Abstract

Much of the attention of philosophy of science, history of science, and psychology in the twentieth century has focused on the nature of conceptual change. Conceptual change in science has occupied pride of place in these disciplines, as either the subject of inquiry or the source of ideas about the nature of conceptual change in other domains. There have been numerous conceptual changes in the history of science, some more radical than others. One of the most radical was the chemical revolution. In the seventeenth century, chemists believed that the processes of combustion and calcination involved the absorption or release of a substance called phlogiston. On this theory, when an ore is heated with charcoal, it absorbs phlogiston to produce a metal; when a metal is burned, it releases phlogiston and leaves behind a residue, or calx. The concept of phlogiston derived from a quite complex Aristotelian/medieval structure that included three concepts central to chemical theory: sulphur, the principle of inflammability; mercury, the principle of fluidity; and salt, the principle of inertness. All material substances were believed to contain these three principles in the form of earths. The phlogiston theory held that in combustion, the sulphurous earth (phlogiston) returns to the substance from which it escaped during some earlier burning process in its history, and that in calcination the process is reversed. However, chemists also knew that a calx is heavier than the metal from which it was derived. So, the theory implies that phlogiston has a negative weight, or a positive lightness. This did not present a problem, though, because it was compatible with the Aristotelian elements of fire and air (the others being earth and water), which were not attracted towards the center of the earth. The development of the oxygen theory of combustion and calcination by Lavoisier in the late eighteenth century has been called the chemical revolution because it required replacing the whole conceptual structure with, for example, different concepts of substance and element and new concepts of oxygen and caloric. In the new system, it was no longer possible to believe in the existence of substances with negative weight. According to the oxygen theory, oxygen gas is released in combustion and absorbed in calcination. Thus calx is metal (substance) plus oxygen, rather than metal minus phlogiston. The concept of phlogiston was eliminated from the chemical lexicon. The reconceptualization of chemical phenomena that took place in the chemical revolution made possible the atomic theory of matter, which, as we know, posits quite different constituents of material substances from the principles central to the earlier conceptual structure. Just what constitutes conceptual change, how it relates to theory change, and how it relates to changes in belief continues to be a subject of much debate. Clearly, though, as the preceding example demonstrates, the three are significantly interrelated.