Gas under pressure is admitted through an orifice formed in the bottom portion of a tank of substance in powder form, and is guided along an internal tube connected to the admission orifice, the tube passes through the powder stored in the tank and entrains powder through openings formed in the wall of the tube, the powder being extracted in the form of a gas-and-powder flow through an outlet at the top portion of the tank. The gas-and-powder mixture traveling along the internal tube is guided to the top portion of the tank where it is directed substantially towards a side wall of the tank so as to set up a gas-and-powder flow traveling through the volume situated in the tank above the powder stored in the tank, prior to the gas-and-powder flow being extracted through the outlet. The gas-and-powder flow can be taken to one or more powdering nozzles, e.g. in a system for powdering a developer in an installation for detecting surface defects by sweating.