Carbon dioxide (CO2) flushing is a technique by which carbon dioxide is added to a storage container of seeds in an effort to replace oxygen (which seeds use to respire) with the carbon dioxide. The idea is that the lack of oxygen will increase the seed storage ability by slowing down the respiration of the seeds, allowing them to remain dormant longer. Carbon dioxide is heavier than oxygen, so introducing it into a container replaces the oxygen using gravity.  This may also have the additional advantage of killing any insect pests which may be present, as the insects also require oxygen to respire and survive (but at a greater quantity than seeds).