* 
’ 
9 
SECTIONAL TRANSACTIONS.—A. 347 
about 8 millicuries, placed directly above the chamber. Evidence for the 
disintegration 
H? + hv —> H!+ n! 
was obtained in the form of short tracks of the protons emitted in this 
process. About 60 of these tracks have been measured and their angular 
distribution, with respect to the direction of incidence of the quantum, has 
been investigated. ‘These data are discussed in respect of their bearing 
upon the mass of the neutron and the nature of the neutron-proton inter- 
action. 
Dr. P. B. Moon.—The influence of temperature on the properties of 
slow neutrons. 
Fermi and his collaborators in Rome, having discovered the effect of fast 
neutrons in converting many elements into their radioactive isotopes, and 
the enhancement of this effect when the neutrons are made slow by collisions 
in hydrogen-containing materials such as paraffin-wax, suggested that many 
of the neutrons might lose so much energy as to reach thermal equilibrium 
with the material through which they diffuse. This suggestion was sup- 
ported by the work of Bjerge and Westcott, whose investigations of the 
scattering, absorption and diffusion of slow neutrons showed that a neutron 
passing through wax or water should make very many collisions with 
hydrogen nuclei before becoming absorbed. 
The existence of ‘ thermal ’ neutrons was finally placed beyond reasonable 
doubt by the discovery that the amount of artificial radioactivity produced 
by slow neutrons in elements such as silver and copper depends upon the 
temperature of the medium through which the neutrons have been passing. 
For example, the artificial B-activity of silver may be increased by at least 
30 per cent. if the neutrons producing it are cooled from room temperature 
to the temperature of liquid oxygen. No such change could occur with 
neutrons whose velocities were very much greater than those of thermal 
agitation. 
Friday, September 6. 
PRESIDENTIAL ApprEss by Dr. F. W. Aston, F.R.S., on The story of 
isotopes (10.0). (See p. 23.) 
Dr. G. Hertz.—The separation of isotopes by diffusion (11.0). 
The lecturer reported on the experiments he has carried out for separating 
gaseous isotope-mixtures. By a single diffusion through a porous wall the 
ratio of the isotope concentrations can, in general, only be varied by a factor 
which is equal to the square root of the ratio of the molecular weights. 
_ There are two possibilities of arriving at a greater variation of the concentra- 
tion ratio by a single diffusion process. One is: to let the mixture pass 
_ through a tube of porous material, with its outer wall adjoining a vacuum 
_and the other: to apply the method of diffusion in flowing gas. In either 
case it has been possible, with neon-isotopes, to attain a 20 per cent. higher 
degree of concentration of the heavy isotope by a single diffusion. By 
combining a large number of separating units an apparatus has been created 
for the separation of the isotopes. By both methods a practically complete 
Separation of the isotopes of neon and of hydrogen has been achieved. 
Neon 22 has been prepared containing less than 1 per cent. of neon 20. 
In experiments with water-vapour and with methane only a low degree of 
