346 MODES OF DIFFUSING HEAT. 



542. The Third Mode of Heat Diffusion. When a 



hand is held over a heated stove, heat is carried to the hand by con- 

 vection and given up to the hand by conduction. But when the 

 hand is held before the stove it is also heated, not by conduction, for 

 fluids have little conducting power ; not by convection, for convec- 

 tion currents are ascending. How then does the heat get to the 

 hand ? The query comes to us with still greater force when we 

 consider the transmission of the sun's heat to the earth, for the 

 atmosphere can carry it by neither conduction nor convection. 

 More important yet, how does the sun's heat reach the earth's 

 atmosphere? This heat passes through the atmosphere without 

 heating it. If along a poker thrust into the fire the hand be moved 

 toward the stove, the temperature increases. If a person ascend 

 through the atmosphere toward the sun the temperature diminishes. 

 We have here a wholly new set of thermal phenomena, heat pass- 

 ing through a substance and leaving the condition of that substance 

 unchanged. 



543. Liiiminiferous Ether. In the case of actual, 

 mechanical energy, the rapid motion of bodies, e. g., a 

 vibrating guitar string, is partly carried off by the air in 

 the shape of sound. When the sound reaches the auditory 

 nerve it represents a certain amount of mechanical energy 

 of motion which has been carried from the string by the 

 air. ^ere is sufficient reason for believing that 

 there is a medium pervading all space which car- 

 ries off part of the invisible motions of molecules, 

 just as the air carries off a portion of the motion 

 of moving masses. This medium, called the luminiferous 

 ether, occupies all space. The gaps between the sun, the 

 planets and their satellites are filled with this ether. " It 

 makes the universe a whole and renders possible the inter- 

 communication of light and energy between star and star." 



544. Density and Elasticity of the Ether. This ether 

 is wonderful, not only in its incomprehensible vastness but equally 

 so in its subtleness. While it surrounds the suns of unnumbered 

 systems and fills all interstellar space, it also surrounds the smallest 



