The present disclosure relates to a syringe for retaining and/or mixing two substances which have been retained separately inside the syringe. In particular the present disclosure relates to a syringe for 1) retaining a dry composition in a vacuum, and 2) mixing the dry composition with an aqueous medium to form a flowable substance. One embodiment relates to a syringe for mixing first and second substances comprising a barrel comprising a vacuum chamber for holding a first substance, a plunger incorporating a reservoir chamber for holding a second substance and configured to be axially displaced in the vacuum chamber, and a valve for controlling and/or establishing a fluid connection between the vacuum chamber and the reservoir chamber.