"Ghosts are nervous creatures by nature, and have been known to utilize a number of mechanisms when they feel threatened. They will attempt to flee by means of invisibility or phasing, though when these are not options some specimens have also been recorded to excrete a high density gas from wounds on their bodies. This fluid is dark, thick, clotted, making it resemble expired blood. This gas will fall in accordance with gravity, and will dissipate on impact with a solid object. This phenomenon has only been observed in ghosts with unhealed wounds at the time of death." - Vincent Bowdern, Field Guide to Spirits, 1969