A PREAMBLE ON ENERGY 6i 



bombardment, but we neglect this, so far, very exceptional 

 result.) 



19^. As Free, or Available Energy. This is energy in 

 such a mode that it may become the reason, or cause, or condition 

 of the occurrence of physical changes, we know it as — 



The energy of massive material bodies such as winds, running 

 water ; heavy bodies that are actually falling to earth, or gravitat- 

 ing ; material things in motion such as the flywheel of an engine, 

 a locomotive or a motor-car ; the actual movements of uncoiling 

 of a spring ; 



the energy of the molecules of material bodies that are moving 

 so rapidly as to be hot ; 



the pressure of the molecules of a gas that is confined, such as 

 steam in a boiler ; 



the mechanical pressure of radiation, such as light ; gravita- 

 tional energy such as that of a mass of water contained in an 

 elevated tank, or the energy of the weights of a clock ; 



the chemical energy of many substances such as coal, oil, 

 oxygen, etc. When such substances react chemically energy 

 in the form of heat, electricity, etc., becomes manifest ; 



the energy of a magnetic body ; electric energy ; 



radiant energy such as light, heat, electromagnetism. X-rays, 

 etc. ; the energy of radio-active substances. 



All such energies are free to become transformed and, in so 

 doing, they set up physical changes. 



igc. Unavailable, or Dissipated Energy. This is energy 

 which, from the present, human point of view, cannot be made 

 to transform so as to set up what we call here physical changes. 

 It is — for instances : 



the energy of motion of the earth and moon in their revolutions 

 and rotations (but see Sections 5, igd) ; 



the energy of low-temperature heat such as that of the ocean ; 



much cosmic radiation (see Section 89). 



igd. Relativity of the Modes of Energy. Further con- 

 sideration will show that the modes of energy are, to some extent, 

 relative to human power of control. Before 1900 there was no 

 available energy in uranium (or man did not know that there 

 was available energy in this material — and this statement means 

 the same thing as the previous one). At present there is no 

 available energy in lead, but this statement may not be true at 



