176 SCIENCE AND THE HUMAN MIND 



As we have seen, the caloric theory, by which heat 

 was pictured as an imponderable fluid, played a 

 Heat and the useful P art in suggesting and making in- 

 Theory of telligible experiments on the measurement 

 Energy. Q q uan tities of heat. But, as a physical 

 explanation, it had always seemed insufficient to the 

 more acute natural philosophers. Investigations by 

 Count Rumford on the boring of cannon, and by 

 Sir Humphry Davy on the rubbing together of two 

 lumps of ice, had shown that heat was developed 

 by friction to an unlimited extent in a way 

 inexplicable on the caloric theory. But more direct 

 experimental proof was needed. James Prescott 

 Joule, during the years between 1840 and 1850, 

 measured accurately the amount of heat produced 

 by the expenditure of a known quantity of work, and 

 proved that, however the work was expended, the 

 same amount of heat was produced. When these 

 experiments became known, an exact equivalence 

 between heat and work was generally recognized, and 

 heat accepted as a form of motion. It was now 

 regarded as the average energy of motion of the 

 molecules of a body, and when, as in a gas, the 

 molecules are supposed to be free from each other's 

 influence, it is easy to obtain a vivid physical con- 

 ception of the state of affairs. Owing to moment- 

 ary collisions, at any instant there will be molecules 

 moving with any number of different velocities in 

 all sorts of directions. The impact of the molecules 

 on the walls of the vessel constitutes the pressure 

 of the gas, while the average energy of the molecules 

 measures its temperature. On these lines Joule 

 gave an elementary mathematical kinetic theory 



