Encyclopaedia Britannica > Volume 8, DIA-England

620
ELECTRICITY.
Phenome¬
na and
Laws.
on Mount
Breven;
storm in a circular building roofed with sheet iron, and
one of the ladies on entering the hut expressed her alarm
lest the lightning should be attracted by the iron roof.
T They had scarcely entered their retreat, and were about
to partake of some refreshment, when a violent storm of
thunder and lightning came on from the west. About forty-
five minutes past two, a gentleman who stood at the eastern
entrance saw a ball of fire which seemed to him moving
on the surface of the ground. It instantly entered the
hut, forcing him several paces forwards from the doorway.
On his recovering from the shock, he found his sisters on
the floor of the hut, fainting, as he imagined, from terror.
Two of the ladies had died instantly; another lady, and
the rest of the party, were much injured. The explosion
which followed the flash of lightning was said by the in¬
habitants of the village to have been terrific. Mr Wil¬
liams, who immediately examined the hut, found a large
crack in the west side of the building, which passed up¬
wards from near the ground to the frame of a small win¬
dow, above which the iron roof was a little indented. Mr
Williams conceived it to be quite clear, from the place of
the fragments of stone and other appearances, that the
clouds were negatively electrified during this storm.
Electrical Various electrical phenomena of a very interesting kind
phenomena have been observed by travellers when ascending lofty
on moun- mountains. in 1767, MM. Saussure, Pictet, and Jalla-
bert, when on the top of Mount Breven, received small
electric shocks at their finger ends by stretching out their
arms, and a whistling noise even accompanied them. The
gold button on M. Saussure’s hat yielded distinct sparks,
on Mount In 1814, a party of Englishmen experienced similar effects
iEtna; on Mount iEtna during a storm of thunder and lightning
accompanied by a heavy fall of snow. One of the party
felt his hair moving, and upon raising his hand to his
head a buzzing sound issued from his fingers. The rest
of the party experienced the same sensations, and by
moving their hands and fingers they produced a variety
of musical sounds, audible at the distance of forty feet,
on Ben- On the 27th of June 1825, Dr Hooker and a party of
Nevis. botanists witnessed effects like those described, during a
fall of snow on Ben-Nevis when there was no thunder¬
storm. The snow fell very heavily for nearly two hours.
Soon after it began, a hissing sound was heard everywhere
around them, and continued about an hour and a half.
It seemed to proceed from every point in the vicinity; and
on arriving at the cairn on the summit of the mountain,
they could almost determine the stones from which the
electricity issued. The hair of several of the party ex¬
hibited, when touched, the usual electrical phenomena.
On the Before quitting the subject of lightning, we must sub¬
transport mit to our readers a brief account of the remarkable ob-
°|^ Poni^r" servations made by M. Fusinieri on the ponderable sub-
stances bv stances transported by lightning in its passage, and which
lightning. *t deposits in a permanent state on the bodies which ob¬
struct its passage. When we consider the magnitude of
the scale on which the great electrical machine of our at¬
mosphere enables us to study its effects, it appears strange
that so little attention has been paid to those interesting
phenomena which accompany the electric stroke. M. Fusi¬
nieri is the only person who has made this an object of spe¬
cial investigation ; and the results to which he has been led
possess, as might have been expected, a very peculiar inte¬
rest. The following are the general results which he ob¬
tained: Lightning contains, like the common electric spark,
matter in a state of extreme division, and in a state of ig¬
nition and combustion. In the matter deposited by light¬
ning on houses and on trees which have been struck by it,
he has found iron, sulphur, and carbon. Lightning divides
and subdivides itself indefinitely into spai’ks, which end in
being not much larger than those of ordinary machines;
Laws
and each of these sparks contains ponderable substances Phenomt
in the state of extreme division already mentioned. The na and
lightning deposits the substances with which it is charged
while it passes through them, and while it breaks hard
bodies ; and it deposits them on the surface by which it
enters the body, as well as on that by which it escapes,
and also on the surfaces of fracture. When the resistance
to its passage is not great, it leaves no perceptible deposit;
and the quantity of matter deposited increases, and is
proportional to the difficulty with which the lightning
traverses the body. At the same time that lightning de¬
posits the matter which it contains, it takes up new mat¬
ter from the combustible bodies, such as iron, charcoal,
&c. through which it passes. The deposited matter tends
always to expand itself in thin films on the surface which
receives it, and it does this most readily on surfaces that
are smooth and free from all asperities.
In examining the traces left by lightning when it fell at
Vicenza in 1829, and at Padua in 1831, M. Fusinieri made
the following observations: It deposited on the surface
of a wall by which it entered the house, a thin layer of
pulverulent matter, of a brown colour at its centre, and
yellowish and much less deep at its margin. When this
matter was collected and carefully examined, it proved to
be iron in different degrees of oxidation. Upon some
stones which the lightning had detached from the wall
there was found a stratum the fiftieth of an inch thick,
and of a brownish colour, which seemed to have under¬
gone a species of fusion. This stratum was sulphuret of
iron, which gradually changed into a sulphate of the same
metal. M. Fusinieri indeed had previously found small
crystals of sulphuret of iron upon an iron rod which the
lightning had struck, and also upon a stone to which it
had passed from the iron. The position of these crystals
indicated that they had been formed in the middle of the
passage of the lightning; a fact which he considered as
proving that the electric matter could transport sulphur
across metal itself. When the lightning escaped from the
wall, it deposited upon the wood a dust composed of small
aggregated grains, which had all the qualities of ferrugi¬
nous matter. In pursuing the passage of the lightning,
it was found to have divided itself into a great number ot
sparks more or less voluminous upon the windows, formed
of pieces of rectangular glass united in a leaden frame.
The traces left on the glass and on the lead were very
slight, and there were only a few marks on the glass very
near its contact with the lead. The traces on the lead
were small cavities, round which there had been a fusion
of the metal. Some of these cavities passed through the
whole thickness of the lead, and their diameters varied
with the size of the sparks that had produced them. In
general, each cavity of any size was surrounded with se¬
veral smaller cavities, which seemed to prove that each
discharge was accompanied by smaller electric sparky dis¬
seminated around it. Besides these cavities, the light¬
ning had Spread on the surface of the metal a stratum of
pulverulent matter, which adhered so strongly to the lead
that none of it could be detached without removing at the
same time a portion of the metal. Each large cavity was
the centre of one of these strata, which appeared to be
composed of globules of lead in the central part, and fer¬
ruginous dust on the margin. The glass, though an in¬
sulating body, was, as we have mentioned, marked also by
the lightning. The origin of the thin strata formed on its
surface was at those points where it had been in contact
with the lead; but they extended much beyond this, and
were composed at first of a powdery matter, sometimes
blackish and sometimes whitish ; and beyond this they
terminated in continuous and diaphanous laminae, which re¬
flected the colours of thin plates. The central and pulve-