SECTIONAL TRANSACTIONS .—B. 291 
vaporising nozzle. This permits the investigation of a great variety of 
molecules which could not have been measured by the older methods. 
(6) This nozzle, either in conjunction with the oven or by itself when 
using substances with a high vapour pressure at room temperature, coupled 
with the use of Ilford X-ray emulsions, enables one to record many more 
interference maxima than had previously been possible. 
(c) These new high order maxima are particularly sensitive to changes 
in chemical structure. Hence we can now make an accurate determination 
of interatomic distances and the molecular architecture of many substances 
whose spatial configuration has hitherto been unknown. 
(d) The high (sin 0/2)/A values of the maxima that are now available 
permit the use of a simplified method of calculating the theoretical scattering 
curves, with which the experimental results are compared, with a conse- 
quent saving in time. 
These technical improvements have turned the electron-diffraction 
method into a quick, reliable, and accurate tool for the determination of 
chemical structure in the vapour phase. It should now be possible to clear 
up most of those debatable points in chemistry where a knowledge of the 
spatial structure is essential. 
Mr. S. F. Boys.—The origin of optical rotatory power (12.0). 
Measurements on the refractive indices, etc., of pure compounds have 
led to the view that the atoms in a molecule are polarised under the action 
of the electric field of a light wave. The polarisation constants of particular 
elements are well known, and are tabulated under the name of refractivities. 
If an asymmetric molecule is examined and the atoms are assumed to 
have the usual polarisibilities, it is possible to calculate the complete optical 
properties of the liquid composed of such molecules. If the liquid only 
contains one of the two enantiomorphs, the calculation shows that, in 
general, the liquid must rotate the plane of polarisation of a transmitted 
light wave. 
The predicted values of the optical rotatory powers of certain simple 
compounds, e.g. amyl alcohol, have been found, and these agree with the 
experimental values. 
The calculation makes it possible to state the conditions which determine 
whether a given simple molecule is dextrorotatory or levorotatory. This 
relation can be used in the reverse sense and it is possible to determine the 
absolute configuration of some optically active compounds. 
Mr. E. Eastwoop and Dr. C. P. SNow.—Opftical properties of conjugated 
compounds : (a) The absorption spectrum of acrolein (12.40). 
Unlike the saturated aliphatic aldehydes, which give very complex band 
spectra, acrolein, CH,=CH—CH=O, gives a band spectrum in which 
the rotational structure is as sharp as in the diatomic gases. This structure, 
however, presents the unique anomaly that the moment of inertia deduced 
for the ground state of the molecule, instead of being constant throughout, is 
different for each vibrational band. 
Dr. C. B. ALtsopp.—Optical properties of conjugated compounds : (b) The 
origin of optical exaltation in conjugated hydrocarbons (12.45). 
Contrary to the prediction of Briihl, but in confirmation of observa- 
tions by Willstatter, the conjugation of two double bonds in 1 : 3-cyclo- 
