Investigation of Superficial Resonance of Sodium Vapour. 1027 



(3) The probable width of t'he spectrum lines emitted by 

 the resonating vapour. 



Apparatus employed. 



The source of light for exciting the resonance was a Meker 

 burner surrounded by an iron chimney perforated with a 

 rectangular aperture. The burner was fed at the base with 

 a spray formed by an atomizer operated by compressed air. 

 A nearly saturated solution (30 grs. to the litre) of NaCl 



was diluted to ~ ± i, ^ , -^ ^ and -L_ an d these solu- 



tions introduced in turn into the bulb of the atomizer, 

 previously well rinsed out with a solution of the concentration 

 employed. 



An image of the window in the iron chimney was formed 

 on the wall of the bulb by an aplanatic condenser of the type 

 described recently by one of us *, having a diameter of 

 11 centimetres and a focus of 3 2 cm. for parallel light. 

 For divergent light, as in the present case, the source and 

 image are each 25 cm. from the lens. 



As the sodium bulbs used in these experiments are very 

 easily made, and are extremely convenient for illustrating 

 resonance radiation, it may be well to 

 Fig- 1- devote a few words to the manner of 



preparing them. The bulbs are 5 cm. 

 in diameter, drawn down to 1 mm. 

 capillaries as shown in fig. 1. The 

 sodium must first be heated to fusion 

 in a small test-tube and poured out on 

 a cool surface. A piece about 2x2x2 

 mm. is introduced into a small piece 

 of very thin- walled glass tubing, closed 

 at one end, and this capsule placed in 

 the lower tube A, which serves as the 

 distillation chamber. The lower end of 

 A is now closed in the flame of a blast- 

 lamp, and the tube sealed to the 

 mercury pump. After exhausting to a 

 pressure of '001 mm. the bulb is heated 

 for four or five minutes with a large 

 Bunsen flame, the pump working all 

 the while. It should be heated as hot as possible without 

 collapsing. After the bulb has cooled off, the flame is care- 

 fully applied to the chamber A, and the sodium distilled into 



* Dunoyer, Journal de Physique, iii. p. 4(38 (1913). 



To Po/hp 



