The modern concept of temperature would not have come to be without the thermometer. This statement can hardly be doubted, and it was not the intention of this project to do so. However, the thermometer—both the device and its name—already existed at the latest in 1624, while the modern concept of temperature cannot be dated earlier than the late eighteenth century.
This research project concentrated both on the beginning and on the end of the historical period mentioned above, trying to grasp the elements involved in the historical narratives of the invention of the thermometer, on the one end, and of the definition of empirical temperature, on the other.
Francis Bacon's Novum Organum (1620) contains one of the earliest detailed descriptions of the "vitrum calendare," a device also known at the time as "weatherglass," "thermoscopium," "thermomètre," or as a "perpetuum mobile." The apparatus can be described as a non-sealed air thermoscope, in which air is trapped in glass by water, and it responds to changes both in temperature and in atmospheric pressure. However, this distinction established itself only after 1650.
Its first appearance was as an anonymous experiment in pneumatics in Giambattista Della Porta's "Magiae naturalis libri IV" (1558). In Della Porta's description, glass was the new, early modern ingredient of an experimental set-up known since antiquity. Through glass, the observer viewed the surface where water met air slowly rise and fall. What one could see was a holistic response to environmental changes taking the easily quantifiable form of variations in height of a water column, and it could stimulate the imagination in different ways.
For example, the weatherglass could be seen as a "perpetuum mobile" mirroring the tides or demonstrating the origin of winds. It could be a device to predict the weather or an alternative to weight-driven clock-work as a model for world order. For others, it was an instrument allowing a visible quantification of the invisible, namely of Aristotelian-Galenic temperatures and temperaments. Francis Bacon proposed to use it to test the sensitivity of air with respect to the heat of spirit of wine or to the cold of opium. For Galileo's friend Sagredo, it simply "measured heat and cold with a pair of compasses" and could be used to question Aristotelian theories.
In ancient and medieval times, heat and cold were thought of as complementary qualities, whose balance—"temperature"—determined the behaviour of the material world. For modern scientists, heat is a form of energy and cold is its absence, so that an interaction between them is a contradiction in terms. However, the existence of a phenomenon described as "thermal equilibrium" is a key assumption of modern thermodynamics, allowing one to define the concept of "empirical temperature", i.e., the quantity measured by a thermometer. Thus, in the science of heat and cold, the term "equilibrium" seemingly migrated from the explanatory to the descriptive level. How did this happen?
In the late eighteenth century, although interest in heat/cold polarity had faded, thermal phenomena could still be explained as resulting from two independent, attractive and repulsive forces which did not cancel out, but balanced each other. Meanwhile, thermometers had established themselves as a valuable tool in studying natural phenomena, even though thermometric readings remained open to widely diverging interpretations.
In the late eighteenth century, attention focused on the relationship between thermometric readings and the idea of thermal equilibrium between systems. Joseph Black is usually credited with having been the first to explicitly state this connection, namely the fact that a thermometer gives the same reading when applied to objects that have been left in thermal contact for a sufficiently long period of time. In the following decades, this phenomenon played an important role in the heat theories of Johann Heinrich Lambert, Pierre Prévost, and Sadi Carnot. It was the aim of this project to take a closer look at their reflections on the subject, as well as those of other contemporaries.
In the course of the nineteenth century, heat came to be regarded as a form of energy. At the same time, the existence and properties of thermal equilibrium came to be increasingly taken for granted and, in today's textbooks, they are introduced as facts known thanks to everyday experience. Yet, in the science of heat, the importance of thermometrically determined equilibrium states cannot be overestimated, even though the physical significance of such states still poses a problem both for rational thermodynamics and for statistical mechanics. Perhaps significantly, the property of transitivity of thermal equilibrium, although often presented as a trivial observation, has the semi-official status of a "zeroth law of thermodynamics," whose importance was realized only after the official count had been established.