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XXXTX. On the Cause of the Interrupted Spectra of Gases. 

 By G. Johnstone Stoney, M.A., F.R.S., %c* 



IN the Philosophical Magazine for August 1868, there is a 

 paper " On the Internal Motions of Gases" f, by the author 

 of the following communication, in which a comparison is insti- 

 tuted between these motions and the phenomena of light, from 

 which the conclusion is drawn that the lines in the spectra of 

 gases are to be referred to periodic motions within the individual 

 molecules, and not to the irregular journeys of the molecules 

 amougst one another. 



Mr. Stoney thinks it possible now to advance another step in 

 this inquiry, and has given to the Royal Irish Academy an ac- 

 count, of which the following is an abstract, of the grounds upon 

 which he founds this hope. 



A pendulous vibration, according to the meaning which has 

 been given to that phrase by Helmholtz, is such a vibration as 

 is executed by the simple cycloidal pendulum. It is, accord- 

 ingly, one in which the relation between the displacement of each 

 particle and the time is represented by the simple curve of sines, 

 of which the equation is 



yrrCo+C^Sm (a? + Ot) , 



where y — C is the displacement of the particle from its central 

 position ; C 1 is the amplitude of the vibration ; re stands for 



2-7T -, where t is the time from a fixed epoch, and r the period of 



a complete double vibration; and a is a constant depending on 

 the phase of the vibration at the instant which is taken as the 

 epoch from which t is measured. 



Now we may not assume that the waves impressed on the 

 aether by one of the periodic motions within a molecule of a gas 

 are of this simple character. We must expect them to be usually 

 much more involved. And whatever may happen to be the in- 

 tricacy of their form near to their origin, they will retain sub- 

 stantially the same complex character so long as they advance 

 through the open undispersiug aether, in which waves of all 

 lengths travel at the same rate. But it would seem that a very 

 different state of things must arise when the undulation enters a 

 dispersing medium, such as glass. 



Let us suppose that the undulation J before it enters the glass 



* From the Proceedings of the Royal Irish Academy, read January 9, 

 18/1. Communicated by the Author. 



t In reading that paper, the reader is requested to correct 16 2 into Vlo" 

 at the end of paragraph 2. 



% By the term undulation is to be understood a series of waves. 



