An apparatus for compressing a gas and its uses are disclosed. The apparatus comprises a fixed-volume container having a hollow and a moveable element subdividing said hollow into a first variable-volume portion and a second variable-volume portion, the second variable-volume portion having an opening for introducing therein a hydraulic and/or pneumatic fluid under pressure, for causing an increase in the volume of said second variable-portion by moving said moveable element, thereby, consequently, decreasing the volume of the first variable-volume portion and compressing a gas contained therein.