Language: Concepts and Conventions
Much of the language used to describe the effects of heat was developed at a time when it was thought that heat was itself an invisible and indestructible fluid-like substance. This caloric was able to flow into (or out of) a body, thereby raising (or lowering) its temperature. In addition, caloric could sometimes flow into a body without raising its temperature, as when ice melts into water of the same temperature. This water was then thought to contain a quantity of hidden, or latent, heat which must be extracted in order to re-freeze the water. Perhaps unsurprisingly, the rise of the mechanical theory of heat in the mid-nineteenth century posed a number of conceptual and linguistic problems. First, should one even speak of heat flow? Remarkably, such language is still commonplace in the twenty-first century, despite the fact that almost nobody believes in the caloric theory of heat from which it was derived. Second, if heat itself is not a conserved substance, then what is the fate of the (so-called) heat which is added to a body—for example when a container of gas is heated over a fire? How much of the added heat goes into motion of the gas molecules (internal kinetic energy)? How much of it goes into changing the configuration of the molecules (internal potential energy)? How much of it goes into moving the walls of the container which confine the gas (external work)? And how much of it simply escapes into nearby bodies, heating them up? In the reading selection below, taken from the appendix to the sixth memoir of The Mechanical Theory of Heat, Clausius attempts to introduce terminology which more accurately reflect the new mechanical theory of heat. Has Clausius’ terminology stuck?