SECT. XXVII. TERRESTRIAL HEAT. 257 



SECTION XXVII. 



Terrestrial Heat Radiation Transmission Melloni's experiments Heat 

 in Solar Spectrum Polarization of Heat Nature of Heat Absorp- 

 tions Dew Rain Combustion Expansion Compensation Pen- 

 dulum Transmission through Crystals Propagation Dynamic 

 Theory of Heat Mechanical equivalent of Heat Latent Heat is the 

 Force of Expansion Steam Work performed by Heat Conserva- 

 tion of Force Mechanical Power in the Tides Dynamical Power of 

 Light Analogy between Light, Heat, and Sound. 



THAT heat producing rays exist independently of those of light 

 is a matter of constant experience in the abundant emission of 

 them from "boiling water. They dart in divergent straight lines 

 from flame and from each point in the surfaces of hot bodies, in 

 the same manner as diverging rays of light proceed from every 

 point of those that are luminous. According to the experiments 

 of Sir John Leslie, radiation proceeds not only from the surface 

 of substances, but also from the particles at a minute depth 

 below it. He found that the emission is most abundant in a 

 direction perpendicular to the radiating surface, and that it is 

 more rapid from a rough than from a polished surface : radiation, 

 however, can only take place in air and in vacuo ; it is altogether 

 imperceptible when the hot body is enclosed in a solid or liquid. 

 Heated substances, when exposed to the open air, continue to 

 radiate heat till they become nearly of the temperature of the 

 surrounding medium. The radiation is very rapid at first, but 

 diminishes according to a known law with the temperature of the 

 heated body. It appears, also, that the radiating power of a sur- 

 face is inversely as its reflecting power ; and bodies that are most 

 impermeable to heat radiate least. Substances, however, have 

 an elective power, on]y reflecting heat of a certain refrangibility. 

 Mr. Grove gives paper, snow, and lime as instances, which, 

 although all white, radiate heat of different refrangibilities, while 

 metals, whatever their colour may be, radiate all kinds alike. 



Rays of heat, whether they proceed from the sun, from flame, 

 or other terrestrial sources, luminous or non-luminous, are in- 



