In modern science, it is generally accepted that will-o'-the-wisp phenomena (ignis fatuus) are caused by the oxidation of phosphine (PH3), diphosphane (P2H4), and methane (CH4). These compounds, produced by organic decay, can cause photon emissions. Since phosphine and diphosphane mixtures spontaneously ignite on contact with the oxygen in air, only small quantities of it would be needed to ignite the much more abundant methane to create ephemeral fires. Furthermore, phosphine produces phosphorus pentoxide as a by-product, which forms phosphoric acid upon contact with water vapor, which can explain "viscous moisture" sometimes described as accompanying ignis fatuus.