200 



CHAPTER XII 



HEAT AND ENERGY. THE KINETIC THEORY 



133. In a previous chapter we have briefly dis- 

 cussed the nature of heat and it was there stated that 

 heat was a form of energy, i.e. that a given quantity 

 of heat is able to perform a definite quantity of work. 

 We already know that a weight of 10 lb., situated 

 at a height of 10 feet from the ground, possesses 100 

 foot-pounds of potential energy. Further, we are aware 

 that this potential energy may be transformed into 

 kinetic energy if the weight be permitted to fall, and 

 this kinetic energy may be made to do work by suitable 

 means, e.g. the weight may be attached to a string 

 passing over a pulley to the axle of which is fixed a 

 small mill, in which coffee or other substance may be 

 ground during the fall of the weight. A hot body 

 resembles the weight in its elevated position. The 

 hot body possesses energy and can perform work such 

 as turning water into steam, the latter being caused 

 to make a grindstone go round by some convenient 

 mechanical contrivance. In thus effecting the perform- 

 ance of work, the hot body loses heat, and, unless 

 a further supply is afforded, its temperature falls and 

 at length reaches that of its surroundings, when its 

 capacity for doing work is exhausted. It then resembles 

 the weight resting on the -ground, at the end of its fall, 

 when the latter no longer possesses either potential or 

 kinetic energy, or, in other words, has parted with 

 its ability to do work. 



