Using a small sample of the liquid in a narrow vertical tube, and observing its rate of evaporation into stream of air passed across the top of the tube can conveniently be used to study the diffusion of vapour of a volatile liquid into air. The set up consists of a glass tube placed in a water bath. A horizontal glass tube is fixed to the upper end of the tube and air is drawn through this by a small air pump included within the unit. Air flows over this tube maintaining partial pressure difference. A traveling microscope with sliding vernier scale is provided to measure the rate of fall of solvent within capillary. A stirrer is fitted to maintain constant temperature inside the bath.