chemistry began a dizzying rise, particularly in the second half of the century, in the synthesis of new materials that, in a certain sense, exceeded natural products. All of this will be discussed in the next issue.