WOOD. — SPECTRA OF SODIUM VAPOR. 



237 



the plane of polarization for wave-lengths agreeing with that of the 

 absorption lines, not all of the absorption lines showing this rotatory- 

 power, however. The arrangement of the apparatus for showing the 

 bright-line magnetic spectrum of sodium is shown in Figure A, 



A piece of thin seamless steel tubing of such a diameter as to slip 

 easily through the hollow cores of the electromagnet, from which the 

 conical pole-pieces have been removed, is procured. A short piece of 

 small brass tubing is brazed into one end, through which the tube is 

 exhausted. 



A lump of sodium the size of a walnut is melted in an iron crucible, 

 and poured out into a V-shaped trough made of thin sheet iron. As 

 soon as the bar is solid it is placed in the iron tube, one end of which 



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Figure A. 



has been previously closed with a small piece of plate glass cemented 

 on with sealing-wax. The tube is introduced into the magnet, the 

 sodium bar pushed to a position midway between the helices, and the 

 other end closed with a piece of glass in a similar manner. The ends 

 of the tube should be coated while hot with sealing-wax before the 

 introduction of the sodium. One has then only to wave a Bunsen 

 flame over them and press on the piece of glass, previously heated ; 

 the sealing-wax should come into optical contact with the glass to 

 insure an air-tight joint. The tube is now connected with an air- 

 pump which will produce a vacuum of a millimeter or two. If the 

 air-pump leaks, it is a good plan to place a glass stopcock between 

 the pump and tube to prevent the entrance of traces of air after ex- 

 haustion. For purposes of demonstration it is sufficient to heat the 

 tube gradually with a Bunsen burner turned down low. In the present 

 work, however, where constancy of temperature was essential, electrical 

 heating was invariably used. 



The light from an arc-lamp, made parallel by a lens, is passed 

 through a Nicol prism, the steel tube, and a second nicol, after 

 which it is brought to a focus by means of a second lens upon the 



