NATURE OF HEAT 67 



its density was decreased, but its mass remained the same. 

 Since the mass remained the same as before heating, and its 

 distance from the earth's center was unchanged, it weighed 

 the same as before. 



When heat was first studied it was thought to be an 

 invisible fluid without weight which worked itself into 

 bodies and caused them to expand in the same way that 

 water affects a sponge or a piece of wood. This fluid was 

 supposed to be driven out by pounding or rubbing. Even 

 the primitive savages knew that fire could be obtained by 

 rubbing two dry sticks together. 



About the close of the eighteenth century an American, 

 Count Rumford, who was boring some cannon for the 

 Bavarian government, showed that the amount of heat 

 developed seemed to be entirely dependent upon the amount 

 of grinding or mechanical energy expended. The old theory 

 of a fluid prevailed, however, until about the middle of the 

 nineteenth century, when a great English experimenter 

 by the name of Joule showed conclusively that the amount 

 of heat developed was due entirely to the amount of energy 

 which apparently disappeared into the heated body. 



We learned in Chapter III that all matter consists of 

 constantly moving particles, or molecules, with spaces be- 

 tween them. When a substance is heated the molecules 

 move more rapidly and strike each other harder. This 

 drives the molecules farther apart and causes the substance 

 to expand. Heat is a form of energy which manifests itself 

 in the motion of these molecules of matter. If a condition 

 could be reached where there was no molecular motion, there 

 would be no heat. 



If we apply sufficient heat to ice, the molecules hit against 

 one another so rapidly and so hard that the ice loses its defi- 



