Ignis Fatuus (a flickering light seen at night over the surface of marshy grounds or graveyards. Sometimes it moves quietly along, resembling the light of a lantern carried in the hand; and again it appears not alone, but two or three together dancing merrily together up and down. In the night mists it seems like the light from some neighboring house; and many a traveller has been led by it into dangerous bogs, from which he found no escape till the appearance of the morning light. It is not strange that a character of mystery should have attached to this luminous appearance, and that the ignorant should have ascribed it to some evil spirit. They called it " Will o' the wisp" and "Jack with a lantern," and this imaginary person is often alluded to by the old English poets. It is commonly believed that the light retires before one who pursues it; this notion is confirmed by the statements of some observers, and disproved by those of others. In Milner's "Gallery of Nature," p. 544, is recorded a statement of Mr. Blesson, who carefully investigated the phenomena in the forest of Gorbitz, in Brandenburg. On a marshy spot he observed bluish purple flames at night, where bubbles of air issued during the day.

These flames retired as he approached, in consequence, he supposed, of the air being agitated by his movement. When he stood perfectly still they soon appeared within reach; and then, carefully guarding against disturbing the air by his breath, he succeeded in singeing a piece of paper, which became covered with a viscous moisture. At last a narrow slip of paper took fire. By disturbing the air over the spot he caused the flames to disappear entirely, but in a few minutes after quiet was restored they appeared again over the air bubbles, apparently without having communication with any known source of flame. On suddenly introducing a torch after extinguishing the flame, a kind of explosion was heard, and a red light was seen over 8 or 9 sq. ft. of the marsh, which diminished to a small blue flame from 2 1/2 to 3 ft. high. He concluded that the cause of the ignis fatuus was the evolution of inflammable gas from the marsh, and that the flames existed by day as well as at night, though not then visible. The lights seen occasionally over churchyards are of similar appearance to those described. These meteors are supposed to be the result of the spontaneous combustion of inflammable gases generated by the decomposition of vegetable or animal bodies.

Phosphu-retted hydrogen, it is well known, bursts into flame as it is allowed to escape into the air from the vessels in which it is prepared. It is produced by the decay of animal matters, and, if thinly diffused here and there over the surface of a marsh, may present the changing, flickering light of the ignis fatuus, as difficult to locate as the illumination of the fireflies, for which it has been mistaken by several eminent naturalists. What is known as marsh gas is a highly inflammable carburetted hydrogen, which bubbles up through the water that covers boggy places, and may be inflamed on the surface. This may be ignited by phos-phuretted hydrogen, and add to the extent and permanence of the flames. The small quantity of these combustible matters present in the air will account for the feebleness of the flames, which have rarely been known to set fire to other substances; and the varying quantity and purity of that exhaled would explain the constantly shifting brightness of the light. According to the account in the " Gallery of Nature" referred to, in the middle of the last century the snow on the summit of the Apennines appeared enveloped in flame; and in the winter of 1693 hay ricks in Wales were set on fire by burning gaseous exhalations.