IGNIS 
two or three acres of the adjacent mountains. The at- 
mofphere from the beginning of the evening had been 
remarkably thick and hazy ; and the dew, as they felt 
it on the bridles of their horfes, was very clammy and unc¬ 
tuous. 
Lights refembling the ignis fatuus are foinetimes ob- 
ferved at Tea, Ikipping about the malts and rigging of flips ; 
and Dr. Shaw informs us, that he has feen thefe in fuch 
weather as that juft mentioned when he faw the ignis fa¬ 
tuus in Palelline. Similar appearances have been obferved 
in various other fituafcions; and we are told of one which 
appeared about the bed of a woman in Milan, furround¬ 
ing it as well as her body entirely. This light fled from 
the hand which approached it; but was at length entirely 
dil'perfed by the motion of the air. Of the fame kind 
alfo, molt probably, are thofe fmall luminous appearances 
which fometimes appear in houfes 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 harmlefs, though not always; for we have 
accounts of fome luminous vapours which would encorn- 
pafs flacks of hay and corn, and fet them on fire; fo that 
they became objects of great terror and concern to the 
country people. Of thefe it was obferved, that they would 
avoid a drawn fword, or fharp-pointed iron inftrument, 
and that they would be driven away by a great noife; 
both which methods were made ufe of to diflipate them ; 
and it was likewife obferved, that they came from fome 
diflance, as it were on purpofe 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 
imagined that the light was occafioned by a number of 
fliining infeCts ; but this opinion was never fupported in 
fuch a manner as to gain much ground. The ignis fa¬ 
tuus feen by Dr. Derham above-mentioned, as well as ail 
the other inflances we have related, feem to demonftrate 
the contrary. Sir Ifaac Newton calls it a vapour Jhining 
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 va¬ 
pours which' let fire to the flacks of corn. Dr. Prieftley 
fuppofes that the light is of the fame nature with that 
produced by putrefcent fubftances ; and others are of 
opinion, that the eleClrical fluid is principally concerned; 
but none have attempted to give any particular folution 
of: the phenomena. 
From the frequent appearance of the ignis fatuus in 
marfhes, inoift ground, burying-places, and dunghills, we 
are naturally led to conclude, that putrefaction is con¬ 
cerned in the production of it. This procefs, we know, 
is attended with the emiffion of an aqueous fleam, toge¬ 
ther with a quantity of fixed, inflammable, phlogifticated, 
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 ful- 
plnir particularly, which become luminous, with a degree 
of heat much lets than that fufficient to fet fire to combuf- 
tible bodies. There is no inconfiftency, therefore, in fup- 
pofmgthattheputrid vapour may be capable of (tuning, with 
a ftill fmaller degree of heat than that of fulphur, and con- 
fequently become luminous by that which putrefaftion 
alone affords. This would account for the ignis fatuus, 
were it only a-fteady luminous vapour arifing from places 
where putrid matters are contained ; but its extreme mo¬ 
bility, and flying from one place to another on the ap¬ 
proach of any perfon, cannot be accounted for on this prin¬ 
ciple. If one quantity of the putrid vapours becomes lu¬ 
minous by means of heat, all the reft ought to do fo like¬ 
wife ; fo that, though we may allow heat and putrefaction 
to be concerned, yet, of neceflity, we muft have recourfe 
to fome other agent, which cannot beany other than cUq- 
FATUUS. 831 
tricky. Without this it is impoffible to conceive how any 
body of moveable vapour fhould not be carried away by 
the wind; but, fo far is this from being the cafe, that the 
ignes fatui, defcribed by M. Beccari, were but little af¬ 
fected by the wind. It is befldes proved by undoubted 
experiment, that eleClricity always is attended with fome 
degree of heat; and this, however (mall, may be fufficient 
to give a luminous property to any vapour on which it 
ads ftrongly; not to mention, that the eleCtric fluid itfelf 
is no other than that of light, and may therefore by its 
aCtion eaftly produce a luminous appearance independent 
of any vapour. 
We have a ftrong proof that eleClricity is concerned, 
or indeed the principal agent, in producing the ignis fa¬ 
tuus, from an experiment related by Dr. Prieftley of a 
flame ot this kind being artificially produced. A gentle¬ 
man, who had been making many electrical experiments 
for a whole afternoon in a fmall room, on going out of 
it, obferved a flame following him at fome little diflance. 
This, we have no realon to doubt, was a true ignis fa¬ 
tuus, and the circumftances necelTary to produce it were 
then prefent, viz. an atmolphere impregnated with ani¬ 
mal vapour, and likewife ftrongly electrified. Both thefe 
circumftances undoubtedly mult 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 eleClricity, would be collected by reafon of the 
fmallnefs of the room. In this cafe, however, there 
feems to have been a confiderable difference between the 
artificial ignis fatuus and thofe commonly met wfith; for 
this flame followed the gentleman as he went out of the 
room ; but the natural ones commonly fly from thofe 
who approach them. This may be accounted for, from 
a difference between the eleClricity of the atmofphere. in 
the one room and the other; in which cafe the flame 
would naturally be attracted towards that place where the 
eleClricity was either different in quality or in quantity ; 
but, in the natural way, where all bodies may be fuppofed 
equally electrified for a great way round, a repulfion will 
as naturally take place. Still, however, this does not 
feem to be always the cafe. In thofe inflances where tra¬ 
vellers have been attended by an ignis fatuus, we cannot 
fuppofe it to have been influenced by any other power 
than what w r e call attradion, and which eleClricity is very 
capable of producing. Its keeping at fome diflance is 
likewife eafily accounted for; as we know that bodies 
poffeffed of different quantities of eleClricity may be made 
to attraCt one another for a certain fpace, and then repel 
without having ever come into contaCl. On this princi¬ 
ple we may account for the light which furrounded the 
woman at Milan, but fled from the hand of any other 
perfon. On the fame principle we may account for thole 
mifchievous vapours which fet fire to the hay and corn- 
flacks, but were driven away by prefenting to them a 
pointed iron inftrument, or by making a noife. Both 
thefe are known to have a great efteCl upon the eleCtric 
matter ; and, by means of eidier, even lightning may oc- 
cafionally be made to fall upon, or to avoid, particular 
places, according to the circumftances by which the ge¬ 
neral mafs happens to be affeCted at that time. 
On the whole, therefore, it feems molt probable, that 
the ignis fatuus is a collection of vapour of the putrefcent 
kind, very much affeCted by eleClricity ; aecordingto the 
degree of which, it will either give a weak or itrong light, 
or even fet fire to certain lubltances dilpofed to receive 
its operation. This opinion feems greatly to be confirm- 
ed from fome luminous appearances obferved in privies,, . 
where the putrid vapours have collected themfelves into 
balls, and exploded violently on the approach of a candle. 
This laft effeCt, however, we cannot fo well afcribe to the 
eleClricity, as to the accenfion of the inflammable air which 
frequently abounds in fuch places. 
In the Appendix to Dr. Prieltley’s third volume of Ex¬ 
periments and Obfervatioiis on Air, Mr. Warltire gives 
an account of fome very remarkable ignes fatui, which 
