﻿Magnetic Rotation Spectra of Sodium Vapour. 501 



The arrangement of the apparatus for showing the bright- 

 line magnetic spectrum of sodium is shown in fig. 1. 



Fisr. 1. 



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Z"-h, 



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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 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 1 as 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-pnmp which will pro- 

 duce a vacuum of a millimetre 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 

 exhaustion. 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 tem- 

 perature 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 slit of a spectroscope. With the steel 

 tube cooled below the point at which sodium vapour forms, 

 the nicols are set for complete extinction, and the field of 

 the spectroscope becomes dark. The tube is now heated and 



