126 THE BEASON'WHY. 



" Te are the light of the world. A city that is set on a hill cannot be hid." 

 MATTHEW v. 



541. Are light and lieat combined, in the solar ray ? 



Yes. A ray of light, as well as containing elementary rays thai 

 produce colours under refraction, contains also chemical rays, and 

 heat rays. 



542. How do we know that light and heat are separate, 

 elements ? 



Because we have heat rays, as from dark hot iron, from various 

 chemical actions, and from friction, which are unattended by the 

 development of light. And we have light, or luminosity, such as 

 that of phosphor esence, which is unaccompanied by any appreciable 

 degree of heat. 



But, besides this confirmation, further proof is afforded by the 

 fact, that in passing rays of solar light through media that are 

 transparent to heat, but not to light, the heat rays may be 

 separated from the luminous rays, and vice versa. 



Black glass, and black mica, which are nearly opaque to light, are 

 transparent to heat to the extent of ninety degrees out of a hundred. While 

 pale green glass, coloured by oxide of copper, and covered with a coating of 

 water, or a thin coating of alum, will be perfectly transparent to light, but will 

 be almost quite opaque to heat. These remarks- apply, in a greater or less 

 degree, to various other substances. 



543. In what respects are light and heat similar? 



Both heat and light have been referred to minute vibratory 

 motions which occur, under exciting causes, in a very subtile elastic 

 medium. 



They are both united in the sun's rays. 



They are both subject to laws of absorption, radiation, reflection, 

 and refraction. 



They are both essential to life, whether animal or vegetable. 



Both may be developed in their greatest intensity by 

 electricity. 



They are both imponderable. 



544. In what respects are light and heat dissimilar f 



Heat frequently exists without light. 

 Light is usually attended with heat. 

 Light ma." be instantly extinguished, but 



