In the 1700s, most scientists thought heat was a fluid called caloric that flowed between objects. In 1798, the American-born scientist Benjamin Thompson (1753–1814), also known as Count Rumford, challenged this concept of heat. Rumford managed a factory that made cannons. Figure 1 shows how a brass cylinder was drilled to make the cannon barrel. Water was used to cool the brass so that it did not melt. Rumford observed that the brass became hot as long as the drilling continued, producing enough heat to boil the water. Soon after the drilling stopped, however, the water stopped boiling. When the drilling resumed, the water again came to a boil. Based on his observations, Rumford concluded that heat could not be a kind of matter, but instead was related to the motion of the drill.

Figure 1  Count Rumford supervised the drilling of brass cannons in a factory in Bavaria. From his observations, Rumford concluded that heat is not a form of matter.