Early Seventeenth-Century Atomists on Heat and Fire

In the early seventeenth century, new empirical knowledge together with the revival of ancient atomism prompted philosophers and physicians to reconceptualise theories of heat and fire in corpuscular terms. Two atomists, i.e., Isaac Beeckman and Sébastien Basson, abandoned the Peripatetic doctrine of forms and qualities and advanced a kinetic theory of heat, whereby the shape, size, and motion of material particles explained heat and fire. Galilei, who adopted a mechanical theory of qualities, argued (The Assayer) that heat is merely a name for the action of minute corpuscles that strike the sense organs. Heat is caused by tiny particles entering our flesh; depending on their quantity and speed, they create either pleasant warmth or painful burning. Galileo explored the relationship between light and heat, and maintained that light is made of extremely subtle, almost immaterial atoms that can expand infinitely. Benedetto Castelli, Galileo’s pupil and collaborator, carried out experimental research on the relationship between heat and light and concluded that a brick’s absorption of heat from the sun’s rays depended on the colour of its surface. Castelli’s experiment was repeated by Robert Boyle, who thoroughly explored this topic in his History of Colours (1664)