would appear again; fometimes, running fwiftly along, it would expand itself at certain intervals over more than two or three acres of the adjacent mountains. The atmosphere from the beginning of the evening had been remarkably thick and hazy; and the dew, as they felt it on the bridles of their horses, was very clammy and unctuous.

Lights resembling the ignis fatuus are sometimes observed at fea, fkipping about the mafts and rigging of fhips; and Dr. Shaw informs us, that he has feen thefe in fuch weather as that just mentioned when he faw the ignis fatuus in Paleftine. Similar appearances have been observed in various other fituations; and we are told of one which appeared about the bed of a woman in Milan, furrounding it as well as her body entirely. This light fled from the hand which approached it; but was at length entirely dispersed by the motion of the air. Of the fame kind alfo, most probably, are those small luminous appearances which fometimes appear in houses or near them, called in Scotland'elf-candles, and which are fuppofed to portend the death of fome perfon about the houfe. In general thefe lights are harmless, though not always; for we have accounts of fome luminous vapours which would encompass ftacks of hay and com, and fet them on fire; fo that they became objects of great terror and concern to the country people. Of these it was obferved, that they would avoid a drawn fword, or sharppointed iron inftrument, and that they would be driven away by a great noife; both which methods were made ufe of to diffipate them; and it was likewife obferved, that they came from fome diftance, as it were on purpose to do mifchief.

Several philofophers have endeavoured to account for thefe appearances, but hitherto with no great fuccefs; nor indeed does there feem to be fufficient data for folving all their phenomena. Willoughby, Ray, and others, have magined that the light was occafioned by a number of

shining

OR THE JACK-A-LANTERN.

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hining infects; but this opinion was never fupported in fuch a manner as to gain much ground. The ignis fatuus seen by Dr. Derham above mentioned, as well as all the other inftances we have related, feem to demonstrate the contrary. Sir Ifaac Newton calls it a vapour fhining without heat; and fuppofes that there is the fame difference between the vapours of ignis fatuus and flame that there is between the fhining of rotten wood and burning coals. But, though this feems generally to be the cafe, there are ftill fome exceptions, as had been inftanced in the vapours which fet fire to the stacks of corn. Dr. Priestley fuppofes that the light is of the fame nature with that produced by putrefcent substances; and others are of opinion, that the electrical fluid is principally concerned; but none have attempted to give any particular solution of the phenomena. From the frequent appearance of the ignis fatuus in marshes, moist ground, burying places, and dunghills, we are naturally led to conclude, that putrefaction is concerned in the production of it. This process, we know, is attended with the emiffion of an aqueous fteam, together with a quantity of fixed, inflammable, phlogisticated, and alkaline, airs, all blended together in one common vapour. It is likewife attended with fome degree of heat; and we know that there are fome vapours, that of fulphur particu larly, which become luminous, with a degree of heat much lefs than that fufficient to set fire to combustible bodies. There is no inconfiftency, therefore, in fuppofing that the putrid vapour may be capable of fhining with a ftill fmaller degree. of heat than that of fulphur, and confequently become luminous by that which putrefaction alone affords. This would account for the ignis fatuus, were it only a steady luminous vapour arifing from places where putrid matters are contained; but its extreme mobility, and flying from one place to another on the approach of any perfon, cannot 3D 2

be

be accounted for on this principle. If one quantity of the putrid vapours becomes luminous by means of heat, all the reft ought to do fo likewife: fo that, though we may allow heat and putrefaction to be concerned, yet of neceffity we must have recourse to fome other agent, which cannot be any other than electricity. Without this it is impoffible to conceive how any body of moveable vapour should not be carried away by the wind; but, so far is this from being the cafe that the ignes fatui described by M. Beccari were but little affected by the wind. It is befides proved by undoubted experiment, that electricity always is attended with fome degree of heat; and this, however fmall, may be fufficient to give a luminous property to any vapour on which it acts strongly; not to mention, that the electric fluid itself is no other than that of light, and may therefore by its action eafily produce a luminous appearance independent of any vapour.

We have a strong proof that electricity is concerned, or indeed the principal agent, in producing the ignis fatuus, from an experiment related by Dr. Priestley of a flame of this kind being artificially produced. A gentleman, who had been making many electrical experiments for a whole afternoon in a small room, on going out of it, observed a flame following him at fome little diftance. This, we have no reason to doubt, was a true ignus fatuus, and the circumstances neceffary to produce it were then present, viz. an atmosphere impregnated with animal vapour, and likewife strongly electrified. Both these circumstances undoubtedly must have taken place in the prefent cafe; for the quantity of perfpiration emitted by a human body is by no means inconfiderable; and it as well as the electricity would be collected by reafon of the smallness of the room. In this cafe, however, there feems to have been a confiderable difference between the artificial ignis fatuus and those com

monly