582 Stoney — Cause of Double Lines in Spectra. 



CHAPTER III. 



THE PROBLEM TREATED FROM THE STANDPOINT OF THE ELECTRO-MAGNETIC 



THEORY OF LIGHT. 



Whether we proceed under the crude dynamical hypothesis which we have 

 hitherto adopted, or under the electro-magnetic theory to which we are now to 

 direct our attention, we must distinguish between the motions of or in the 

 molecules which do not affect the luniiniferous aether, and certain others which 

 set up an undulation in it — an undulation which consists of transverse oscillations 

 under the dynamical hypothesis, but of alternations of electro-magnetic stresses 

 under the electro-magnetic theory. Among motions of the first kind, those that 

 do not affect the aether and are not affected by it, we are to include the following : 

 the progressive journeys of the molecules as they dart about between the 

 encounters ; the much swifter translation which carries a molecule of the gas 

 through the eether at the rate of 30,000 metres per second, in common with the 

 rest of the earth; and other motions of a like kind. There are also probably 

 motions in the molecule of a swiftly periodic kind that do not affect the aether, but 

 there are certainly some that do, and it is these that we have to investigate. 



The simplest hypothesis for our purpose is to disregard the motion of the 

 molecule through the aether, whether that which it has in common with the earth, 

 or that which is peculiar to it, such as its darting about in the gas. We may 

 simplify the problem by disregarding these, and may treat the molecule as though 

 it remained at one station in the aether, undergoing internal periodic motions, 

 some of which are of parts that carry charges of electricity with them, and, there- 

 fore, act on the aether and are acted on by it ; so that periodic motions, when set 

 up in these parts, will cause a synchronous motion in the ^ther. Correspondingly, 

 an undulation in the aether of suitable periodic time will set these parts of 

 the molecule in motion, and through them, perhaps other parts of the molecule. 

 The distinction between the motions which do, and the motions which do not, 

 affect the Esther, requires to be taken into account equally on the dynamical 

 hypothesis and on the electro-magnetic theory. 



To pass from the dynamical investigation to the electro-magnetic, attention 

 must be given to Faraday's " Law of Electrolysis," which is equivalent to the 

 statement that in electrolysis a definite quantity of electricity, the same in all 

 cases, passes for each chemical bond that is ruptured. The author called attention 

 to this form of the Law in a communication made to the British Association in 



