214 Dr. H. Hudson on Wave-Theories of 



every indication is opposed to the existence of matter in the or- 

 dinary sense. 2nd. To suppose an setherial wave impinging on 

 a body to act primarily on its material niolecules (instead of upon 

 the associated aether) appears to me a most gratuitous and un- 

 likely assumption. 3rd. In the case of free transmission of light 

 or heat-^Yaves through any substance vrithout produciug (so far 

 as we can perceive) the slightest disturbance of the molecules of 

 the body (evidenced by phosphorescence or increase of tempera- 

 ture)^ I find it impossible to admit TyndalFs conclusion^ viz. 

 "that free transmission indicates discord and absorption con- 

 cord."'^ If the vibrations of an impinging setherial wave 

 interfere with those of the aether (associated in the molecules), 

 there must be disturbance of the latter, which will, no doubt, 

 cause disturbance of the molecules also, and hence, on my 

 view, "phosphorescence or increase of temperature.''^ 4th. The 

 existence of very intense light without any appreciable heat 

 (as iu moonlight concentrated by lenses or mirrors), as also 

 of great heat from perfectly dark matter, compelled me to dis- 

 card the idea that light and heat could only differ in refran- 

 gibility or wave-length. The fact also that a compound beam 

 of light and heat can be so easily deprived of either of its con- 

 stituents by transmission through different substances, as well 

 as the different march of the luminiferous and calorific rays (of 

 the same refrangibility) in respect to their intensities, all pointed 

 to the same conclusion, viz. " that both sets of waves could not 

 be ascribed to the same function of the setherial vibrations ;^^ and 

 from this consideration arose the idea that calorific waves are 

 probably due to getherial vibrations in the direstion of the wavers 

 progress. 



A theory, however, may be plausible (as I hope this will 

 appear) and yet require proof, v/hich the theory itself, if true, 

 ought to supply. I have already indicated an experimental test 

 in reference to two media, and beg now to suggest that Foucault^s 

 experiment of the revolving mirror (vvith lenses of rock-salt and 

 a thermopile) may be applied to ascertain the velocity of calorific 

 waves. The resistance to wave-progress where the vibrations 

 are normal must be considerably greater than in the case of 

 transversal vibrations ; and, as a consequence, the progress of the 

 calorific wave ought to be considerably slower than that of the 

 luminiferous wave. 



I may add that the " conduction of heat ^' in crystals is, so far 

 as I am aware, better in the direction oi i]iQ\v planes of cleavage 

 (as in bismuth) than in any other direction, and, with wood, in 

 the direction of the fibre, facts v/hich appear to me in favour of 

 my views, that calorific vibrations are in the direction of wave- 

 progress ; the molecules gliding more easily and attaining greater 



