92 SCIENTIFIC RECREATIONS. 



The reflection and refraction of heat are ruled by the same laws as the 

 reflection and refraction of light. A convex lens will bring the heat or 

 light to a focus, and will act as a burning-glass if held in the sunlight. 

 Gunpowder has been ignited by a lens of ice, and more than one house has 

 been mysteriously set on fire at midday in summer by the sun's rays shining 

 through a glass globe of water containing gold fish, and falling upon some 

 inflammable substance. Professor Tyndall performed a series of experi- 

 ments of a very interesting nature, described in his book, " Heat considered 

 as a Mode or Motion," and showed the transmutation of invisible heat rays; 

 into visible rays, by passing a beam of electric light through an opaque 

 solution, and concentrating it upon a lens. The dark heat rays were thus 

 brought to a focus, all the light was cut off, and at the dark focus the heat 

 was found to be intense enough to melt copper and explode gunpowder. 

 This change of invisible heat into light is termed Calorescence. 



It was Sir William Herschell who discovered that there were heat rays 

 beyond the red end of the spectrum. When light is split up into its com- 

 ponent rays, or decomposed, Sir William found that the heat increased as 

 the thermometer passed from violet ta indigo, and so on to blue, green, 

 orange, and red, and the last were the hottest, while beyond the spectrum 

 there was heat even greater. A Heat Spectrum was thus discovered, and by 

 comparing, by means of the thermometer, the various degrees of heat within 

 certain limits, Professor Tyndall found that the invisible Heat Spectrum is 

 longer than the visible Light Spectrum. 



