30 president's address. 



the results of his classic researches, as detailed in his published 

 lectures, may still be considered as the clearest and most illumi- 

 nating account of the phenomenon in question."^ The cause of 

 this difference in behaviour of the two kinds of gases lies in the 

 fact that the simple gases do not respond to the vibrations of the 

 portion of the spectrum carrying the heat waves; they behave to 

 these much as they do to the still shorter waves of visible light: 

 while on the other hand, the larger molecules constituting the 

 compound gases, while quite as transparent to the light waves as 

 those of the simple gases, are capable of vibrating in unison with 

 the obscure heat waves, and so, by transferring the energy of 

 these to themselves, offer an efiectual barrier to their progress. 

 The action of the compound gases towards obscure heat rays is 

 much the same in character as that of a sheet of metal w hen 

 placed so as to intercept the heat from a fire; the metal is 

 capable of responding to the vibrations of the heat waves, and so 

 absorbs their energy to produce heat vibration in its own 

 substance. 



We may now very briefly consider a very interesting 

 phenomenon, the acceleration or retardation of the waves of 

 radiant energy when the body emitting them is moving in a 

 direction to or from the observer. A very commonly noticed 

 analogous case is the sharpening or flattening of the pitch of a 

 railway whistle when approaching or receding. If the source 

 from which the radiant energy is being emitted be approaching 

 the observer, the waves right along the sptctrum are accelerated 

 by the precise amount of the forward motion, while the radiation 

 from a receding body will be drawn out or retarded. A ray 

 from a stationary body, which reaches the eye as yellow, will, 

 from a body approaching at a sufficiently rapid rate, be accelerated, 

 and appear as some colour nearer the blue, while from a receding 

 source it will be retarded in the direction of the red. Obviously, 

 any absorption lines in the light from a body moving in the line 

 of sight, will be displaced in one direction or another, according 



* Tyndall, " Heat a Mode of Motion," 6th ed., 1880, p.B21 et seq. 



