The nature of heat As we saw in Chapter 4 (p. 74) Bacon thought he had shown that heat was a form of motion. But despite his prestige another theory came to be favoured l . It was held that heat was a material substance, probably some sort of fluid and possibly an element. Some philosphers, for example Robert Boyle, tried to show that heat had weight so that bodies were heavier when they were hot. Others, who did not consider weight to be an essential property of matter, (see quotation on p. 67) suggested that heat was weightless so that bodies remained the same weight however hot or cold they became. There was no general agreement; indeed for a short time there was yet another theory which suggested that there were two different fluids, one producing heat and the other cold, though by the eighteenth century it had become generally accepted that cold was a deficiency of heat. The fluid responsible for heat was called 'caloric' and the caloric theory of heat survived into the nineteenth century.