SECTIONAL TRANSACTIONS. 
(For reference to the publication elsewhere of communications entered in the following 
lists of transactions, see end of volume, preceding index.) 
SECTION A. 
MATHEMATICAL AND PHYSICAL SCIENCES. 
Thursday, September 24. 
PresipENTIAL AppREss by Sir J. J. Toomson, O.M., F.R.S., on Lhe 
Growth in Opportunities for Education and Research in Physics in the 
past Fifty Years. (See page 19.) 
Prof. H. L. Brosz.—The Cross-section of Gas-Molecules with respect to very 
slow Electrons. 
The effective cross-sections (Q) of the molecules of the saturated hydrocarbons 
with respect to slow electrons have been stated by Briiche to vary to an extent com- 
parable with that which occurs in the case of argon. By using Townsend’s diffusion 
method for determining the Q- /v-curves it has been possible to establish that 
the values of the Townsend temperature factor & for electrons and of their drift 
velocity W are widely different irom the corresponding values for argon. When, 
however, the values of the cross-sections are calculated from the values of k and W 
Z pape — 
5-109 = Vi it is found that the Q- Vv-curves 
assume an argon-like character. Investigation thus shows that in spite of this simi- 
larity, the collisions of electrons with molecules of the methane series of gases are 
yery different from those between electrons and argon atoms. This is borne out by 
the 2-Vv-curves, where A denotes the average fractional loss of energy of the 
electrons during collisions and v, as above, denotes the volt velocity of the electrons. 
The results for methane and ethane are discussed in detail. 
by means of the formula Q = 
Prof. G. P. THomson.—Some recent Experiments on Electron Diffraction. 
In these experiments the diffracted cathode rays are recorded on a photographic 
plate ; the markings on the plate when homogeneous cathode rays are used depend 
on the nature of the diffracting crystal or polycrystalline aggregate, according to the 
laws of wave optics. The etched surface of a single crystal of metal gives a pattern 
of spots, which is regular if the surface is crystallographically simple. The experiment 
is the analogue with electron waves of a well-known optical experiment with crossed 
gratings. To explain it, the surface must be covered with small lumps of metal of the 
order 10-6 cm. in size. It is apparently very difficult to prepare a surface of metal 
of smoothness comparable to that of the cleavage face of an ionic crystal. Thus 
 rocksalt shows quite a different type of diffraction, characterised by a few spots, and 
numerous lines of the kind described by Kikuchi. In both cases the results are in 
full agreement with the de Broglie theory of electron waves, and no abnormalities 
appear, such as the spectra of half orders which have been reported by some experi- 
menters using slow electrons. 
Some other results indicate the possibility of using electron diffraction to measure 
‘the size of crystal aggregates on thin surface layers. This may have interest in the 
study of catalysts. 
