52 On the Cause of the Interrupted Spectra of Gases. 



of each line is in excess or falls short of the mean periodic time. 

 Thus, if the line we have taken as our zero-line be, as we sup- 

 pose, the 628th harmonic of the fundamental motion, and if, as 

 is likely, its inverse wave-length be 1697*0 (the observed amount) 

 rather than 1697*3 (the calculated amount), it will follow that 

 this line arises mainly from motions with the period 



t x 629 t 



1697-0 2-6979 

 which is somewhat longer than the duration we have assigned* to 



T 



the mean periodic time, 'viz. r It would thus seem pro- 



bable that this line is due principally to those disturbed motions 

 in the molecules of the vapour which continue sufficiently long 

 in a phase in which they have a periodic time about 1*0002 

 times the normal periodic time. 



22. To this branch of the subject belongs also that shading 

 between the lines which is occasioned by still wider departures 

 from the mean periodic time. This shading evidently follows 

 laws intimately associated with the laws which determine the 

 general pattern of the spectrum • for we have noticed that marked 

 excesses and defects in the shading recur between the corre- 

 sponding lines of several successive sections of the pattern. But 

 this is a part of the phenomenon to which we have not yet had 

 time to give sufficient attention, and we therefore merely men- 

 tion it here. 



Section V. Conclusion. 



23. We think that our measures satisfactorily confirm the 

 theory which was recently laid down by one of us as to the 

 cause of the lines which present themselves in the spectra of 

 gases, and that we have ascertained with considerable precision 

 the normal periodic time of one of the motions in the molecules 

 of the vapour of chlorochromic anhydride at the temperature 

 and pressure of the atmosphere. 



In reference to the other matters upon which we have ven- 

 tured to touch, the results at which we have as yet arrived 

 are defective and less secure • and we have entered into them 

 only so far as appeared necessary to explain the methods we 

 pursue. 



* See equation (3) above. 



