The substances to be brought together are placed in a vertical cell, at least that substance which is placed at the bottom of the cell being in the liquid state at the moment of contacting. A hydraulic seal consisting of a liquid having a density of higher value than the liquid-state density of the substance which is located immediately beneath said liquid is pierced by means of a perforator and caused to tilt downwards. The entire seal thus reaches the bottom of the cell and the reactants are thus brought together. The perforator is formed from magnetic material and set in motion as a result of the force produced by a magnetic induction gradient which is created by external means.