32 SECTIONAL ADDRESSES 
DIFFRACTION OF MOLECULAR Rays AND ELECTRONS. 
Bombardment need not, however, be used only as an agent of 
destruction, since Dr. Fraser will tell you how gentle beams, in the form 
of molecular rays, travelling with the velocity of thermal agitation, 
instead of with velocities comparable with that of light, can be used to 
demonstrate the presence or absence of magnetic or electrostatic moments, 
to study the character of ‘free radicals,’ or to test the variability of 
‘dipole moments’ with temperature ; and Dr. de Laszlo will describe 
some applications of the method devised by Mark and Wierl for studying 
the structure of molecules by the orderly scattering of beams of electrons. 
The results thus obtained are so similar to those given by Debye’s study 
of the diffraction of X-rays as to be almost identical. 
DIFFRACTION OF X-RAYS. 
The applications of X-rays to chemistry are so numerous that I may 
be excused for selecting only a few examples that have interested me 
personally. ‘The influence of Cox’s X-ray analysis in vetoing an incorrect 
formula for ascorbic acid will perhaps be referred to in the joint dis- 
cussion on this vitamin ; but 1 may mention here that, in the case of 
another product of the same general class, Bernal was able to obtain a 
complete X-ray analysis by using a crystal weighing only 0-oo0015 mg., 
and was only prevented from making an exact determination of mole- 
cular weight by the Brownian movement, which prevented a precise 
determination of the density of the crystal by flotation—a difficulty which 
he suggests could be overcome with the help of a centrifuge. In a totally 
different field, I was during the war deeply interested in the polymor- 
phism of ammonium nitrate, which melts at 169°, but also has transition- 
temperatures at 125°, 84°, 32° and —16°. The heaps of nitrate from the 
driers in a shell-filling factory were therefore almost always either at 84° 
or at 32°, on account of the arrest of cooling at these transition-points. 
It is fascinating now to be told that these transitions are associated with 
the spinning of the ions in a rigid crystal lattice. As a result of this 
spinning, a tetrahedral ammonium ion and a triangular nitrate ion finally 
acquire complete spherical symmetry, and take up the same positions as 
the monatomic ions of sodium and chlorine in a crystal of rock salt, so 
that the substance crystallises in the cubic system in the range from 125° to 
169° C. 
MUTAROTATION. 
In accordance, I believe, with well-established custom, I pass on 
now to consider those examples of ‘ Physical Methods of Chemistry ’ 
with which I have been most closely concerned during a long period of 
years. 
Nearly forty years ago, as a student of organic chemistry under Prof. 
Armstrong, I undertook my first research, on the stereochemistry of the 
«-derivatives of camphor. ‘The earliest experiments (5) showed that the 
bromination of «-chlorocamphor and the chlorination of «-bromo- 
