Butane gas is dropping from this flask, even though this picture does not capture it well. (Click here for a video of this.) It can be seen (in person, at least) pouring from a flask such as this into air. Butane (2.6 g/L) is denser than air (1.2 g/L). Therefore, it is a good idea to dispose of the butane near an open door or a fume hood. These dense vapors are slow to disperse!