The Anomalous Dispersion of Sodium Vapour. 159 



sure, repay any one who takes the trouble to repeat them, and will be 

 extremely useful in illustrating the subject of anomalous dispersion to 

 small classes of advanced students, I shall describe in some detail the 

 manner of preparing and using these dispersion tubes which I have 

 found most advantageous. The ends of the tubes are first warmed 

 and thickly coated with sealing-wax ; one of the glass straws is then 

 placed in position, and a small piece of plate glass, previously warmed, 

 pressed against the wax, any crevices around the straw being closed 

 with wax. The leading-in tube is next placed in position, and a piece 

 of freshly cut sodium (about 5 mm. on a side) inserted. The other 

 window is then cemented to the tube, and the current of hydrogen 

 started as soon as possible. Some experience is necessary properly to 

 regulate the hydrogen stream during the experiment. When the tube 

 is first heated much white smoke forms. If a stream corresponding to 

 about one bubble per second is allowed to flow, the smoke will usually 

 clear up in a few minutes and give little trouble. The tube should be 

 heated by means of a Bunsen burner turned down low, the tip of the 

 flame playing against the bottom of the tube. If a sodium flame is 

 placed behind the tube the formation of the vapour can be watched, 

 for it appears almost jet black against the flame, though quite colour- 

 less in white light. The behaviour of the vapour is somewhat 

 peculiar. It grows out from the sodium globule as a dark atmosphere 

 with a sharply defined surface, which clings to the globule with great 

 tenacity. It resembles at first a thick growth of mould more than 

 anything else that I can think of, and a sudden gust of hydrogen 

 scarcely moves it at all. A wire pushed up through it drags a certain 

 amount above the free surface in much the same manner as a stick 

 pushed up through the surface of thick molasses would do. If the 

 tube be inverted the black cloud clings to the upper surface, behaving 

 on the whole like a very viscous mass. It is even possible to dip some 

 of it up on a wire. 



These peculiar physical properties of the metallic vapour I have as 

 yet only studied in a very superficial manner, and I mention them 

 now, only because it appears to me that there is some connection 

 between them and the optical behaviour of the medium. 



I am of the opinion that the apparent viscosity is an illusion, and 

 that the sharply-defined surface is merely the boundary at which 

 either condensation or chemical action (the hydrogen not being pure) 

 is taking place. The process of dipping the vapour up on the wire 

 might be explained by condensation on the wire followed by vapori- 

 sation. A more careful study of the physical behaviour of the vapour 

 will be made some time in the future. 



The apparatus employed in the study of the dispersion of the 

 vapour was essentially identical with that used by Becquerel. The 

 light of an arc lamp was focussed on the horizontal slit of a collimator, 



