1 6 TJie Realm of Nature CHAP. 



not appeal so directly to our senses as matter does ; fifty 

 years ago it was unknown and a long course of reasoning 

 was necessary to convince investigators of its existence and 

 reality. Nothing appears more readily produced or destroyed 

 than motion, heat, or light. Motion is destroyed in a 

 railway train by applying the brake, in a bullet by contact 

 with the target. Heat can be destroyed by using it up in 

 a steam-engine ; the visible motion of an engine can be 

 destroyed in turning a dynamo -electric machine ; electric 

 currents can be destroyed in an incandescent lamp ; light can 

 be destroyed by allowing it to fall on a black surface. Hence 

 none of these things is real in itself. But when motion is 

 stopped in a train heat is invariably produced, the wheels 

 sometimes becoming red-hot. When heat is destroyed in 

 a steam-engine, visible motion is produced ; when motion 

 is destroyed in a dynamo -electric machine, electricity is 

 produced ; when electricity is destroyed in a lamp, light is 

 produced ; and when light is destroyed by falling on a black 

 surface, heat is produced. More than this, the amount of 

 heat, motion, electricity, light produced is the precise 

 equivalent of what is destroyed in producing it. All are 

 capable of doing work of some kind, and this power of 

 doing work can neither be created nor destroyed, its amount 

 can neither be increased nor diminished. Energy is the 

 name given to this real thing. 



26. Matter and Energy in Nature. Besides matter 

 and energy nothing has been proved to have an independent 

 existence. The whole of Nature consists of the two grand 

 parts, that which works and that which is worked on. The 

 two are quite inseparable, for work of every kind has been 

 proved to necessarily involve motion through a large or a 

 small space in straight or curved lines, and motion is in- 

 comprehensible except as some piece of matter moving. It 

 is only through matter that we recognise energy, and only 

 through energy that we recognise matter. It has been 

 proved in some cases, and is possibly true in all, that the 

 properties which distinguish different kinds of matter from 

 each other are due to the different amounts of energy with 

 which they are associated. 



