RECENT ADVANCES IN SCIENCE 289 



thermionic current due to ionisation of the gas molecules by 

 collisions. 



Optics. — In the Proc. Roy. Soc. (A. 91, A. 630, June) 

 the Hon. R. J. Strutt describes experiments carried out on 

 the resonance radiation of sodium vapour. It appears that the 

 centres emitting the resonance radiation of sodium vapour 

 excited by the D lines are not persistent enough to be carried 

 along with the vapour when it is distilled away from the 

 place of excitation. This is in sharp contrast, not only with the 

 behaviour of sodium vapour excited electrically, but also with 

 the behaviour of mercury vapour whether excited optically 

 or electrically. The resonance radiation is invisible through 

 even a very dilute layer of sodium vapour placed in front of 

 it ; moreover it is changed in intensity when the vapour is 

 placed in a magnetic field, a result which is quite in accordance 

 with the Zeeman effect. 



The same number contains an account of some experiments 

 by T. R. Merton on the origin of the " 4686 " series. A con- 

 siderable controversy has been raging for some time around 

 the type of atom responsible for this series. The earlier view 

 ascribed it to hydrogen. Lately, under the influence of the 

 Bohr theory of the atom, the view that the series is produced 

 by helium has been gaining ground, and some spectroscopic 

 work by Mr. Evans (referred to in the last number of Science 

 Progress) supports it. Mr. Merton employs a vacuum tube 

 containing helium and hydrogen at low pressure giving the 

 helium lines, hydrogen lines, and 4686, and determines the 

 highest order of interference of the spectrum lines at which 

 the fringes produced by the method of Fabry and Perot 

 remain visible. This method enables the observer to calculate 

 the ratio of the mass of a known atom emitting a radiation 

 to the mass of an atom emitting another radiation, when he 

 knows the limiting orders at which these two radiations show 

 interference fringes. The author, while pointing out possible 

 sources of error, arrives at the conclusion that the 4686 line is 

 due to systems of subatomic mass, having a mass about 

 one-tenth of that of hydrogen. 



Heat. — T. Carlton Sutton (Phil. Mag. April) discusses the 

 validity of two formulae which have been used in recent work 

 for determining the latent heat of vaporisation of liquids. 

 By applying the Clausius-Clapeyron equation to the Biot 



