218 GENERAL SCIENCE 



are made incandescent by the heat of combustion as the 

 vapors and gases burn. In the incandescent and arc electric 

 lights the incandescence is caused by electrical energy being 

 transformed into heat. Much resistance is offered to the 

 passage of the electricity in the filaments of one, and through 

 the air gap separating the carbon pencils of the other. The 

 intimate relationship believed to exist between heat and 

 light as forms of energy is shown in the explanation that as 

 the temperature of a body rises by reason of increase of its 

 molecular activity there comes a time when the ether dis- 

 turbances originating in the quickened movements of its 

 molecules, and radiating outward from the body in straight 

 lines in every direction, become sufficiently frequent per 

 second to affect the optic nerves in our eyes, resulting in a 

 sensation of light and enabling us "to see." Bodies from 

 which these light waves originate are said to be luminous. 

 Most bodies about us are non-luminous, and we see them 

 only because light coming to them is reflected to us and into 

 our eyes. Much of the expense of artificial lighting systems 

 lies in the fact that there is large waste involved in production 

 of the heat necessary to incandescence. Illumination by 

 "cold light," i.e., at temperatures as low as that of the light 

 emitted by fireflies, would greatly increase the comfort and 

 cheapen the cost of household illumination. 



SUMMARY 



The plumbing, heating, and lighting of modern American homes 

 exhibits in a striking way the contributions of scientific achievement 

 and inventive genius to the comfort and well-being of this generation. 

 The homes of our grandparents, and the palaces of kings a century 

 ago, lacked these things. What we now consider necessary to our 

 well-being they had not dreamed of. 



Coal like the other natural resources of timber, ores, petroleum, and 

 natural gas, is not exhaustless in quantity. As a source of power by 

 reason of the heat liberated when it is burned, coal is at present indis- 



