460 Resonance Radiation of Sodium Vapour. 



photographs the two outside records were obtained by the 

 light reflected from the MgO, and the middle record from 

 the resonance. 



The change resulting from the presence of hydrogen in 

 the bulb was also very marked. In this case both lines were 

 present even when the bulb was at a low temperature. 



Similar investigations were made using D, as the exciting- 

 radiation. The difficulties are greater in this case, as D x has 

 an intensity only half as great as that of D 2 , and it is more 

 difficult to completely exclude the light of the brighter line : 

 moreover, the resonance is only half as bright as with D 2 

 excitation. 



With pure sodium at 210° we obtained a faint trace of D 2 , 

 while with O'l mm. of hydrogen D 2 is about half as bright 

 as Dj. 



On some plates recording the effects with more hydrogen 

 in the bulb, the intensity of tbe two lines was nearly equal. 

 All of the results were verified by repetitions, about fifty 

 plates being taken in all. 



The observations may be summarized as follows: — 



D 2 Excitation. 



Bulb as free from hydrogen as possible. 



At 210° (no trace of D x ) intensity ratio of D 2 to D x 



at least 20 : 1. 

 At 300° ratio of D 2 to Dj about 5. 



Bulb containing hydrogen at 0'25 mm. 

 At 220° ratio of Do to D x about 4. 

 At 300° „ „ 3. 



D 1 Excitation. 



Bulb free from hydrogen. 

 At 220° a trace o£ D 2 . 

 At 300° ratio of B 1 to D 2 about 3. 



Bulb containing 0*1 mm. hydrogen. 

 At 220° ratio of Dj to D 2 about 2. 



Bulb containing 0*25 mm. hydrogen. 

 At 250° ratio about 3/2. 



In conclusion it seems safe to say that the transfer of 

 energy from the excited line to its companion results from 

 molecular collision, either of sodium with hydrogen or sodium 



