Abstract: The apparatus is of the type comprising a vertical cylindrical vessel containing a flexible tubular bag closed at one end and fixed by the periphery of its open opposite end to the periphery of the upper part of the vessel and in which mercury is injected in the vessel around the bag when the member to be treated is introduced in the bag. The apparatus comprises two coaxial flexible bags (3,21) extending one inside the other within the vessel (2) and defining therebetween a small closed space (22), means (23) for introducing a liquid of high density in the space (22) and means (24) for achieving in the vessel, around the outer bag (3), a solid molding, which is capable of being disaggregated, of a member of any size introduced in the inner bag (21), the vessel (2) being vertical or inclined.