SECTIONAL TRANSACTIONS. 



SECTION A.— MATHEMATICAL AND PHYSICAL 



SCIENCES. 



(For reference to the publication elsewhere of communications entered in the 

 following list of transactions, see p. 684.) 



Thursday, September 6. 



Prof. H. S. Allen. — Progress in Band Spectra. 



The fluted spectrum known as a ' band spectrum ' is resolved under high dispersion 

 into groups of fine lines in close sequence. The empirical relations found by Deslandres 

 between the frequencies of the lines have received an explanation by applying the 

 principles of Bohr's quantum theory of line spectra. It is now accepted that band 

 spectra originate in molecules containing more than one atom. Such spectra may be 

 classified as (1) pure rotation bands, the motion of the molecule being solely a 

 rotation about an axis through the centre of mass ; (2) vibration-rotation bands, 

 this rotation being accompanied by a vibration of the nuclei along the line joining 

 them ; (3) electronic bands involving in addition to motions of rotation and vibration 

 a change in the electronic configuration. The energy change in an electronic shift 

 is so large that the spectrum may be in the visible or ultra-violet. The basic work 

 in the application of Bohr's theory to such bands was done by Schwarzschild (1916) 

 and Heurlinger (1918). It is assumed that the frequency of the radiation absorbed 

 or emitted in a transition between two stationary states of the molecule is deter- 

 mined by Bohr's well-known frequency relation. The energy in a stationary state 

 may conveniently be divided into (1) rotational energy E„, ; (2) vibrational energy, 

 E„ ; (3) electronic energy, E^. 



Since 1926 considerable progress has been made in the interpretation of electronic 

 shifts through the work of Birge, Hund, MulUken and others. Hund has applied to 

 band spectra the conception of electron spin which has proved fruitful in accounting 

 for the multiple energy levels in atomic spectra. Birge and Mulliken have laid stress 

 on the close similarity between the electronic levels of molecules and those of the 

 ' corresponding ' atoms, i.e. atoms with the same number of outer electrons. Mulliken 

 has extended the analogy by assigning inner quantum numbers to these levels, the 

 values assumed being the same as Sommerfeld's values for corresponding atomic 

 states. 



The new methods are illustrated by the consideration of the band spectrum of 

 water vapour which has been studied in the St. Andrews laboratory by D. Jack. 

 Using Mulliken's notation he has given an interpretation of these bands, including the 

 satellites and singlet series, and has obtained interesting results as to the selection 

 rules. 



The application of the new quantum mechanics has removed outstanding 

 difficulties as regards quantum numbers in band spectra. Further progress in this 

 direction is to be anticipated from the recent work of Dirac and of Darwin based on 

 Schrodinger's wave mechanics. 



Dr. EzER Griffiths, F.K.S., and Mr. J. H. Awbery. — The Measurement of 



Flame Temperatures. 



Two methods have been employed for the measurement of the temperature of a 

 homogeneous flame. 



In the one a refractory metal in the form of wire is heated electrically in vacuo, 

 and the relation between temperature and heating current determined by an optical 

 pyrometer. The same wire is then inserted in the flame, and the relation between 

 temperature and heating current again determined. When the results are plotted 

 graphically the point of intersection of the two lines will give the temperature of the 

 flame, for at the temperature represented by this point the electrical supply is 

 sufficient to balance the radiation loss, wliether the wire is in vacuo or in the flame, 

 so that the surrounding gas in the flame neither imparts nor abstracts heat from the 

 wire. 



