One of the most circumstantial accounts of these objects is that published in the Belgian journal “Ciel et Terre” for July-August, 1920, by a retired army surgeon, Jules Rossignol, who observed them repeatedly in the autumn of 1908 in and about some marshy woods near Grupont. They were generally seen to rise from the ground, at first in the shape of little white clouds, which changed to luminous globes on attaining an altitude of a dozen yards, and returned by a circular path toward the ground. They lasted from one to several minutes before disappearing in the air. It is astonishing that the phenomenon of _ignis fatuus_, though reported from so many parts of the world, has not yet been made the subject of direct scientific examination. Nobody has ever studied its light with the spectroscope, for example. Chemists have, indeed, attempted to reproduce the phenomenon, yet the chemical explanations of it that have appeared in reference books down to a recent date are quite untenable. It has sometimes been attributed to marsh gas (methane, CH4), and sometimes to phosphureted hydrogen (phosphine, PH3). But marsh gas, besides not being spontaneously combustible, diffuses too rapidly in the air to produce the effects described, while phosphureted hydrogen, though it takes fire spontaneously in the air, produces thick wreaths of smoke when burning and has a powerful odor--features never reported in connection with will-o’-the-wisp. At least two more plausible explanations of _ignis fatuus_ have been offered in the last few years. Mr. F. Sanford (“Scientific Monthly,” Oct., 1919) believes that it is due to “swarms of luminous bacteria which are carried up from the bottom of the marsh by rising bubbles of gas.” A Belgian chemist, M. Léon Dumas (“La Nature,” Dec. 11, 1909), claims to have produced little luminous clouds, corresponding to the traditional descriptions of will-o’-the-wisp, by combining the two gases sulphureted hydrogen and phosphine. Both these substances are produced in the decay of animal matter, especially of the brain and spinal cord. The body of an animal, buried in some wet place, would accumulate the two gases under pressure in the skull and spinal canal, and their escape, simultaneously, would fulfill the conditions of M. Dumas’ experiments. (At the request of the present writer, these experiments were repeated at the Bureau of Standards, in Washington, with only partial success. Further trials with these and other gases due to putrefaction are desirable.) 2. THE RAIN TREE From time to time the newspapers publish accounts of a wonderful tree, said to grow wild in Peru or elsewhere, from the leaves of which falls a continuous shower of rain, even in the driest weather. The writers generally urge the introduction of this tree in regions where the rainfall is deficient, and a so-called “rain tree” has actually been sold for this purpose by nurserymen.