48 Mr. Jameff Rice on 



substances which enter into combination with one another are 

 also examples of molecular potential enei'gy, and all are capable 

 of being converted into molecular kinetic energy — it is this con- 

 version w^e are dealing with when we speak of heat of reaction, 

 combination, combustion, and so on. Indeed we measure such 

 energies by the amounts of heat procurable by their conversion 

 under well-defined conditions. The work of Joule and his 

 followers has given us, as it were, a currency in which we can 

 measure any amount of energy and in any form, just as in the 

 world of economics we measure the value of all commodities in 

 terms of one commodity — money. All forms of energy are 

 ultimately measurable in terms of heat energy, although in actual 

 experience we may measure an amount of energy in terms of an 

 energy which is not thermal but one whose value in thermal 

 units is known. 



We have l)y no means exhausted all our stores of energy in 

 the short statement just made. Energy, for instance, reaches us 

 from the sun in a form whose importance is manifest to all. It 

 is established beyond doubt that it requires some time to reach 

 us — about 8 minutes in fact. It is decidedly inconvenient for us 

 to conceive a quantity of energy leaving the sun at an instant 

 and turning up on the earth's surface 8 minutes later, if we can- 

 not make it give some account of itself in the meanwhile. We 

 absolutely refuse to believe that it is non-existent in the interim. 

 There is no material substance in the intervening space to attach 

 it to ; so we invent one, put the energy into it, and call it 

 radiant energy. A very arbitrary act, no doubt, and many 

 scientists, even the great Newton himself, had serious doubts 

 about it. Yet the " proof of pudding, etc." Its justification lies 

 in the coordination of our knowledge thereby effected, a coor. 

 di nation whose completeness can only be appreciated by those 

 engaged in the study of physical science. 



To the ])hysicist, indeed, it is not a question of the existence 

 of the ethereal medium ; his difficulty lies in its nature ; his 

 doubts are centred around the properties he is to ascribe to it. 



