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LVII. Fluorescence and Magnetic Rotation Spectra of Sodium 

 Vapour, and their Analysis. By R. W. Wood *. 



[Plates XI.-XV.] 



PUEVrOUS work, which has been recorded in the Philo- 

 sophical Magazine f, convinced me that a careful study 

 of the remarkable optical properties of the vapour of metallic 

 sodium would, in time, furnish the key to the problem of 

 molecular vibration and radiation. This opinion has been 

 strengthened by the work of the past year, and though much 

 remains to be done, it seems best to place the results already 

 obtained on record. In no other case that I know of is the 

 molecular mechanism so completely under the control of the 

 operator. Its periodicities can be studied in a variety of ways : 

 by absorption, by cathode-ray stimulation, by excitation with 

 light, either white or monochromatic, and lastly by its 

 remarkable selective magnetic rotation of the plane of 

 polarization. 



The vapour is, in every case, that obtained by heating 

 metallic sodium in steel or porcelain tubes, usually highly 

 exhausted. From a study of the dispersion of the vapour, 

 it seems probable that we may be dealing with clusters 

 of molecules with which a certain amount of hydrogen may 

 be associated. 



As I have shown in a previous paper {, if a pool of sodium 

 is heated in a highly exhausted horizontal tube, the top of 

 which is cooler than the bottom, the vapour has an enormous 

 optical density close to the surface of the pool, and a very 

 small density along the roof, the non-homogeneous layer 

 acting as a prism. The only way in which I can reconcile 

 this state of things with the kinetic theory, is to assume that 

 the vapour leaves the metal in the state of molecular clusters, 

 which gradually break up into smaller clusters and eventually 

 into molecules. This is of course only an hypothesis, and I 

 mention it in the present paper merely to indicate that our 

 vibrating mechanism may be an aggregate and not a single 

 molecule. It is also possible that hydrogen atoms are associated 

 with the sodium, for the work on the dispersion indicated that 

 there was present always a small trace of some gas other than 

 sodium, which no amount of pumping would remove ; that 



* Communicated by the Physical Society : read October 26, 1906. 



t " Magneto-Optics of Sodium Vapour/' Phil. Mag. Oct. 1905. The 

 " Fluorescence of Sodium Vapour," Phil. Mag. Nov. 1905. 



\ " A Quantitative Determination of the Dispersion of Sodium Vapour," 

 Phil. Mag. vol. viii. p. 293 (Sept. 1904). 



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