Existence of the Luminiferous ^Etlxer. 491 



in continuity from the comparatively gross oscillations which 

 produce sound up to those which give rise to radiant heat and 

 light. Why, then, should our aether theory produce a break ? 

 Yet this is what the aether theory does, because all suppose 

 that sound is transmitted by the vibrations of the molecules 

 of matter itself. If, then, we imagine heat and light to be 

 transmitted by the undulations of particles of aether, at some 

 point which we do not know we must have that the trans- 

 mission of the vibratory movement by matter ceases, and its 

 propagation by aether begins. It is unnecessary to say that 

 there is no experimental evidence whatever of the existence 

 of this point. The close relationship between the waves pro- 

 ducing radiant heat and those producing sound is also fully 

 shown in a communication recently made to the Royal Society 

 by Prof. Tyndall*. In this paper it is shown that, like solids, 

 gases and vapours are capable of producing the degradation 

 of waves of light and heat, and converting them into sound f . 

 This view of the phenomenon is also held by Prof. Tyndall, 

 who speaks of the rays transmitted by iodine as being " con- 

 verted into sound." 



In the foregoing pages we have considered some of the de- 

 ductions to be drawn from recent researches. These deduc- 

 tions, one and all, lend support to the molecular theory, while 

 they give to the aether theory a great degree of improbability, 

 this improbability in some cases amounting to almost an abso- 

 lute proof of its non-existence. Into the much discussed 

 question of action at a distance it is not necessary to enter. 

 Whether action at a distance does or does not take place, so 

 far as our present purpose is concerned, is beside the ques- 

 tion. All that it has been the object of the author to show is, 

 that the phenomena of light can be explained simply and com- 

 pletely without calling into existence a hypothetical substance. 



In conclusion, the chief results of this inquiry are briefly 

 stated as follows: — 



(a) The action of the radiometer, depending, as it does, 

 upon the movements of the residual gas, is unaffected by the 

 presence of the aether. 



(/3) The action of heat upon crystalline bodies, whereby 

 their optical properties are changed, is difficult of explanation, 

 unless we assume that the same molecules are concerned in 

 the oscillations which produce heat and also those which pro- 

 pagate light. 



* "Action of an Intermittent Beam of Radiant Heat upon Gaseous 

 Matter." Read before the Royal Society, Jan. 13, 1881. 



t May not the term " sonorescence " be applied to this phenomenon ? 

 See paper in the May Number of this Journal. 



