RADIATION. 47 



but a dark colored glass is previously placed before the 

 prism, to absorb the light and allow the heat only to 

 pass. Emerging from the prism, it forms an invisible 

 spectrum of rays beyond. These rays correspond to 

 the different colored rays of light, and have different 

 capacities of passing through different substances, as 

 before stated. But, strictly speaking, they have no 

 color ; they were called blue and red, simply to de- 

 signate their relative position. Heat from very intense 

 sources is mostly violet, and violet heat passes more 

 readily than the other rays through most substances. 

 This accounts for the fact that the heat of the sun is 

 not stopped by glass, and many other substances 

 which stop the heat of a fire. 



97. EFFECT OF DIFFERENT HEAT RATS 

 xrid of the IN MELTING SNOW. Snow melts compara- 

 meiting of tively slowly in the heat of the sun, for 



snow? J J 



the reason mentioned in the last paragraph. 

 Being from a highly heated source, it passes through 

 the snow instead of stopping to melt it. But near a 

 fallen tree melting proceeds more rapidly, because the 

 heat absorbed as violet, is radiated again from the mod- 

 erately heated source as red heat, which, falling on the 

 snow in- its vicinity, is readily absorbed, instead of be- 

 ing transmitted. 



98. BURNING GLASS OF ICE. A lens 



How can gun- 

 powder be iff- sufficiently powerful to ignite gunpowder 



may even be made of ice. In using any 

 lens, it is first to be placed near the object to be ignited, 

 and then withdrawn till the spot of light which it 

 casts is round and very small. The focus to which 



