[ 41 ] 



IV. An Inquiry into the Cause of the Interrupted Spectra of 

 Gases. — Part II. On the Absorption- spectrum of Chloro chromic 

 Anhydride. By Gr. Johnstone Stone y, M.A., F.R.S., Se- 

 cretary to the Queen's University in Ireland, and J. Emerson 

 Reynolds, M.R.C.P.E., Keeper of the Mineral Department, 

 and Analyst to the Royal Dublin Society*. 



Contents. 

 Section I. Introductory. 

 Section II. On the periodic time of one of the motions in the molecules 



of Chlorochromic Anhydride . 

 Section III. On the character of this molecular motion. 

 Section IV. On the perturbations which it suffers. 

 Section V. Conclusion. 



Section I. Introductory, 



1. /^VNE of the authors of this communication endeavoured f 

 ^~J in 1867 to call attention to the circumstance that 

 wherever the spectrum of a gas consists of lines of definite wave- 

 lengths there must he periodic motions in the gas, and that mo- 

 tions of this kind can exist only within the individual molecules 

 of the gas; and more recently J he has pointed out that from 

 each periodic motion there will usually arise several lines, and 

 that the lines which thus result from one motion will have periods 

 that are harmonics of the periodic time of the parent motion. 



2. In our endeavours to bring this theory and its various con- 

 sequences to the test of observation, we commenced with absorp- 

 tion-spectra, for the examination of which the apparatus at our 

 immediate command was best suited. The apparatus consisted 

 mainly of the great Grubb spectroscope of the Royal Dublin 

 Society, and of the appliances in the laboratory of the Society 

 for keeping up an abundant supply of the oxhydrogen limelight. 



3. The chief obstacles we anticipated were those that arise 

 from the extreme closeness with which the lines are often found 

 to be ruled, and those to be expected from the complexity of 

 spectra; for usually lines belonging to several distinct systems 

 are presented together to the eye within the same field of view, 

 and this makes an apparently confused maze of lines, from 

 which it is difficult to pick out those that are to be referred to 

 any one motion in the gas. Accordingly, as a preliminary step, 



* Communicated by the Authors, having been read before the Royal Irish 

 Academy, June 12, 1871. 



t Phil. Mag. vol. xxxvi. (1868) p. 132. 



X Phil. Mag. vol. xli. (1871) p. 291, and Proceedings of the Royal Irish 

 Academy of January 9, 1871. Report of the British Association for 18/Oj 

 p. 41. 



