Lightj Heat y and Electricitij. 215 



amplitude of vibration in the interstices of the planes of cleavage, 

 in which direction v/e must necessarily conclude that the setherial 

 waves (which act on the molecules) must also have their vi-- 

 brations. 



I have classed fluorescence with phosphorescence: — -1st, because 

 none of the fluorescent rays are transmitted ; and 2nd, because 

 they are not polarized even when the light which produced them 

 was polarized. Now 1 find it difficult to admit the mere change 

 of refrangibility of the incident rays ; but to believe them to be 

 also depolarized is, in my mind, impossible. 



So far as I am aware, the alleged ^^ polarization of heat '^ is 

 the only serious objection to my views j if calorific waves be 

 normal (not transverse) there can he no polarization. Melloni has 

 shown that Forbes^s experimental proofs cannot be accepted ; and 

 I have never been satisfied with Melloni^s own experiments. The 

 velocity of heat-waves is, 1 believe, the point which must decide 

 the question ; but I conceive it to be possible that the conside- 

 ration of those waves whose vibrations are intermediate between 

 the direction of progress and its perpendicular may account for 

 some small indications of heat polarization, as such waves un- 

 doubtedly would partake in some degree of the qualities of both 

 luminiferous and calorific undulations. 



Having thus discarded molecular vibrations as causing ^^ calo- 

 rific waves,''^ in order to satisfy the experimental researches of 

 Eumford, Davy, Joule, Tyndail, and other philosophers we have 

 only to substitute ^'temperature^'' in the place of heat; for, to 

 my thinking, temperature is simply an accident of matter, and 

 represents the state of motion of its molecules under the influence 

 of their associated £8ther. I look, therefore, on ^' temperature of 

 space ^^ as a pure myth ; and I believe a zero of temperature to 

 be an utter impossibility j for, in a world constituted like our 

 universe, absolute rest is unattainable, and equally absurd with 

 '^perpetual motion.'^ 



I believe that the dark lines in the solar spectrum are due to 

 the interference of waves of like refrangibility, which difi'er in 

 their periods by an odd number of semiundulations, and consider 

 the same explanation applicable to certain phenomena of heat 

 which have generally been referred to the theory of exchanges. 



According as the quality of the radiation (as regards velocity 

 of vibration) approaches more nearly to that of the aether asso- 

 ciated with the molecules of matter through which we transmit 

 the rays, the greater will be the tendency, I conceive, to an in- 

 crease of interference. Thus, for instance, the most transparent 

 glass loses almost entirely its power of transmitting light when 

 its temperature is so raised that it becomes "self-luminous;" 

 and even below a red heat its transparency is greatly diminished. 



