THERMODYNAMICS. 355 



sity and Elasticity ; Radiant Heat, and Ra- 

 diation ; Diathermancy; Selective Absorp- 

 tion; Reflection from plane and concave surfaces; 

 Refraction ; the Change from radiant into sensible 

 heat; the determination of Absorbing, Reflecting 

 and Radiating Powers, and their Mutual Re- 

 lations ; Sympathetic Vibrations. ' 



V, 



THERMODYNAM ICS. 



561. Definition of Thermodynamics. Ther- 

 modynamics is the branch of science that considers 

 the connection between heat and mechanical work. 

 It has especial reference to the numerical relation between 

 the quantity of heat used and the quantity of work done. 



5O2. Correlation of Heat and Mechanical Energy. 



We know that heat is not a form of matter because it can be 

 created in any desired quantity. We must continually remember 

 that it is a form of energy. When heat is produced some other 

 kind of energy must be destroyed. Conversely, when heat is de- 

 stroyed, some other form of energy is created. Considered as heat 

 merely, this agent may be annihilated ; considered as energy, it 

 may only be transformed. The most important transformations of 

 energy are those between heat and mechanical energy. The process 

 of working these transformations will be considered directly. It is 

 to be noticed, however, that while we may be able to effect a total 

 conversion of mechanical energy into heat, we are not able to bring 

 about a total conversion of heat into mechanical energy. 



563. Heat from Percussion. A small iron rod 

 placed upon an anvil may be heated to redness by repeated 

 blows of a hammer. The energy of ths moving mass is 



