and the lie so nance Radiation of Electrons. 515 



it is probable that the sodium would eventually eat its way 

 through brass. The latter metal can, however, be used if a 

 large steel tube cannot be procured. The tube should be 

 2 or 3 inches in diameter and about 2 feet long. A small 

 piece of brass tubing is brazed into one end, through which 

 the tube can be exhausted to a pressure of a £ew T millimetres, 

 for it is only at low pressures that a fluorescence of any 

 brilliancy can be obtained. A piece of clean sodium, obtained 

 by melting the metal in a crucible and pouring it out on a 

 plate, is introduced into the tube, and the ends closed with 

 plate glass, cemented on with sealing-wax. One window 

 should be attached before the introduction of the sodium. 

 The following method gives the best results : — Heat the end 

 of the tube and apply the melted sealing-wax smoothly to 

 the rim, building it up to a depth of about 2 mms. Stand 

 the tube on end, heat the glass plate, wave the flame over 

 the wax until the surface melts, and then immediately apply 

 the plate. The wax must be in optical contact with the 

 metal and the glass, otherwise the joint is sure to leak. It 

 is a good plan to go around the joint with a very minute 

 pointed gas-flame, heating the wax until it runs freely over 

 the surfaces. A mercurial or mechanical pump can be used 

 for the exhaustion. If the pump leaks ever so slightly, a 

 glass stopcock should be put between it and the tube, and all 

 the joints made of glass and sealing-wax. With a little 

 practice the whole apparatus can be set up in fifteen minutes ; 

 and the tube, once prepared, can be put away for future 

 demonstrations, care being taken to close it hermetically with 

 w r ax. 



Sun or arc-light is focnssed in a slightly oblique direction, 

 by means of a lens, directly over the lump of sodium, which 

 should be brought to the centre of the tube. On heating 

 the tube, the brilliant spot of green fluorescent light can be 

 observed through the opposite end, care being taken to keep 

 the eye out of the emergent beam of white light. Tubes of 

 this description, while perfectly satisfactory for demonstration, 

 are unsuitable for long continued spectroscopic study, and 

 various devices w^ere accordingly tried. 



The apparatus in its final form consisted of a seamless tube 

 of thin steel 3 inches in diameter and 30 inches long, with a 

 steel retort at its centre in which a large amount of sodium 

 could be stored. The retort was made by fitting two circular 

 disks of steel to a short piece of tubing, just large enough 

 to slip snugly into the larger tube. The circular ends of the 

 retort were provided with oval apertures as shown in fig. 1 

 (p. 517). The retort was half filled with sodium, the molten 



2N2 



