212 Historical Review of Experiments 



The quantity of the heat excited in this manner is 

 exactly proportional to the force with which the two 

 surfaces are pressed together, and to the rapidity of the 

 friction. When this force was equal to the pressure of 

 ten thousand pounds, and when the rod was turned 

 with such rapidity about its axis that it revolved thirty- 

 two times a minute, the quantity of heat generated by 

 the continual rubbing of the two surfaces together was 

 extraordinarily great. It was equal to the quantity given 

 off by the flame of nine wax-candles of moderate size 

 all burning together. 



The quantity of heat generated in this manner dur- 

 ing a given time is manifestly the same, whether the 

 globular vessel D is filled with water, and the surfaces 

 of the two hemispheres rub on each other in this liquid, 

 or whether there is no water in the vessel, and the ap- 

 paratus by which the friction is produced is simply sur- 

 rounded by air. 



The source of the heat which is generated by this 

 apparatus is inexhaustible. As long as the rod C is 

 turned about its axis, so long will heat be produced by 

 the apparatus, and always to the same amount. 



If the globe-shaped vessel D is filled with water, this 

 water becomes hotter and hotter, and finally begins to 

 boil. I have myself in this way boiled a considerable 

 quantity of water. 



If this experiment is performed in winter when the 

 temperature of the air is but little above the freezing- 

 point, and if the vessel D is filled with a mixture of 

 water and pounded ice, the quantity of heat caused 

 in a given time by the rubbing together of the two sur- 

 faces can be expressed very exactly by the amount of 

 ice melted by this heat. 



