MODES OF DIFFUSING HEAT. 347 



particles of matter and fills intermolecular space as well. It is 

 called luminiferous because it is the medium by which light is 

 propagated, it serving as a common carrier for both heat and light. 

 We have seen ( 426) that the velocity of sound depends upon two 

 considerations, the elasticity and the density of the medium. The 

 enormous velocity with which the ether transmits heat and light as 

 wave motion (about 186,000 miles per second), compels us to assume 

 for the ether both extreme elasticity and extreme tenuity. 



545. Radiant Heat. We have seen that the mole- 

 cules of a heated hody are in a state of active vibration. 

 The motion of these vibrating molecules is communicated 

 to the ether and transmitted by it, as waves, with wonder- 

 ful velocity. Thus, when you hold your hand before a fire, 

 the warmth that you feel is due to the impact of these 

 ether-waves upon your skin ; they throw the nerves into 

 motion, just as sound-waves excite the auditory nerve, and 

 the consciousness corresponding to this motion is what we 

 popularly call warmth. Heat thus propagated by the 

 ether, instead of ~by ordinary forms of matter, is 

 Radiant Heat. TJie process of propagation 



is called radiation. t 



546. The Transmission through a 

 Vacuum. Iladiant heat will traverse a 

 vacuum. We might infer this from the fact 

 that the sun radiates heat to the earth. It may 

 be also shown experimentally. 



(a.) A thermometer is sealed air-tight in the bottom 

 of a glass globe in such a way that the bulb is near the 

 centre of the globe. The neck of the flask is to be p IGi 2 6i. 

 about a yard long. The apparatus being filled with 

 mercury and inverted over a mercury bath, a Torricellian vacuum 

 is formed in the globe and upper part of the tube. The tube is 

 then melted off above the mercury. When the globe is immersed 

 in hot water, the thermometer immediately indicates a rise of tern- 



