Abstract: A gas sensor for use in monitoring gases such as H.sub.2 S, NO.sub.2 or SO.sub.2 comprises a film or layer of non-protonated polyaniline as the gas sensing material.

Abstract: A barrier assembly for a Langmuir trough is arranged for a substrate to be carried automatically to-and-fro on a dipping head over a fixed barrier into monomolecular layers A, B on a common subphase so as to stack layers ABABAB . . . on the substrate. The layers A, B are confined by the barrier and two constant perimeter areas formed by a single flexible band passing over rollers. By adjusting the positions of bridges that carry some of the rollers, the confined areas can be contracted to compress the layers A, B. In an alternative mode the substrate receives a layer from layer A while a new layer B is being formed, the substrate then being transferred to layer B while a new layer A is being formed. This involves an oscillating rather than a rotating movement of the substrate.