354 MODES OF DIFFUSING HEAT. 



that a body that can absorb any particular kind 

 of heat rays can radiate the same kind. 



Note. It will be seen further on, that obscure heat rays differ 

 from light only in the matter of wave length. Most of the phenomena 

 of one may be shown to pertain to the other. Absorption, radiation, 

 reflection, transmission and refraction of rays follow the same laws, 

 whether the agent be called heat or light. Other phenomena, such 

 as interference and polarization, more satisfactorily studied with 

 luminous rays, have been produced with obscure rays. It should 

 be borne in mind that the most delicate instruments yet made are 

 far less sensitive to obscure heat than is the eye to light. A candle 

 flame may be seen a mile away ; any one might well be pleased with 

 an instrument that would detect its heat at the distance of a rod. 



QUESTIONS. 



1. Good conductors feel warmer or cooler to the touch than poor 

 conductors of the same temperature. Why ? 



2. Why is it so oppressively warm when the sun shines after a 

 Bummer shower ? 



3. Why is there greater probability of frost on a clear than on 

 a cloudy night ? 



4. Can a good absorbent be a good reflector of heat ? Is a good 

 absorbent a good radiator, or otherwise ? 



5. Explain why the glass covering of a hot-bed or conservatory 

 renders the confined air warmer than the atmosphere outside. 



6. From your own experience, decide which is the better con- 

 ductor of heat, linen or woolen goods, oil cloth or carpet. 



7. Why are the double walls of ice-houses filled with sawdust ? 

 Why do fire-proof safes have double walls inclosing plaster-of- 

 Paris or alum ? 



8. Why do furnace men, firemen and harvesters wear woolen 

 clothing ? Explain the use of double windows. 



9. How may heat be diffused ? How is the surface of the earth 

 and how is the atmosphere heated ? Can you boil water in a vessel 

 with heat applied from above ? Why? 



Recapitulation. In this section we have considered 

 Conduction; the conductivity of Fluids; Con- 

 vection; the Luminiferous Ether, its Den- 



