RADIATION OF ENERGY. 351 



The principle of the conservation of energy is a principle of equivalence rather 

 than of equality. It states that when a given amount of mechanical work is 

 done a given amount of heat or electrical energy will appear. The equivalence 

 of heat and electrical energy is known only through their respective equivalence 

 to mechanical work. In fact, when we show that every time we do 4.2 joules of 

 work we obtain 1 calorie of heat, we do not prove that 1 calorie is equal to 4 2 

 joules, but simply that they are equivalent quantities of energy. 



Energy appears to me to have no real existence independently of physical 

 objects. Knowledge of the universe comes to each individual only through his 

 senses, and by means of sense impressions. Physical bodies or objects are 

 concepts based upon groups of sense impressions. 1 A physical object is that 

 which appeals to my senses through its attributes. I can feel, touch, taste or 

 see it. A chair, a table or a piece of sugar has a real existence. For the take of 

 brevity, a thing which has real existence may be called an entity. The energy of 

 a body is defined as the power or ability of the body to do work, and is measured 

 by the work done. The ability of a body to do work is a measure of its state 

 or condition. Energy therefore is a measure of the condition of a body. Velocity 

 and momentum are also measures of the condition of a body, and not entities. 



Time and space, also, are not entities, but are the modes under which we 

 perceive things apart. Motion is a combination of space with time and is also 

 a mode of perception. 2 



Matter is a generic or group name for all physical objects. It is not in itself 

 an entity but is the group name for all entities. 



It is impossible to conceive of energy apart from an object. It is a measure 

 of the physical condition of a body or of a system of bodies, and a measure of a 

 condition is certainly not an entity. 



Another property of energy which seems to me to show that it has no existence 

 independently of bodies is that of its complete relativity. For instance, when 

 two elastic bodies are moving with different speeds if they come together they can 

 do work on each other and therefore have kinetic energy with respect to each 

 other. If, on the other hand, they are both moving with the same speed they 

 can never come together and do work on each other and consequently have no 

 relative kinetic energy, however fast they may be moving. 



What do we mean, then, when we say that radiant energy is propagated 

 through space? We simply mean that some of the energy of a certain body 

 disappears, and that an equal or less amount of energy appears in another body 

 at a later time. How it goes from one place to the other, or even if it goes at all, 

 we do not know; we simply know that it disappears and reappears according to 

 certain laws which are known as the wave theory of light. If space were empty 

 there would be no light. The phenomena of light can only exist where there are 

 bodies to emit it, and bodies to receive it. 



1 Pearson, Grammar of Science, pp. 178, 281. 

 1 Ibid., pp. 156, 191, 193, 293. 240. 



