Louis Dufour (1832-1892), professor at the Academy of Lausanne (Switzerland), had observed in 1874 a pressure difference across a porous wall separating moist air and a vessel containing either water or a desiccant, pointing to the possibility to use this phenomenon to build an hygrometer. A long period of almost complete oblivion followed. Heinrich Greinacher (1880-1974), then physics professor at the University of Bern (Switzerland) and Director of the Meteorological Observatory of this city, took in 1944 a sudden interest in instruments to measure the atmosphere moisture content. The performances of the hygrometers then in current use were certainly not satisfactory. Greinacher rediscovered the work of Dufour, claimed to have built the first Diffusion Hygrometer, proposed a theory and even got a patent for it. In the course of the 1950s, two different models were produced by young firms of Lausanne, Rüeger and TESA. We found one exemplar of each maker in the collections of our Physics Museum and tested them. Eventually, the Diffusion Hygrometers were a commercial failure. Their drawbacks were too numerous, and they were becoming superseded by novel electrical/electronic types of instruments.