Abstract: Apparatus for contacting various samples with water vapors has a vessel which is sealable by a removable cover and contains an insert above a supply of liquid electrolyte. The insert has an annulus of sockets for containers each of which confines a different sample and can be sealed by a removable lid. A vertical shaft is mounted in the insert with minimal friction for rotary and axial movement and is driven by a stirring device which rotates a permanent magnet serving to rotate a permanent magnet or a magnetizable body which is secured to the shaft. The shaft carries an impeller which agitates the gas in the interior of the vessel while the lids are off the respective containers. The supply of electrolyte is agitated by a bar which is rotated by the magnet of the stirring device or which constitutes the magnet or magnetizable body on the shaft.