Principles, elements and substances

La influència filosòfica en la revolució química

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When Lavoisier spoke of oxygen he did not have the same idea in mind as a contemporary chemist would. The difference between one concept and another is a fundamental problem when studying chemical theories from before and after the chemical revolution. Empiricist philosophy –the idea that all knowledge of the world comes through the senses– led Lavoisier to redefine the goals of chemistry and insist that it deal only with tangible substances. It is this shared assumption that chemistry only deals with the materials of our experience that enabled this science to progress continuously despite further theoretical changes and allows us to say that the oxygen we observe today is the same substance described during the chemical revolution.

A major theme of the Enlightenment movement of the eighteenth century was a distrust of the elaborate but highly speculative system building that had been the pride of seventeenth-century rationalist natural philosophers. Prominent thinkers such as Denis Diderot and the abbé de Condillac pushed for the natural sciences to become more empirical, to rely more on experiment and less on pure reason.

Condillac argued that knowledge is built entirely from sense-experience but formed via logical analysis, involving the decomposition of an idea into its simplest parts as well as the reverse, the synthesis of more complex things from simpler ones. He recognised that language played an important role in this process and that a clear logical language is necessary for the complex ideas involved in scientific theories.

Antoine Lavoisier (1743-1794) was heavily influenced by empiricist thinkers such as Condillac and Gabriel-François Venel, incorporating their ideas into his method of research. His elegant experiments were crucial in deciding the fate of chemical theories that posited invisible principles, such as the phlogiston theory of combustion. Furthermore, Lavoisier put Condillac’s method of analysis by decomposition and re-composition into practice in the laboratory, for instance, when demonstrating that water is composed of more basic components. He showed that it could be decomposed into hydrogen and oxygen but also made a point to form a new batch of water out of those gases in order to make his conclusion undeniable.

Other reformers, such as Louis-Bernard Guyton de Morveau, Antoine-François Fourcroy and Claude Louis Berthollet, recognised that chemical nomenclature was in desperate need of standardisation. When Lavoisier joined this revolutionary movement he brought to it more specific goals –to realise Condillac’s ambition to produce a pure scientific language and a dedication to the new phlogiston-free chemistry–.

Elements, Principles and Theory-Change

Experimental discoveries of the late eighteenth century completely overturned the way chemists saw familiar materials –substances were not just renamed, the elements became compounds and many compounds became elements–. The dramatic theoretical changes that constituted the chemical revolution are sometimes played down as a simple inversion of the earlier theory. In re-characterising the process of combustion, for example, the release of phlogiston (the older principle of fire) can be replaced by the absorption of oxygen (and release of caloric, the new principle of heat). This seems particularly simple for the combustion and reduction of metals, but the conceptual change that came with this inversion was really quite radical: Indeed, before the chemical revolution ores were regarded as simple substances and metals were compounds; after the revolution, metals came to be seen as the simpler substances and ores were the compounds. According to Georg Ernst Stahl’s (1659-1734) phlogiston chemistry, the ore of a metal (its «calx») could be brought to a metallic state (its «regulus») by the addition of phlogiston (or some material rich in phlogiston, such as charcoal): Calx + Phlogiston → Metal (regulus).

Conversely, a metal in its unnatural metallic state could rust or burn, that is, revert to the natural «calx», by releasing phlogiston: Metal (regulus) → Calx + Phlogiston. The oxygen theory of combustion turned this process on its head. Indeed, the reduction of metals came to be seen as the elimination of oxygen from a compound ore (its oxide): Metal oxide + Carbon → «Oxide of carbon» + Metal. And the combustion of a metal was simply the addition of oxygen from the air: Metal + Oxygen → Metal oxide (+ Caloric).

This reform is not as simple as it first appears; the change in theoretical representation is also a change in worldview. Although most metals did not change their names, the chemical concepts before and after were quite different.

This re-conceptualisation of familiar processes was accompanied by an even more radical change in belief about chemical elements. Throughout the Middle Ages, Aristotle’s Earth, Fire, Air and Water was the only orthodox set of elements. Yet certain alchemists had proposed alternative ensembles of simple principles: Paracelsus and his followers believed that all chemical changes were due to the combination of mercury, sulphur and salt; and many French alchemists held that there were five principles: spirit (or mercury), oil (or sulphur), salt, phlegm (or water) and earth. (Proponents of these systems usually admitted that Aristotle’s four elements were the absolute building blocks but that their principles, built from the elements, were fundamental enough to explain all chemical phenomena). Although they used mostly everyday names, the alchemists and early-modern chemists who used these systems of principles did not believe that they were ordinary water, salt, mercury etc. nor even highly purified samples of these common substances. Rather, the principles were supposed to be mystical archetypes: the alchemist’s philosophical or «sophic» mercury was the principle of volatility, found in ordinary mercury but also in all other volatile substances; sophic sulphur was found in all flammable bodies and sophic salt in all solid bodies. Even as late as the seventeenth century, principles such as these were part of chemistry –«phlogiston» was simply Stahl’s name for philosophical sulphur, the principle of flammability–.

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© Mètode
Étienne Bonnot, abbé de Condillac (1715-1780) embraced John Locke’s empiricism –the philosophical view that all knowledge comes originally from the senses–. Condillac claimed that it is language that enables us to convert our sensations into knowledge by putting them in order and thus inspired Lavoisier to reform chemical nomenclature.

«Experimental discoveries of the late eighteenth century completely overturned the way chemists saw familiar materials»

 

 

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Lavoisier’s apparatus for the decomposition of water. Elaborating on experimental work by James Watt and Henry Cavendish in Britain and Gaspard Monge in France, Lavoisier showed conclusively that water was not an element by decomposing it and then recomposing water from its constituent parts.

«Antoine Lavoisier was heavily influenced by empiricist thinkers such as Condillac and Gabriel-François Venel, incorporating their ideas into his method of research»

Lavoisier Keeps It Simple

Although French chemists continued to pay lip-service to Aristotle’s four-element system right up to the generation preceding the chemical revolution, Lavoisier did away with that once and for all when he demonstrated that water could be decomposed into two gases, which he later named hydrogen and oxygen. The nomenclature reform that took place during the chemical revolution was not just about standardising names, Lavoisier replaced earlier systems of principles with his own table of simple substances, but he made no claim about which of these might be fundamental elements. Like Condillac, he doubted that anyone really knew what the smallest parts of matter are, and instead took a pragmatic stance by giving an empirical definition of an element: «if we apply the term elements or principles of bodies, to express our idea of the last point that analysis is capable of reaching, then all the substances that we have not yet been able to decompose are for us elements». Lavoisier’s innovation here was the insistence that the things we call elements be real, tangible substances, unlike the sophic mercury and phlogiston of earlier generations.

Lavoisier’s empirical definition of «simple substances» allowed him to reset the boundaries of chemistry: «The principal object of experimental chemistry is to decompose natural bodies, so as to examine separately the different substances which enter into their composition». This demarcation between practical chemistry and metaphysical matter-theory is much clearer than in most of Lavoisier’s predecessors. For example, Stahl had tried to define chemistry as the science of mixts (substances made of intimately bound components), as opposed to physics, which was concerned with aggregates (collections of merely juxtaposed components).

This attitudinal shift away from metaphysics contributed greatly to the high degree of continuity that chemistry has enjoyed since this colossal revolution. By denying that he had any reliable knowledge of atoms and confining himself to describing observable phenomena, Lavoisier was able to simply avoid debates over how to reduce chemical interactions to physical mechanisms between intricately shaped particles that took place on the border of chemistry and physics in the late seventeenth and early eighteenth centuries (before the domains were completely separate). Chemistry thus became a laboratory science, increasingly distinct from the more speculative parts of «natural philosophy».

Chemistry’s Philosophical Problem of Reference

Modern chemists speak about combustion with language almost identical to Lavoisier’s, but completely different from the phlogiston-based terminology that preceded the chemical revolution. Nevertheless, there are theoretical differences so great that we must be careful not to interpret post-revolutionary chemistry as conceptually identical to our own. There are three major differences that suggest that we should be reluctant even to say that Lavoisier’s oxygen is the same as ours: we now know oxygen to be composed of molecules of two atoms, with further internal structure; our oxygen is not the principle of acidity that Lavoisier’s was; and our oxygen has nothing to do with caloric, whereas under Lavoisier’s system the two were intimately entwined.

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© Museo Galileo. Istituto e Museo di Storia della Scienza, Firenze
«Table of Simple Substances» from Lavoisier’s Elements of Chemistry (1789). Although many substances retained their old names, they were reconceptualised under the new system, with many elements becoming compounds and many compounds becoming «simple substances». «French chemists continued to pay lip-service to Aristotle’s four-element system right up to the generation preceding the Chemical Revolution»

We now know that oxygen gas is composed of bi-atomic molecules, that each atom is composed of eight protons with a certain number of neutrons and electrons. But that is of little concern. Lavoisier all but admitted that the future might hold this sort of discovery; his refusal to speculate about more fundamental levels of analysis suggests that he expected future study to elucidate a finer structure. Thus we can say that deeper studies of the modern element oxygen alone do not disprove his claims.

But it is not simply that our concept of oxygen is more complex than Lavoisier’s –we should not forget that Lavoisier’s theory of combustion was intimately tied to his theory of acidity–. When Lavoisier first published his theory of combustion in 1777 he called it «the oxygen principle» from the Greek prefix oxy-, meaning «acid», and the suffix –genēs, meaning «former». Burning non-metals such as sulphur had long been known to produce acids (what we now know as acid-anhydrides) so Lavoisier inferred that the oxygen gained through burning caused the acidity. Yet we no longer believe that all acids contain oxygen, nor even that oxygen is the immediate cause of acidity in those acidic compounds that do contain oxygen. If oxygen always meant the principle of acidity, then we would have to transfer the name oxygen to whatever substance was believed to be the active agent of acidity: when Humphry Davy discovered that hydrogen was the relevant component of acids, he would have had to name hydrogen oxygen. Gilbert Lewis defined an acid as any chemical species that can share a pair of elections, so he could have used the word oxygen to refer to a whole host of different electrophiles!

Even if we ignore the question of acids and consider oxygen only in its capacity as the agent of combustion, which we still believe it to be, one cannot say that the modern oxygen-based model of combustion is the same as Lavoisier’s oxygen theory because his model also involved caloric, a «subtle fluid». It would be completely wrong to suggest that, when Lavoisier spoke of caloric, he meant anything like the modern notion of heat; indeed, he was well aware of the theory that heat is kinetic energy and rejected it. The net energy change from breaking and reforming chemical bonds, which chemists now use to explain the heat released in combustion reactions, is a very different notion from Lavoisier’s caloric, supposedly a physical substance that heats by insinuating itself between molecules. Not only was Lavoisier mistaken, his whole notion of oxygen rests upon this mistake.

Empiricism Underwrites Scientific Debate

Nevertheless, modern chemists continue to use Lavoisier’s word oxygen. What gives them the right to use that word? Lavoisier’s empirical definition of an element does.

Before the chemical revolution, proponents of three-element and five-element models had very little meaningful debate because their theoretical entities were somewhat mystical and difficult to pin down and because there were different entities for each theory, playing unique roles in their respective systems. In the same way, it can be very difficult for anyone after the chemical revolution to meaningfully criticise the phlogiston theory.

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© Museo Galileo. Istituto e Museo di Storia della Scienza, Firenze
Cover of Physica Subterranea by J. J. Becher, G. E. Stahl (Lipsiae Glenditsch, 1738) in which Phlogiston Theory’s first formulations were made. It depicts a human figure representing the Earth, whose bowels hold seven metals, always influenced by the seven planets. «Only thanks to Lavoisier’s tangible notion of an element can we believe in continuity in our chemical kinds, even across major subsequent changes in chemical theory»

If oxygen were only a chemical principle like those of the alchemists and earliest chemists, then denying the existence of caloric or refuting oxygen’s role in acidity would be to deny the existence of oxygen, but Enlightenment empiricism changed the nature of chemical theories. By 1789 Lavoisier had dropped the word principle in favour of calling it simply oxygen (oxygène) and included it in his table of «simple substances», in line with his embrace of empiricism. This means that even if past and future chemists disagree about some of its properties, they can still talk meaningfully about oxygen or any other substance they can point at.

Only thanks to Lavoisier’s tangible notion of an element can we believe in continuity in our chemical kinds, even across major subsequent changes in chemical theory. Such progress would not have been possible under three-, four- or five-principle systems, or even under most later versions of phlogiston chemistry. These principles could never have been isolated –even hypothetically– thus no one could ever have pointed to a flask and said: «That is phlogiston»; let alone have the name stick across significant theoretical re-conceptualisations. Practical know-how enabled Antoine Lavoisier to disprove the existence of phlogiston but it was Enlightenment philosophy that changed the way chemistry operates. More than anything else, it was Lavoisier’s move from alchemical principles to simple substances that put chemistry on the path towards the modern conception of an element.

BIBLIOGRAPHY
Bensaude-Vincent, B., 2009. «Philosophy of Chemistry». In Brenner, A. & J. Gayon(eds.). French Studies in the Philosophy of Science. Springer. Heidelberg, Dordrecht, New York.
Hendry, R. F., 2005. «Lavoisier and Mendeleev on the Elements». Foundations of Chemistry,7: 31-48.
McEvoy, J. G., 1988. «Continuity and Discontinuity in the Chemical Revolution». Osiris,4: 195-213.
Pyle, A., 2001. «The Rationality of the Chemical Revolution». In Nola, R. & H. Sankey (eds.). After Popper, Kuhn and Feyerabend: Recent Issues in Theories of Scientific Method. Kluwer. Dordrecht.
Sankey, H., 1991. «Translation Failure between Theories». Studies in History and Philosophy of Science Part A,22: 223-236.
Wise, M. N., 1993. «Mediations: Enlightenment Balancing Acts, or the Technologies of Rationalism». In Horwich, P. (ed.). World Changes: Thomas Kuhn and the Nature of Science. The MIT Press. Cambridge, Mass.

Nicholas W. Best. Department of History and Philosophy of Science, Indiana University (USA).
© Mètode, Annual Review 2012.

 

Bibliografia
Bensaude-Vincent, B., 2009. «Philosophy of Chemistry». In Brenner, A. i J. Gayon(eds.). French Studies in the Philosophy of Science. Springer. Heidelberg, Dordrecht, Nova York.
Hendry, R. F., 2005. «Lavoisier and Mendeleev on the Elements». Foundations of Chemistry,7: 31-48.
McEvoy, J. G., 1988. «Continuity and Discontinuity in the Chemical Revolution». Osiris,4: 195-213.
Pyle, A., 2001. «The Rationality of the Chemical Revolution». In Nola, R. i H. Sankey (eds.). After Popper, Kuhn and Feyerabend: Recent Issues in Theories of Scientific Method. Kluwer. Dordrecht.
Sankey, H., 1991. «Translation Failure between Theories». Studies in History and Philosophy of Science Part A,22: 223-236.
Wise, M. N., 1993. «Mediations: Enlightenment Balancing Acts, or the Technologies of Rationalism». In Horwich, P. (ed.). World Changes: Thomas Kuhn and the Nature of Science. The MIT Press. Cambridge, Mass.

© Mètode 2011 - 69. Online only. Elective Affinities - Spring 2011

Department d’Història i Filosofia de la Ciència, Indiana University (EUA).