A physical-chemistry paper reporting experiments on the heat generated when dry cotton wool is wetted by water or exposed to air saturated with water vapour (the Pouillet effect). Masson measures the temperature rise of cotton wrapped on a thermometer bulb and the moisture absorbed over time, and concludes that the heat comes from condensation of water vapour on the fibres. He gives empirical relations for the temperature-time curve and discusses the fate of the absorbed moisture, favouring a solid-solution view. The paper includes experimental tables and curve diagrams.