Denuder tubes were first considered on a theoretical basis in the 1890s during a study on the diffusion of ions into gases.’ Later, in the 1930s. studies of atmospheric condensation nuclei revealed significant particulate losses from gas flows in sample pipelines and, consequently, the diffusional removal of particulates in both cylindrical and rectangular section pipelines was considered. Subsequent measurements of the diffusional proper ties of micro-particulates were made with multi-tubular assemblies and diffusion batteries, consisting of several flat plates with narrow gaps between them. In an appendix to this work. A mathematical treatment was presented of the diffusion from a stream flowing through a thin rectangular tube.