Chapter 2 ‘Atoms of Fire’: Galileo’s Unachieved Theory of Heat and the Beginnings of Thermometry (c.1603–1638)
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Abstract
Galileo never developed a systematic theory of heat in his works, nor of the atomic structure of matter. All we know about it can be derived from Galileo’s correspondence with Giovan Francesco Sagredo (1612–1615), from Il Saggiatore (1623), from Benedetto Castelli’s letter to Ferdinando Cesarini (1638), and from Vincenzo Viviani’s Racconto istorico (1654). The limits and unresolved issues of Galilean atomism have been thoroughly discussed in the literature. My paper is focused on Galileo’s mechanistic-corpuscular account of heat and related phenomena (water ebullition, air expansion, the glowing of hot charcoal and metals, etc.) as effects produced by subtle, mobile, acute, and indivisible minima of fire penetrating the inter-atomic vacua of material substances. The hypothesis was not original, as the connection between heat and motion at microscopic level already belonged to the tradition of ancient and early modern corpuscularianism. Galileo’s specific contribution is to be found in his indirect quantification of the heat released or acquired by a material body, given the impossibility of a direct method to measure the amount of fire corpuscles emitted or absorbed. Such an attempt exposed Galileo to challenging objections from his contemporaries: if hot bodies did not seem to be heavier than cold ones, heat must not be identified with fire itself, but with the effects of igneous atoms streaming through larger particles (friction, agitation, disruption, and transport). The conceptual shift towards studying the action of fire on material bodies in terms of measuring their temperature, by means of the already known phenomenon of thermal dilatation, was the key step leading to Galileo’s thermoscope (ca. 1603) and to its first medical application as graduated (proto-)thermometer by Sanctorius in the 1610s.