by Douglas Allchin
Perhaps the most renowned case in the history of chemistry is, as dubbed by Conant in his classic classroom case studies, the "overthrow of the phlogiston theory" and its replacement with a theory of combustion based on oxygen--a scientific change so profound that we have labeled it the "Chemical Revolution." Once, burning was explained as the release of an inflammable substance or "principle" known as phlogiston. Chemists in the 18th century might heat (or "calcine") a metal to release its phlogiston, produce the metal's powdered calx (our `oxide'), and then reform the metal by reintroducing phlogiston from another phlogiston-rich source, such as charcoal. When we identified oxygen as a distinct gaseous element, however, and began to understand that it could be "fixed" and could combine with other elements in solid forms, we supposedly adopted a new theory of combustion and abandoned the then-crippled concept of phlogiston. We recall critics who asked, for example, how the loss of phlogiston in combustion could lead to a gain in weight (for us, in the transition from metal to metal oxide). In our conventional interpretations of this episode, the phrase "phlogiston after oxygen" is a contradiction in terms, a droll oxymoron. --And we have sometimes delighted in ridiculing the persons who, according to our scheme, must have imagined phlogiston to have negative weight: "how absurd!"
Recently, historians have deeply reassessed the Chemical Revolution (see Donovan, ed., 1988). They have found that many of the "revolutionary" changes can be viewed in more gradualistic terms. They have noted that Lavoisier's new explanations of acidity, caloric, etc., were replete with problems, recognized even at the time--and that the modern view was not constructed overnight. We have learned the fundamental importance of simply having a coherent system of nomenclature for elements and of seeing the role of gaseous elements, such as oxygen, in chemical reactions (especially striking when reactants and products are weighed). But perhaps the most significant revision to our historical knowledge emerges from the fact that many chemists continued to defend the notion of phlogistion even after the discovery of oxygen. If one traces the concept of phlogiston forward (rather than reading history "backwards" from the answer we now regard as "right"), the picture is very different. It is harder to claim that one view wholly replaced or superceded the other. Though we have tended to cast the phlogistic and anti-phlogistic positions--much as many of the time did--as mutually exclusive rivals, for some chemists the two theories represented complementary and, ultimately, compatible explanations.
Many late defenses of phlogiston are typified by the position of James Hutton. Hutton is more widely known for his geological work. He "found" the unconformity at Siccar Point, for example: a striking case where upended layered rock is capped with yet more horizontal layers. He also promoted a "uniformitarian" view of geological processes. Both contributed to our notions of the age of the earth. But in the late 1700s "science" was not yet fully professionalized and divided into distinct disciplines, and so it is not unusual that Hutton pursued chemical inquiries in addition to natural history. Hutton's broad stylistic emphasis on the context of observations was, in fact, important in perceiving how the notion of phlogiston remained relevant.
Hutton published two dissertations, in 1792 and 1794, where, in direct response to Lavoisier's 1789 Traité, he staunchly defended the doctrine of phlogiston. At the very outset, however, Hutton lauded Lavoisier's discovery of oxygen. He said that it "is to be ranked among the greatest discoveries in physics," one which "does honor to the present age, and infinite credit to the author." Yet, Hutton maintained, there was still a role for phlogiston. He and others even used the term alongside oxygen, sometimes when describing combustion.
Hutton felt "that some important facts, or essential phenomena in the burning of bodies, are not explained in the antiphlogistic theory." When coal burns, for example, he noted:
There are produced two distinct effects; first, by the oxigenating of the gravitating carbonic substance, there is produced fixed air or carbonic acid in an elastic state. Secondly, in thus changing the nature of coal, there is produced a great quantity of light and heat; it is only this last event, or effect, with regard to which there is any difficulty, or any dispute to be made.
"The present chymists, . . ." Hutton claimed, "must necessarily leave some natural appearances unexplained while they give a most accurate analysis with regard to the gravitating matter of bodies." For Hutton and others, the problem was in assuming that the French principles "should be considered as comprehending all the appearance."
The person who translated Lavoisier's Method of Chemical Nomenclature into English, likewise, could not refrain from commenting on the text:
. . . Yet we still allow the absolute existence of a phlogiston. It is still the matter of fire, of flame, of light, and of heat, which is liberated in combustion.
While admitting that this matter of fire might be liberated from "vital air" (oxygen, as Lavoisier claimed), rather than the substance being burned, he insisted:
Yet it is still phlogiston with its most distinguishing attributes. In short, it is still the matter of heat; whether we call it phlogiston, caloric, or in plain English fire.
Since we do not believe in phlogiston today, we are tempted to take belief in it to be pathological. But it is hard to imagine a pathology that permeated a whole community of thoughtful scientists. Rather, we must explore more sympathetically their claims.
Indeed, late advocates of phlogiston were generally interested in the heat and light of combustion, ignition or reactivity, and other aspects of matter and chemical reactions related to what we would call energy. Hutton, for example, was obsessed (it may be fair to say) with light: how it is produced in inflammation; how it can be "stored" in phosphoretic bodies; how it can sometimes generate heat; how it appears in different colors in burning; how it is transformed by plants into animal fuel; and how it ultimately sustains the planet's habitability. Phlogiston, as a form of fixed light--or what Hutton called a variation of the `solar substance'--was central to his thinking. Though Hutton's own ideas were deeply embedded in a natural theology, his concerns about light were shared by others. Joseph Black, Hutton's close friend and an early supporter of the French system, expressed his own reservations: "For my part I now, tho I had reluctance first, find the French theory so easy and applicable that I mostly make use of it, tho it must be confessed that it takes almost no notice of light." Black's acceptance of the new system was withheld specifically where light was concerned.
For Hutton, phlogiston was also important in explaining why coal burned and how energy flowed in nature. He noted how plants serve as fuel for animals producing heat and that they must "compose" or generate phlogiston. He acknowledged how plants fix carbon from fixed air (carbon dioxide) and hydrogen from water, while releasing oxygen. But he emphasized that this process was specifically coupled with sunlight, as shown by Ingenhousz's experiments on plants. That is, light was essential in understanding the process in plants that was the reverse of combustion. Further, Hutton noted the vegetable origin of coal and how the fixed solar substance (or phlogiston) in plants explained why coal burns. Hutton thus used the new system of naming elements to explain what we would call the carbon cycle in nature, while insisting that one must also use phlogiston to explain the far more significant process that biologists today would understand as energy flow through an ecosystem.
Not the least of the considerations regarding energy for late phlogistonists was the relationship between phlogiston and electricity, first suggested in 1758. Electricity was used, for example, as early as 1774 to reduce calxes (oxides) to their corresponding metals. Others used the concept of phlogiston to suggest experiments, such as burning diamonds by electricity or examining how electricity affected the acid of phosphorus. For Hutton, there was a relationship between fire and the release of sparks when electricity crossed through the air. These notions flourished even after the nomenclature using oxygen had been introduced. Even as late as 1809, George Gibbes was claiming that "the principle of the negative side of the galvanic apparatus" is phlogiston.
There were, of course, many for whom the concepts of oxygen and phlogiston were inevitably contradictory. The most noted defenders of phlogiston (Priestley, Cavendish and Kirwan) all advocated positions that could not be reconciled with the new system of elements. But these chemists also interpreted phlogiston specifically in terms of gases. Cavendish and Kirwan, for example, equated phlogiston with inflammable air (or hydrogen) and Priestley saw phlogiston as a component determining the "purity" of the air. Yet the notion of phlogiston did not emerge historically from addressing the role of gases in combustion, nor was it essentially linked to such considerations. Compositionally, one may be able to conceive phlogiston as "negative oxygen." But for many, the important questions were wholly separate from questions about composition and weight. As Hutton declared, the existence of phlogiston was:
a truth that never can appear to those philosophers, who, with the balance in their hands, refuse to admit into the rank of chymical elements substances which do not ponderate, or which are not cognizable in that manner of estimation.
The Chemical Revolution was largely about this shift: honoring the balance as a tool for research, while simultaneously neglecting questions (about energy) that could not be so answered. One problem that was particularly ripe for expansive investigation was pursued, while another that was at the time relatively intractable languished. The Chemical "Revolution," then, was less a conceptual reversal than a practical diversion of focus.
One can easily appreciate that Lavoisier's work led to the Proust's law of constant proportions and then to Dalton's coherent system of elements with atomic weights. Yet this does not discount those who continued to defend phlogiston. Many chemists at the time recognized that the new system using oxygen did not wholly replace earlier explanations of energy relations in reactions. Though Lavoisier introduced the notion of caloric, they found that the concept of phlogiston better underscored the strong relationship and conversions between light, heat and electricity and chemical changes. Even today, it is hard to conceive of the bridge between oxidation, reduction, and combustion without some notion similar to phlogiston. We just call it energy, reducing potential or electrons instead.