ix WORK AND ENERGY 1-25 



Exi'T. 118. Allow air, which having been compressed into a 

 cylinder has again assumed the temperature of the air, to come 

 in contact with a thermopile or other delicate means of measur- 

 ing changes of temperature. Notice the cooling of the com- 

 pressed air when allowed to escape. 



The Energy of Radiant Heat and Light. We shall learn 

 more fully later the reasons we have for regarding these as being 

 of the same nature. We refer under this heading to those forms 

 of energy which travel through space where, as we know, there 

 is no air to convey them in the way in which sound is trans- 

 mitted. But in order to understand the transmission of these 

 forms of energy it is necessary to imagine the existence of a 

 medium which is referred to as the luminiferous ether, or more 

 shortly as the "ether." Certain experiments and observations 

 by different investigators leave no doubt of the existence of 

 this medium, vibrations in which cause light and radiant heat. 

 Sound passes through air by the to and fro vibration, in turn, 

 of the air particles, in the form of a wave as it is called. So 

 radiant heat and light pass through the ether by the succes- 

 sive motions of the constituent molecules of the ethereal 

 medium. 



This takes place with astonishing quickness, for light travels 

 about 186,000 miles per second, or something like 7i times 

 round the earth in this small interval of time. Nor does the 

 ether fill inter-steller space alone, for it must exist in the inter- 

 stices (p. 3.) of those bodies through which radiant heat and light 

 can pass, or how else can light pass through a transparent body, 

 or radiant heat through substances like rock-salt ? 



Radiation can be converted into work, but in a less direct 

 manner than is the case with ordinary heat. It must first be 

 absorbed and heat some material body causing its molecules to 

 oscillate in the manner we have described. This form of heat, we 

 have seen, has a mechanical equivalent, and we can fairly argue 

 that, if the whole radiation is absorbed, the mechanical equiva- 

 lent of the absorbed heat is an exact measure of the energy of 

 the radiation. 



Energy of Electrification. We are careful not to speak of 

 electricity as a form of energy, for whatever electricity may be 

 it certainly is not energy. Though it would be very interesting 

 to discuss the nature of electricity it does not come within the 



