SECTIONAL TRANSACTIONS. 
SECTION A.—MATHEMATICAL AND PHYSICAL 
SCIENCE. 
(For references to the publication elsewhere of communications entered in the 
following list of transactions, see page 464.) 
Thursday, August 7. 
1. Prof. A. Fownrr, F.R.S.—The Spectra of Ionised Elements. 
The paper gives a summary of recent work on the spectra of ionised and 
multiple-ionised atoms. In the case of silicon, four successive spectra have been 
sufficiently observed to permit the complete or partial classification of the lines 
in series. In passing from Si IV to Si I, the effects of adding external electrons 
one by one are exhibited under somewhat simpler conditions than in the regular 
sequence of the neutral atoms of Na, Mg, Al, and Si, since the charge and the 
mass of the nucleus remain constant. Similar changes in the spectra of other 
elements have been observed, and series have been worked out for C II. The 
ionisation potentials deduced from the series of silicon and carbon have an 
important application in connection with the determination of the temperatures 
and densities of stellar atmospheres. 
2. Dr. F. L. Mouter and Dr. P. D. Foore.—Critical Potentials and 
their Interpretation. 
Available data on the normal energy levels of atoms, particularly in the range 
inaccessible to spectroscopic methods, are summarised. Results obtained in this 
laboratory on radiation potentials of gases and stages in the excitation of spectra 
are compared with critical radiation potentials of solids and other data on soft 
X-ray limits. Moseley diagrams of these limits give curves of the type predicted 
by Bohr. 
Some questions as to the interpretation of critical potentials can be settled 
on the basis of this correlation of results. 
3. Prof. J. C. McLennan, F.R.S.—Recent Studies in Band Spectra 
and their Bearing on the Structure of Molecules. 
4. Mr. R. H. Fowxier.—Mechanisms of Excitation, Ionisation, and 
Dissociation in Statistical Theory. 
This paper attempts a systematising survey of current theory and experi- 
mental evidence about the processes occurring, or mechanisms of exchange 
normally acting, in gaseous assemblies, particularily at high temperatures. Two 
different hypotheses form possible starting-points : (A) U’he hypothesis of pre- 
servation, that the actual distribution laws for the systems in any assembly in 
equilibrium must be the same whatever the mechanisms of exchange that are 
setting up this equilibrium; (B) 7he hypothesis of detailed balancing, that in 
equilibrium the effects of any specified process, or element of a process, must be 
balanced exactly by another which is the exact reverse of the one specified, 
differing, that is, only in a reversed time scale. These hypotheses are not 
equivalent, (B) being more restrictive than (A), and a number of mechanisms 
are re-examined explicitly in the light of these hypotheses, including the 
mechanism of excitation by collision (Klein and Rosseland), ionisation and 
dissociation by collision (Becker), excitation by radiation (Einstein), scattering 
by free electrons (Einstein and Ehrenfest), and the photo-electric effect (Kramers, 
Milne). 
