Device for admixing at least one first substance and one second substance to a flowable base substance for forming a mixture by means of a continuous mixing operation, in particular for admixing a first substance and a second substance to a urea melt for forming a dispersion, wherein the device for admixing has the following features: a the device for mixing has at least one first mixing vessel and one second mixing vessel, b the first and second mixing vessels (12, 14) have in each case one inlet opening (26, 32), in each case one introduction opening (46, 48) for admixture of the first substance and second substance, respectively, in each case one outlet opening (28, 34) and in each case one pressure-equalization opening (30, 36), each of the mixing vessels (12, 14) having at its base a further outlet opening, which is intended for complete emptying of the mixing vessels (12, 14), wherein c the first mixing vessel and the second mixing vessel are each provided with a stirrer, d the pressure-equalization openings (30, 36) of the first and second mixing vessels (12, 14) are connected to one another by means of a pressure-equalization line or are open to the surroundings, wherein, during the operation of the device, the pressure-equalization openings (30, 36) are situated above fluid surfaces (42, 44) present in the mixing vessels, and e the outlet opening of the first mixing vessel and the inlet opening of the second mixing vessel are connected to one another by means of a connecting line, wherein, during the mixing operation of the device, the connecting line is open and the outlet opening of the first mixing vessel and the inlet opening of the second mixing vessel and also the connecting line are situated below the fluid surfaces (42, 44) in the mixing vessels (12, 14), so that fluid can pass from the first mixing vessel into the second mixing vessel under the effect of gravitational force. a the device for mixing has at least one first mixing vessel and one second mixing vessel, b the first and second mixing vessels (12, 14) have in each case one inlet opening (26, 32), in each case one introduction opening (46, 48) for admixture of the first substance and second substance, respectively, in each case one outlet opening (28, 34) and in each case one pressure-equalization opening (30, 36), each of the mixing vessels (12, 14) having at its base a further outlet opening, which is intended for complete emptying of the mixing vessels (12, 14), c the first mixing vessel and the second mixing vessel are each provided with a stirrer, d the pressure-equalization openings (30, 36) of the first and second mixing vessels (12, 14) are connected to one another by means of a pressure-equalization line or are open to the surroundings, wherein, during the operation of the device, the pressure-equalization openings (30, 36) are situated above fluid surfaces (42, 44) present in the mixing vessels, and e the outlet opening of the first mixing vessel and the inlet opening of the second mixing vessel are connected to one another by means of a connecting line, wherein, during the mixing operation of the device, the connecting line is open and the outlet opening of the first mixing vessel and the inlet opening of the second mixing vessel and also the connecting line are situated below the fluid surfaces (42, 44) in the mixing vessels (12, 14), so that fluid can pass from the first mixing vessel into the second mixing vessel under the effect of gravitational force.