THE KEASON WHY. 127 



When I consider thy heavens, the work of thy fingers, the moon and the stars 

 which thou hast ordained : 



Heat can only be more gradually reduced, by diffusion. 



The solar rays deliver heat to the earth by day, and the heat 

 remains with the earth when the light has departed. 



Heat diffuses itself in all directions. 



Light travels only in straight lines. 



The colours that absorb and radiate both light and heat do not act 

 in the same degree upon them both. Black, which does not radiate 

 light, is a good radiator of heat, &c., &c. 



The oxy -hydrogen light emits a most intense heat, but glass 

 which will transmit the rays of light, will afford no passage to the 

 rays of the heat. 



Heat is latent in all bodies, but no satisfactory proof has been 

 found that light is latent in substances. 



These are only a few of the analogies and distinctions that exist 

 between the two mysterious agents, light and heat. But they are 

 sufficient to supply the starting points of investigation. 



The importance of the heat that attends the solar rays may be illustrated by 

 the experiments performed a few years ago, by Mr. Baker, of Fleet-street, 

 London, who made a large burning lens, three feet and a half in diameter, and 

 employed another lens to reduce the rays of the first to a focus of half an inch in 

 diameter. The heat produced was so great that iron plates, gold, and stones were 

 instantly melted ; and sulphur, pitch, and resinous bodies, were melted under 

 water. 



545. What is the point of heat at which bodies become 

 luminous ? 



The point of heat at which the eye begins to discover luminosity 

 has been estimated at 1,000 deg. 



546. What is the velocity of artificial liglit ? 



The light of a fire, or of a candle, or gas, travels with the same 

 velocity as the light of the sun, a velocity which would convey 

 light eight times round the world while a person could count " one." 



547. A.t ivhat rate of velocity does the liglit of the stars 

 travel ? 



At the same velocity as all other light. And yet there are stars 

 so distant that, although the light of the sun reaches the earth in 

 eight minutes and a half, it requires hundreds of years to bring 

 their light to us. 



