[en] This thesis deals with some investigations at very low temperatures in which a ⁴He-circulating ³He-⁴He dilution refrigerator and a Pomeranchuk cooling device are used. The main theme is the design and construction of a special device, a Pomeranchuk cell that is precooled and pressurized with a ⁴He-circulating ³He-⁴He dilution refrigerator, that can be used to obtain a lowest temperature of the order of 1.1 mK. Furthermore, some details of the working of such a dilution refrigerator and some properties of ³He-⁴He mixtures at high pressures were investigated. (Auth.)