Ignis Fatuus (Lat. "foolish fire") is an appearance of a flickering light seen after sunset in churchyards and over marshes. Descriptions vary a great deal, and no satisfactory explanation of the, phenomenon has yet been given. The light is generally of a blue-green or yellow colour; it may remain steadily in one spot; it may bound about and rise some feet into the air, or it may recede as the observer approaches it. Perhaps in some cases luminous insects or the phosphorescence of other animal or vegetable matter has produced the effect. Although marsh gas is flammable, and is readily given off in marshy ground, it does not take fire at all readily, and is not likely to offer the best explanation. Phosphoretted hydrogen is possible; it is spontaneously flammable, but its smell when burning is characteristic and could scarcely pass unnoticed. An flammable phosphorescent vapour would explain many of the strange movements, but none is known excepting that of phosphorus itself, and this does not occur free. The folklore of the Jack-o-lantern, the Will-o'-thewisp, etc., which are the popular names of the ignis fatuus, is very extensive and interesting.