Early ideas on the nature of internal energy
It had long been known that nails became hot when hammered into a piece of wood. In the seventeenth century Robert Boyle explained this by saying that the hammer blows set the particles of metal into violent vibration and so concluded that the rise in temperature was simply caused by motion. But there the matter rested, and by the beginning of the eighteenth century a new theory became popular. Heat, as it was called, came to be considered as a weightless fluid known as caloric. This id appealed to men such as the Scottish chemist, Joseph Black, who found it easier to talk about caloric rather than a vague quantity of motion. And so the caloric the gained ground and was destined to occupy a prominent place in the study of t subject for the next 150 years.