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Molecular Scattering in Gases. 627 immediately above the circular orifice of an oven, from which a heavy vapour is effusing molecularly to be condensed on a cooled hood above the oven. The density of the vapour stream is adjusted so that the alkali metal atoms suffer single collisions in passing through the apex of the stream. The scattered atoms are received on a circular ring of tungsten wire, which forms the positive electrode of a surface ionization gauge. The gauge is movable along the axis of the primary beam ; in this way the angle of collection of the scattered atoms can be varied at will over a prescribed range. For a given angle of scattering the number of atoms falling on unit area of the detector per second is inversely as the square of its distance from the centre of scattering ; hence a compara tively large number of scattered atoms is received at the larger angles. This enables measurements to be made with approximately equal accuracy over the whole range of a steeply falling angular distribution curve. This advantageous arrangement is, of course, only possible if the scatterer is a heavy, low velocity molecule ; otherwise the circularly sym metric distribution of the scattered atoms about the axis of the primary beam is lost.
Molecular Scattering in Gases. 029 primary beam as steady as possible over periods of several hours. To this end particular attention was paid to the design of the oven Oj, and to the control of its temperature during an experiment; to the provision of a large clean surface of molten alkali metal in the oven ; and to the vacuum conditions in the oven chamber.
Molecular Scattering Gases. 631 cm./sec. for potassium, under the conditions of the experiments; (3) the asymmetric structure of the molecule, and its slow rate of temperature rotation, are favourable to the observation of intramolecular diffraction effects : assum ing always the possibility that the Maxwell distribution of velocities in the primary beam, and the background of incoherent scattering, do not together mask the diffraction pattern.