SECTIONAL TRANSACTIONS.— A. 295 



(d) Dr. H. W. B. Skinner and Dr. S. H. Piper. — Angular Distribution of 



Phot oelectrons from a Copper Crystal. 



This experiment is an attempt to obtain evidence of the coherence of the photo- 

 electrons which are ejected from a crystal when radiation strikes it. If the photo- 

 electrons are the conduction electrons and form a coherent wave in the crj^stal, the 

 work of Rosenfeld and Witmer has made it seem probable that some coherence should 

 occur. The theory is the exact analogue of the Duane Theory of X-ray (or electron) 

 diffraction. We may therefore hope under suitable conditions to obtain maxima in 

 the angular intensity distribution of the photoelectrons. But the theory is subject 

 to the condition that the mechanism responsible for the electrical resistance of the 

 metal does not cause excessive damping of the electron waves in the lattice. If the 

 maxima are obtained we should have evidence of the lattice-binding (as opposed to 

 atomic-binding) of the conduction electrons. 



To realise the best experimental conditions we are forced into the region of very 

 long X-rays, and this makes the technique complicated. The K radiation of Lithium 

 is used for the light source and falls on a copper crystal round which a Faraday 

 collecting box can be rotated. A retarding voltage analysis is made. 



The X-ray tube is incorporated in the Pyrex bulb which contains the whole 

 apparatus. The anticathode is of copper let in with a copper-pyrex seal for water 

 cooling and Lithium can be distilled on to the copper. The apparatus is treated with 

 the methods of modern high-vacuum technique. 



Apart from the main question, the apparatus will be used for experiments on the 

 general problem of the production of photoelectrons by soft X-rays. 



(e) Mr. E. T. S. Appleyard. — Absorption of Light in Ccesium Vapour. 



The Bergen-Davis experiment on the capture of electrons by a particles suggests 

 that when electrons move with respect to a particles there is a very high probability 

 of capture of the electron into one of the orbits of ionised helium. If, following Bergen- 

 Davis, we calculate the energy change which occurs by recapture into the ground 

 state of ionised helium it proves to be equal to twice the ionisation energy of ionised 

 helium. Hence, if this energy reappears as radiation the emitted radiation must have 

 twice the frequency corresponding to the series limit of ionised helium. 



If the results of Bergen-Davis are correct, the converse of capture should be 

 absorption of this frequency and ejection of an electron with a definite energy, a process 

 which should occur with great probability. Caesium vapour is experimentally 

 convenient to study on account of its low ionisation potential. 



A study of absorption of caesium in the ultra-violet is therefore being made using a 

 flourite spectrograph and a high power hydrogen discharge tube to provide a continuous 

 spectrum. 



(/) Mr. S. H. Piper and Dr. T. Malkin. — X-ray Investigations with Long- 

 chain Compounds. 



Routine investigations of the composition of various natural fats and waxes by a 

 comparison of their long spacings with those of pure synthetic standards and mixtures. 

 Intensity distributions in the various orders of reflection allow the position of a side 

 chain to be determined (Shearer. P.R.S. 1925, 2194). 



(g) Dr. L. C. Jackson and Dr. C. F. Powell. — An Attempt to detect 

 Metastable Molecules in Active Nitrogen. 



When metastable excited atoms or molecules impinge on a metal surface, they set 

 free electrons therefrom, the maximum energy of which is given by the difference 

 between the energy of the metastable atom or molecule and the work function of the 

 metal surface. This effect is being employed in an attempt to detect the " S™6tastable 

 Nj molecules in active nitrogen predicted by Carlo and Kaplan's theory. A molecular 

 beam of active nitrogen falls on a nickel target and sets free electrons from it. These 

 electrons are collected on a nickel box surrounding the target and a velocity analysis 

 of them is made. Preliminary results have been obtained. 



