APPENDIX 357 



energy. There is only one kind, or form, of energy 

 which presents itself to our aided or unaided intuitions, 

 that is kinetic energy. Bodies that move possess this 

 energy represented by their motion : they can be made 

 to do work, that is, their energy can be transformed into 

 other forms of energy. All things are in motion. A 

 gas consists of molecules incessantly moving with high 

 velocity, and colliding and rebounding from each other. 

 The energy of a gas is the sum of one-half of the masses 

 of all the molecules, multiplied by the squares of the 

 velocities of all the molecules, that is, ^mv 2 . This is 

 also the kinetic energy of a projectile, or of a planet 

 revolving round the sun. Kinetic energy is that of the 

 uniform, unchanging motion of some entity possessing 

 mass, but we must extend our notion of mass so as 

 to include immaterial, imponderable entities such as 

 electrons. 



This energy cannot be destroyed or created the 

 law of conservation of energy. This is a principle or 

 mode of our thought. We are unable scientifically or 

 philosophically to think of an entity ceasing to be. 

 Dreams and phantoms show us entities which are real 

 while they last, but which cease to exist. If we do 

 attempt to think of entities that appear from, or dis- 

 appear into, nothing, we surrender the notion of reality. 

 The more we think of it the more clearly we shall see 

 that the things which we call real are the things which 

 are conserved. 



Yet energy, to our immediate intuitions, seems to 

 disappear. A flying bullet strikes against a target and 

 becomes flattened out into a motionless piece of lead. 

 A red-hot piece of iron cools down to the temperature 

 of its surroundings. A golf-ball driven up the side of 

 a hill comes to rest in the grass. A current of electricity 

 passing through water is used up, that is, electricity 



