2 3 o THE POPULAR SCIENCE MONTHLY 



everywhere. In every actual case of motion the moving bodies are 

 subject to friction and to collision, their energy is dissipated and they 

 come to rest. This dissipation of energy is always accompanied by the 

 generation of heat, and experience shows that the amount of heat 

 generated is equivalent to the energy dissipated (first law of thermo- 

 dynamics). It is important to understand that the term dissipation 

 of energy refers to the conversion of mechanical energy into heat by 

 friction or collision. 5 Thus energy is dissipated in the bearing of a 

 rotating shaft, energy is dissipated when a hammer strikes a nail, and 

 so on. The atomic theory enables one to form a clear idea of the 

 dissipation of energy. Thus, the energy of the regular motion of a 

 hammer is converted into energy of irregular molecular motion when 

 the hammer strikes a nail. 



It is worth while to give a statement of the first law of thermo- 

 dynamics reduced to its simplest terms. A given substance is heated 

 by the dissipation of work and brought back to its initial state by 

 being cooled by contact with another (cooler) substance B. Then, 

 if the loss of heat to surrounding bodies is carefully avoided, the 

 thermal effect produced in substance B is exactly the same as would 

 be produced in it if it had been heated directly by the dissipation of the 

 original amount of work. Therefore a substance which is heated by 

 the dissipation of work stores something which is equivalent to the 

 work and which is called heat. 



Gay Lussacs Law and the Air Thermometer. — When a number of 

 closed vessels containing different gases at the same pressure are carried 

 from a cool cellar, for example, to a warm room, they all suffer the 

 same rise of temperature, and all of the gases show the same increase 

 of pressure. That is to say, all gases follow the same law of increase 

 of pressure with increase of temperature, the volumes of the containing 

 vessels being constant. This fact was discovered by Gay Lussac and it 

 is called Gay Lussac's Law. This law affords a convenient basis for 

 the definition of temperature ratios, convenient because not dependent 

 upon any particular gas. The ratio of two temperatures (provisionally 

 defined) is the ratio of the pressures of a constant volume of gas at the 

 respective temperatures. That is, if p and p' are pressures of a con- 

 stant volume of a gas at temperatures T and T, respectively, then we 

 have by definition 



T/T' = V /p' (1) 



The air thermometer is a device for measuring the ratio of two tem- 

 peratures by observing the pressures of a constant volume of dry air 

 at the respective temperatures. 



6 Energy is also dissipated in a wire in which an electric current flows. 



