~U M M A R YDiffusiophoresis is the name of the phenomenon whereby particles of an aerosol, through which a vapour or gas is diffusing, experience a force which moves them in the direction of the diffusive flux of the vapour or gas. Experiments are described which show that the velocity (cm sec-l) imposed on sub-micron particles (0.01 to 0.1 p radius) in the case of water vapour diffusing in air, when the water-vapour pressure is small compared with the total pressure, is governed by the equation : dP v, = -1.9 x 10-4-dx where dp/dx is the water vapour pressure gradient expressed in millibars per cm. This result is shown to be in excellent agreement with theoretical considerations.The absolute efficiency of diffusiophoresis in scavenging radioactive fallout particles from the atmosphere, because of cloud droplets growing by condensation, is deduced to be low. This is confirmed by a cloudchamber experiment in which inactive salt droplets were grown in the atmosphere of a radioactive aerosol.Furthermore, it is concluded that the wash-out due to diffusiophoresis accounts for less than one per cent of the total observed wash-out of nuclear-bomb debris for each condensation cycle occurring in nature. Cloud droplets would have to undergo over one hundred condensation-evaporation cycles on the same condensation nucleus for diffusiophoresis to be responsible for most of the observed wash-out.

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