Chemistry and Physics. 145 



excitation. As Wood has shown, mercury vapour can also emit 

 the line A.2536 without the other mercury lines when light of pre- 

 cisely this wave-length falls upon it. So far the behaviour of 

 mercury and sodium run parallel. Now comes the difference. 

 The luminous centres emitting mercury resonance radiation of 

 wave-length 2356 can be distilled away from the place of excita- 

 tion as well and easily as if they had been excited electrically. 

 The centres emitting the resonance radiation of sodium cannot, 

 as we have seen, be distilled away from the place of excitation at 

 all. I cannot at present make any suggestion as to how these facts 

 should be regarded. It seems very strange that the analogy 

 between the behaviour of sodium and mercury should go so far, 

 and then suddenly break down." 



(b) A layer of very dilute sodium vapor, which is quite trans- 

 parent to white light, does not allow the resonance radiation of 

 sodium to pass through it. This accounts for the fact that the 

 spot of superficial resonance produced next to the inner wall of 

 the glass container, at the place where the exciting light enters 

 the vapor, can only be seen from the front, that is, in such a 

 direction as to enable the light to fall upon the retina without tra- 

 versing sodium vapor. " From the back it cannot be seen, as 

 Dunoyer has observed." 



(c) The intensity of the resonance radiation of sodium vapor 

 is dependent upon the strength of the magnetic field in which the 

 vapor is placed. If the exciting flame contains only a small pro- 

 portion of salt the resonance radiation diminishes as the field 

 increases. If the exciting flame is rich in sodium the resonance 

 radiation first rises to a maximum and then diminishes as the field 

 increases. 



(d) When the stimulating source is placed in the magnetic field 

 a dilute flame gives diminished resonance radiation while a con- 

 centrated flame gives increased resonance radiation. In this con- 

 nection Strutt says : " The experiment serves as a striking 

 demonstration of the Zeeman effect, and of the various experi- 

 ments described it is perhaps the best for this purpose. Since 

 the vessel containing sodium vapour has not to get into the space 

 between the magnet poles, it can be made large, and a large patch 

 of resonance radiation produced upon it. The effect of the field 

 on this could quite well be shown to 10 or 20 persons at once." 



All of the phenomena involved in cases (c) and (d) can be satis- 

 factorily accounted for by elementary considerations involving a 

 knowledge of the number of Zeeman components of each D line, 

 of the wave-length of each component in a given magnetic field, 

 of the breadth of the lines, and of the self-reversals of the lines. 

 — Proc. Roy. Soc, vol. xci (A), p. 388, June, 1915. h. s. tj. 



7. The ft-Rays from Radium P. — By making use of an 

 a-ray electroscope and by taking special precautions Lise Meit- 

 nee has succeeded in demonstrating electroscopically the exist- 

 ence of the /3-rays from RaD. In collaboration with von Baeyer 



Am. Jour. Sci. — Fourth Series, Vol. XLI, No. 241. — January, 1916. 

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