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.