PROCEEDINGS 



OF THE 



CmnMtrjje ||{jU0S0pj)ral Satbtg. 



Note on some Physical Properties of Sodium Vapour. By 

 P. V. Bevan, M.A., Trinity College. 



[Bead 1 May 1905.] 



In the Philosophical Magazine, Vol. ill. 1902, Professor R. W. 

 Wood described some exceedingly interesting experiments with 

 sodium vapour in which he was able to observe and measure the 

 anomalous dispersion of the vapour of this metal. In his paper 

 he describes the formation of a curious cloud of sodium vapour, 

 and in a later paper, Phil. Mag. 1904, Vol. viii. p. 296, he states 

 " the vapour of sodium appears to possess a cohesion almost as 

 great as that of a liquid. Even in a vacuum tube it appears to 

 have a free surface." This behaviour of a vapour was so extra- 

 ordinary that it seemed worth while to see under what conditions 

 this phenomenon took place, and the experiments to be described 

 in this note were made with this purpose. 



It is a very simple matter to observe this cloud. All that is 

 necessary is to heat a piece of metallic sodium in a tube or bulb 

 that is partially exhausted, or that contains hydrogen, and observe 

 through the tube an ordinary bunsen flame which is coloured 

 yellow by sodium. The sodium vapour given off absorbs the 

 sodium light and the cloud is easily visible. 



The cloud possesses in general a very sharply defined boundary 

 across which diffusion of sodium vapour does not appear to take 

 place. The first point investigated was to decide whether this 

 cloud had its definite boundary in vacuo. It was found that a 

 considerable pressure of gas was necessary for the cloud to possess 

 this definite boundary. If the sodium be heated in a high 



VOL. XIII. PT. III. 10 



