Heat and Energy. The Kinetic Theory 203 



to this, substances are all composed of molecules, 

 separated by varying distances, these latter being 

 greatest for gases and least for solids. The molecules 

 are in rapid motion, the freedom and rate of motion 

 being again greatest in the case of gases, least in that 

 of solids. The application of this hypothesis to the 

 explanation of some of the various phenomena observed 

 during the course of our study of heat follows. 



136. Heated, bodies were found to expand under 

 the influence of the heat supplied. It is supposed that 

 the molecules of a substance, when heated, are made to 

 move with greater speed. As a result of this, they repel 

 one another, during their frequent collisions, with 

 greater force, thus exercising greater pressure upon 

 their surroundings. If this increase of pressure be 

 not counterbalanced in any way, the general effect of 

 the combined increase of pressure in the part of the 

 total number of molecules in any given body is to 

 push away its surroundings to some extent, i.e. the 

 body as a whole expands. 



If the heated body be in the solid state, we have 

 found that, as a general rule, it changes its state when 

 a certain definite temperature is reached. Liquid mole- 

 cules possess greater freedom of movement than those 

 of solids, and in order to overcome partially the cohesion 

 of the molecules of a solid substance, a large store of 

 extra energy must be supplied to the solid, which then 

 assumes the liquid state. This corresponds to our 

 observation that a very large quantity of energy in 

 the form of heat is used up in changing a solid into a 

 liquid without rise of temperature. This energy, as 

 already demonstrated, becomes again available in the 

 form of heat when the liquid substance is caused to 

 solidify once more. 



