1871.] Molecules, Ultimates, Atoms, and Waves. 461 



and their connection of some kind with the aurora 

 borealis, have been put forth in many shapes, but as 

 far as I have learned none afford any explanation of the 

 origin of the electrical disturbance. Without this they are 

 like the vortices of Descartes, which explained the move- 

 ments of the planets by supposing another kind of motion 

 still more incomprehensible. 



Explanations which are more difficult to explain than 

 the phenomena they propose to elucidate only obscure the 

 light of true science, and stand as impediments to the 

 progress of sound philosophy. 



II. MOLECULES, ULTIMATES, ATOMS, AND 

 WAVES. 



By Mungo Ponton, F.R.S.E. 



(Concluded from p. 359). 



Part III. 



f~~ N all the foregoing estimates respecting the wave-lengths 

 corresponding to the fixed lines of the spectrum, these 

 have been regarded as they exist in the free ether ; but 

 when the luminous waves traverse a portion of the ether, in 

 which any transparent substance temporarily subsists, the 

 progress of the wave-motion is, according to the experiments 

 of M. Fizeau, retarded in exact proportion to the refractive 

 index of the substance. Thus, if the refractive index be 2, 

 the speed of transmission will be halved. The returning 

 energy, however — that by which the disturbed particles are 

 restored to their points of rest, will be only a fourth of that 

 which subsists in the free ether — the forces being as the 

 squares of the speeds of transmission. Hence, for any given 

 period of vibration, the excursion of the individual particles 

 will be only a fourth of what they are in the free ether. 



The highest ascertained refractive index is that of 

 chromate of lead (2*974); ^ ut ^ * s uncertain whether the 

 extremely short waves produced by aluminium electrodes 

 can be transmitted by that substance. There can be no 

 doubt, however, about quartz, seeing it is by using prisms 

 of this mineral that the existence of those very minute 

 waves has been ascertained. The refractive index of quartz 

 is 1*5484, so that the shortest aluminium wave would, 

 while passing through this medium, be reduced to 151,150 

 billionths of an inch ; while the ratio of the length of the 



