290 SECTIONAL TRANSACTIONS.—A, G; B. 
this way it is possible to show that in the case of the less permeable fabrics 
there is an interchange of heat between the material of the fabric and the 
air stream which flows through the interstices. 
Mr. A. BatLey and Mr. W. F. Cope.—Heat transmission in pipes of square 
and rectangular section. 
The paper describes experiments carried out at the National Physical 
Laboratory. The conclusion is drawn that such pipes behave in a similar 
manner to circular pipes of the same hydraulic diameter. 
SECTION B.—CHEMISTRY. 
Thursday, September 6. 
PRESIDENTIAL ADDRESS by Prof. T. M. Lowry, C.B.E., F.R.S., on 
Physical methods in chemistry (10.0). (See p. 29.) 
Dr. R. G. J. Fraser.—Applications of molecular rays to chemical problems 
(Tihs). 
The Method of Molecular Rays—Modern vacuum technique allows the 
production of beams of neutral molecules, moving with thermal velocities 
in vacuo. Hence molecular properties can be studied directly, without 
the necessity for statistical arguments. 
Applications of the method more immediately touching chemical problems 
are : 
Dissociation.—If a non-magnetic diatomic molecule dissociates into mag- 
netic atoms, and a mixed molecular-atomic beam is sent through an in- 
homogeneous magnetic field, the atoms suffer deflection, the molecules are 
unaffected. ‘Thus the atoms and molecules can be physically separated, and 
their relative numbers determined. 
Molecular cross-sections—The weakening of a beam of molecules on 
traversing a vapour determines their mean free path and collision area Q. 
Methods have been devised which permit extrapolation to beams of negligible 
width. Hence Q values are obtained which are independent of the geometry 
of the apparatus. 5 
Dipole moments.—The deviation of a beam in an inhomogeneous electric 
field determines the molecular dipole moment up. The dipole moment is 
measured outright, at a single temperature. Hence a possible dependence 
of u. on temperature is readily established. 
Free radicals —A molecular ray is collision free; hence the primary 
products of chemical reactions can be isolated in the beam and examined by 
special methods. 
Dr. H. De LaszLo.—Determination of molecular structure by electron- 
diffraction (11.35). 
The technique of obtaining photographic records of the scattering of fast 
electron beams by vapours and gases has been simplified and perfected in 
the following way. 
(a) The interference pattern of the vapour of any substance that will 
vaporise im vacuo up to 1000° C. without decomposition can be photo- 
graphed by means of a small oven, equipped with an original type of 
