460 PHYSIOLOGICAL PHYSICS. [Chap. xxxv. 



Rumford showed that a thermometer suspended 

 in a globe exhausted of air was affected by heat out- 

 side of the globe, just as it would be if the globe were 

 filled with air. Heat can pass, then, through a 

 vacuum. It can be propagated in a way that is 

 neither conduction nor convection, by what is called 

 radiation. Now we have seen that air is not- 

 necessary for the propagation of light, which is held 

 to be due to vibrations of a subtle elastic ether per- 

 vading all space. By the vibrations of the same 

 ether, heat also is held to be propagated. So that 

 heat reaching us from the sun or from any other 

 source of heat is independent of the atmosphere, 

 and travels through it by wave-like movements of 

 ether without necessarily affecting it. The motion is 

 communicated from the heated body, whose particles 

 are in a state of vibration, to the ether by which it is 

 surrounded. So that when we hold our hands before 

 a fire, and experience its warmth, we are to imagine 

 we see countless waves of ether breaking against 

 our hands, and throwing their sensory nerves into 

 a state of agitation, which we become aware of as 

 a sensation of heat. When the agitation is of mode- 

 rate amount it is pleasurable ; when it becomes 

 too intense it is painful, and we have the sensa- 

 tion of burning. Radiant heat is subject to the same 

 laws as light. It undergoes refraction and reflection, 

 as light does. In fact, the difference between light 

 and heat, as we have seen (page 335), is that the vibra- 

 tions of light are much more rapid, and the wave- 

 lengths shorter. Thus the red end of the spectrum, 

 and the part beyond it, are rich in heat rays, which have 

 a rapidity too slow to be perceived as light, but which 

 can be shown to be present by the heating effects. 

 The notable experiment of heating platinum to incan- 

 descence by the dark rays of the spectrum has been 

 mentioned (page 335). Even as the dark rays of heat 



