Common or Static Electricity, or Electricity of Tension—A Dual
Power—Methods of exciting it—Attraction and
Repulsion—Conduction—Electrics and
Non-electrics—Induction—Dielectrics—Tension—Law of the Electric
Force—Distribution—Laws of Distribution—Heat of Electricity—Electrical
Light and its Spectrum—Velocity—Atmospheric Electricity—Its
cause—Electric Clouds—Violent effects of Lightning—Back
Stroke—Electric Glow—Phosphorescence.
ELECTRICITY is a dual power which gives no visible sign of its existence
when in equilibrio, but when elicited forces are developed capable of
producing the most sudden, violent, and destructive effects in some
cases, while in others their action, though generally less energetic, is
of indefinite and uninterrupted continuance. These modifications of the
electric forces, incidentally depending upon the manner in which they
are excited, present phenomena of great diversity, but yet so connected
as to justify the conclusion that they originate in a common principle.
The hypothesis of electricity being a fluid is untenable in the present
advanced state of the science; we only know that it is a force whose
action is twofold; that bodies in one electric state attract, and in
another repel each other; in the former the electricity is said to be
positive, in the latter negative; and thus regarding it as a force, its
modes of action come under the laws of mechanics and mathematical
analysis.
Electricity may be called into activity by the friction of heterogeneous
substances, as in the common electrifying machine, by mechanical power,
heat, chemical action, and the influence of magnetism. We are totally
ignorant why it is roused from its neutral state by these means, or of
the manner of its existence in bodies; but when excited it seems to
produce a molecular polarity or chemical change in the ultimate
particles of matter.
The science is divided into various branches, of which static or common
electricity comes first under consideration, including that of the
atmosphere. Substances in a neutral state neither attract nor repel.
There is a numerous class called electrics in which the electric
equilibrium is destroyed by friction; then the positive and negative
electricities are called into action or separated; the positive is
impelled in one direction, and the negative in another. Electricities of
the same kind repel, whereas those of different kinds attract each
other. The attractive power is exactly equal to the repulsive power at
equal distances, and when not opposed they coalesce with great rapidity
and violence, producing the electric flash, explosion, and shock; then
the equilibrium is restored. One kind of electricity cannot be evolved
without the evolution of an equal quantity of the opposite kind. Thus
when a glass rod is rubbed with a piece of silk, as much positive
electricity is elicited in the glass as there is negative in the silk.
The kind of electricity depends more upon the mechanical condition than
on the nature of the surface; for when two plates of glass, one polished
and the other rough, are rubbed against each other, the polished surface
acquires positive and the rough negative electricity. The manner in
which friction is performed also alters the kind of electricity. Equal
lengths of black and white ribbon applied longitudinally to one another,
and drawn between the finger and thumb so as to rub their surfaces
together, become electric. When separated the white ribbon is found to
have acquired positive electricity, and the black negative; but if the
whole length of the black ribbon be drawn across the breadth of the
white, the black will be positively and the white negatively electric
when separated. The friction of the rubber on the glass plate of the
electrifying machine produces abundance of static electricity. The
friction of the steam on the valve of an insulated locomotive
steam-engine produces seven times the quantity of electricity that an
electrifying machine would do with a plate three feet in diameter,
worked at the rate of 70 revolutions in a minute. Pressure is a source
of electricity which M. Becquerel has found to be common to all bodies;
but it is necessary to separate them to prevent the reunion of the
electricities. When two substances of any kind whatever are insulated
and pressed together they assume different electric states, but they
only show contrary electricities when one of them is a good conductor.
When both are good conductors they must be separated with extreme
rapidity to prevent a return to equilibrium. When the separation is very
sudden the tension of the two electricities may be great enough to
produce light. M. Becquerel attributes the light produced by the
collision of icebergs to this cause. Iceland spar is made electric by
the smallest pressure between the finger and the thumb, and retains it
for a long time. All these circumstances are modified by the temperature
of the substances, the state of their surfaces and that of the
atmosphere. Several crystalline bodies become electric when heated,
especially tourmaline, one end of which acquires positive, and the other
negative electricity, while the intermediate part is neutral. If the
tourmaline be broken through the middle, each fragment is found to
possess positive electricity at one end and negative at the other.
Electricity is evolved by substances passing from a liquid to a solid
state, and by chemical action during the production and condensation of
vapour, which is a great source of atmospheric electricity. In short, it
may be generally stated, that when any cause whatever tends to destroy
molecular attraction there is a development of electricity; if, however,
the substances be not immediately separated, there will be an
instantaneous restoration of equilibrium.
Electricity may be transferred from one body to another in the same
manner as heat is communicated, and like it too the body loses by the
transmission.
Although no substance is altogether impervious to electricity, nor is
there any that does not offer some resistance to its passage, yet it
moves with more facility through a certain class of substances called
conductors, such as metals, water, the human body, &c., than through
atmospheric air, glass, silk, &c., which are therefore called
non-conductors. The conducting power is affected both by temperature and
moisture. The terrestrial globe is a conductor on account of its
moisture, though dry earth is not. Though metals are the best conductors
of electricity, it affects their molecular structure, for the heat which
accompanies its passage acts as a transverse expansive force, which
increases their breadth by diminishing their length, as may be seen by
passing electricity through a platinum wire sufficiently thick to resist
fusion. Through air the force is disruptive on account of its
non-conducting quality, and it seems to act chemically on the oxygen,
producing the substance known as ozone during its passage through the
atmosphere. If a conductor be good and of sufficient size the
electricity passes imperceptibly but it is shivered to pieces in an
instant if it be a bad conductor or too small to carry off the charge.
In that case the physical change is generally a separation of the
particles, or expansion from the heat, as in trees, where it turns the
moisture into steam, but all these effects are in proportion to the
obstacles opposed to the freedom of its course.
Bodies surrounded by non-conductors are said to be insulated, because
when charged the electricity cannot escape. When that is not the case,
the electricity is conveyed to the earth: consequently it is impossible
to accumulate electricity in a conducting substance that is not
insulated. There are a great many substances called non-electrics in
which electricity is not sensibly developed by friction unless they be
insulated, because it is carried off by their conducting power as soon
as elicited. Metals, for example, which are said to be non-electrics can
be excited, but being conductors they cannot retain this state if in
communication with the earth. It is probable that no bodies exist which
are either perfect non-electrics or perfect non-conductors. But it is
evident that electrics must be non-conductors to a certain degree,
otherwise they could not retain their electric state.
A body charged with electricity, although perfectly insulated, so that
all escape of electricity is prevented, tends to produce an electric
state of the opposite kind in all bodies in its vicinity. Positive
electricity tends to produce negative electricity in a body near to it,
and _vice versâ_, the effect being greater as the distance diminishes.
This power which electricity possesses of causing an opposite electrical
state in its vicinity is called induction. A Leyden jar, for example, or
glass jar coated half way up both outside and in with tin foil, when
charged with positive electricity, immediately induces negative
electricity on the tin foil outside. Notwithstanding their strong mutual
attraction they are prevented from coalescing by the glass, which is a
non-conductor; but if the tin inside and out be connected by a
conducting wire they instantly unite. When a body in either electric
state is presented to a neutral one, its tendency in consequence of the
law of induction is to disturb the condition of the neutral body by
inducing electricity contrary to its own in the adjacent side, and
therefore an electrical state similar to its own in the remote part.
Hence the neutrality of the second body is destroyed by the action of
the first, and the adjacent parts of the two, having now opposite
electricities, will attract each other. The attraction between
electrified and unelectrified substances is a consequence of the altered
state of their molecules. Induction depends upon the facility with which
the equilibrium of the neutral body can be overcome, a facility which is
proportional to its conducting power. Consequently the attraction
exerted by an electrified substance upon another substance previously
neutral will be much more energetic if the latter be a conductor than if
it be a non-conductor.
It is clear that one body cannot act upon another at a distance without
some means of communication. Dr. Faraday has proved that the intervening
non-conducting substance or dielectric has a great influence upon
induction. Thus the inductive force is greater when sulphur is
interposed between the two bodies than when shellac is the dielectric,
and greater when shellac is the dielectric than glass, &c. Professor
Matteucci has proved by the following experiment that the intervening
substance is itself polarized by induction. A number of plates of mica
in contact were placed between two plates of metal, one of which was
electrified, so that the whole was charged like a Leyden jar. On
separating the plates with insulating handles, each plate of mica was
electrified; one side of it was positive and the other negative, showing
decidedly a polarization by induction throughout the whole intervening
non-conducting substance; and thus, although the interposed substance or
dielectric is incapable of conducting the electrical force from one body
to the other, it becomes by induction capable of transmitting it. In the
atmosphere induction is transmitted by that of the intervening strata of
air. It is true that induction takes place through the most perfect
vacuum we can make, but there always remains some highly elastic air;
and even if air could be altogether excluded, the ethereal medium
cannot, and it must be capable of induction, since, however attenuated,
it must consist of material atoms, otherwise it would be a nonentity.
The law of electrical attraction and repulsion has been determined by
suspending a needle of gum-lac horizontally by a silk fibre, the needle
carrying at one end a piece of electrified gold leaf. A globe in the
same or opposite electrical state when presented to the gold leaf will
repel or attract it, and will therefore cause the needle to vibrate more
or less rapidly according to the distance of the globe. A comparison of
the number of oscillations performed in a given time at different
distances will determine the law of the variation of the electrical
intensity, in the same manner that the force of gravitation is measured
by the oscillations of the pendulum. Coulomb invented an instrument
which balances the forces in question by the force of the torsion of a
thread, which consequently measures the intensity; and Sir William Snow
Harris has constructed an instrument with which he has measured the
intensity of the electrical force in terms of the weight requisite to
balance it. By these methods it has been found that the intensity of
electrical attraction and repulsion varies inversely as the square of
the distance. However, the law of repulsive force is liable to great
disturbances from inductive action, which Sir William Snow Harris has
found to exist not only between a charged and neutral body, but also
between bodies similarly charged; and that, in the latter case, the
inductive process may be indefinitely modified by the various
circumstances of the quantity and intensity of the electricity and the
distance between the charged bodies.
The quantity of electricity bodies are capable of receiving does not
follow the proportion of their bulk, but depends principally upon the
form and extent of their surface. It appears from the experiments of Sir
W. S. Harris that a given quantity of electricity, divided between two
perfectly equal and similar bodies, exerts upon external bodies only one
fourth of the attractive force apparent when disposed upon one of them;
and if it be distributed among three equal and similar bodies, the force
is one ninth of that apparent when it is disposed on one of them. Hence,
if the quantity of electricity be the same, the force varies inversely
as the square of the surface on which it is disposed; and if the surface
be the same, the force varies directly as the square of the quantity of
electricity. These laws however do not hold when the form of the surface
is changed. A given quantity of electricity disposed on a given surface
has the greatest intensity when the surface has a circular form, and the
least intensity when the surface is expanded into an indefinite straight
line. The decrease of intensity seems to arise from some peculiar
arrangement of the electricity depending on the extension of the
surface. It is quite independent of the extent of the edge, the area
being the same; for Sir W. S. Harris found that the electrical intensity
of a charged sphere is the same with that of a plane circular area of
the same superficial extent, and that of a charged cylinder the same as
if it were cut open and expanded into a plane surface.
The same able electrician has shown that the attractive force between an
electrified and a neutral uninsulated body is the same whatever be the
forms of their unopposed parts. Thus two hemispheres attract each other
with precisely the same force as if they were spheres; and as the force
is as the number of attracting points in operation directly, and as the
squares of the respective distances inversely, it follows that the
attraction between a mere ring and a circular area is no greater than
that between two similar rings, and the force between a sphere and an
opposed spherical segment of the same curvature is no greater than that
of two similar segments, each equal to the given segment.
Electricity may be accumulated to a great extent in insulated bodies,
and so long as it is quiescent it occasions no sensible change in their
properties. When restrained by the non-conducting power of the
atmosphere, its tension or the pressure it exerts is proportional to the
coercive force of the air. If the pressure be less than the coercive
force, the electricity is retained; but the instant it exceeds that
force in any one point it escapes, and that more readily when the air is
attenuated or saturated with moisture, for the resistance of the air is
proportional to the square of its density, but the inductive action of
electricity on distant bodies is independent of atmospheric pressure.
The power of retaining electricity depends also on the shape of the
charged body. It is most easily retained by a sphere, next to that by a
spheroid, but it readily escapes from a point, and a pointed object
receives it with most facility.
The heat produced by the electric shock is proportional to the square of
the quantity of electricity discharged, and is so intense that it fuses
metals and volatilizes substances, but its intensity is not felt to its
full extent on account of the shortness of its duration. It is only
accompanied by light when the electricity is obstructed in its passage
through substance.
Electrical light when analysed by a prism differs very much from solar
light. Fraunhofer found that, instead of the fixed dark lines, the
spectrum of an electric spark is crossed by numerous bright lines; and
Professor Wheatstone has observed that the number and position of the
lines differ with the metal from which the spark is taken, and believes
the spark itself results from the ignition and volatilization of the
matter of the conductor.
According to the experiments of Sir Humphry Davy, the density of the air
has an influence on the colour. He passed the electric spark through a
vacuum over mercury, which from green became successively sea-green,
blue, and purple, on admitting different quantities of air. When the
vacuum was made over a fusible alloy of tin and bismuth, the spark was
yellowish and extremely pale. Sir Humphry thence concluded that
electrical light principally depends upon some properties belonging to
the ponderable matter through which it passes, and that space is capable
of exhibiting luminous appearances, though it does not contain an
appreciable quantity of matter. He thought that the superficial
particles of bodies which form vapour, when detached by the repulsive
power of heat, might be equally separated by the electric forces, and
produce luminous appearances in vacuo by the destruction of their
opposite electric states.
The velocity of electricity is so great that the most rapid motion which
can be produced by art appears to be actual rest when compared with it.
A wheel revolving with celerity sufficient to render its spokes
invisible, when illuminated by a flash of lightning, is seen for an
instant with all its spokes distinct, as if it were in a state of
absolute repose; because, however rapid the rotation may be, the light
has come and already ceased before the wheel has had time to turn
through a sensible space. This beautiful experiment is due to Professor
Wheatstone, as well as the following variation of it, which is not less
striking: If a circular piece of pasteboard be divided into three
sectors, one of which is painted blue, another yellow, and a third red,
it will appear to be white when revolving quickly, because of the
rapidity with which the impressions of the colours succeed each other on
the retina. But, the instant it is illuminated by an electric spark, it
seems to stand still, and each colour is as distinct as if it were at
rest. This transcendent speed of electricity has been ingeniously
measured, as follows, by Professor Wheatstone, who has ascertained that
it much surpasses the velocity of light.
In the horizontal diameter of a small disc, fixed on the wall of a
darkened room, are disposed six small brass balls, well insulated from
each other. An insulated copper wire, half a mile long, is disjointed in
its middle, and also near its two extremities; the six ends thus
obtained are connected with the six-balls on the disc. When an electric
discharge is sent through the wire by connecting its two extremities,
one with the positive, and the other with the negative coating of a
Leyden jar, three sparks are seen on the disc, apparently at the same
instant. At the distance of about ten feet a small revolving mirror is
placed so as to reflect these three sparks during its revolution. From
the extreme velocity of the electricity, it is clear that, if the three
sparks be simultaneous, they will be reflected, and will vanish before
the mirror has sensibly changed its position, however rapid its rotation
may be, and they will be seen in a straight line. But if the three
sparks be not simultaneously transmitted to the disc—if one, for
example, be later than the other two—the mirror will have time to
revolve through an indefinitely small arc in the interval between the
reflection of the two sparks and that of the single one. However, the
only indication of this small motion of the mirror will be, that the
single spark will not be reflected in the same straight line with the
other two, but a little above or below it, for the reflection of all
three will still be apparently simultaneous, the time intervening being
much too short to be appreciated.
Since the number of revolutions which the revolving mirror makes in a
second is known, and the angular deviation of the reflection of the
single spark from the reflection of the other two can be measured, the
time elapsed between their consecutive reflections can be ascertained.
And, as the length of that part of the wire through which the
electricity has passed is given, its velocity may be found.
The number of pulses in a second, requisite to produce a musical note of
any pitch, are known; hence the number of revolutions accomplished by
the mirror in a given time may be determined from the musical note
produced by a tooth or peg, in its axis of rotation, striking against a
card, or from the notes of a siren attached to the axis. It was thus
that Professor Wheatstone found the mirror which he employed in his
experiments made 800 revolutions in a second; and, as the angular
velocity of the reflected image in a revolving mirror is double that of
the mirror itself, an angular deviation of one degree in the appearance
of the two sparks would indicate an interval of the 576,000th of a
second; the deviation of half a degree would, therefore, indicate more
than the millionth of a second. The use of sound as a measure of
velocity is a happy illustration of the connexion of the physical
sciences.
The earth possesses a powerful electrical tension, and the atmosphere
when clear is almost always positively electric. Its electricity is
stronger in winter than in summer, during the day than in the night. The
intensity increases for two or three hours from the time of sunrise,
comes to a maximum between seven and eight, then decreases towards the
middle of the day, arrives at its minimum between one and two, and again
augments as the sun declines till about the time of sunset, after which
it diminishes and continues feeble during the night. The mere
condensation of vapour is a source of atmospheric electricity; but
although it is also produced by the vapour that rises from the surface
of the earth, it is not under all circumstances. M. Pouillet found that
electricity is only developed when accompanied by chemical action: for
example, when the water whence the vapour proceeds contains lime, chalk,
or any solid alkali, negative electricity is produced; and when it holds
in solution either gas, acid, or some of the salts, the vapour is
positively electric. Besides, the contact of earth with salt and fresh
water generates positive electricity, and the contact of fresh and salt
currents of water negative, so that the ocean must afford a great supply
to the atmosphere; hence thunderstorms are most frequent near the
coasts: but as electricity of one kind or another is developed whenever
the molecules of matter are deranged from their natural state of
equilibrium, there must be many partial variations in the electric state
of the air. When the invisible vapour rises charged with electricity
into the cold regions of the atmosphere, it is condensed into cloud, in
which the tension is increased because the electricity is confined to a
smaller space; and if the condensation be sufficient to produce drops of
rain, they carry the electricity to the ground, so that in general a
shower is a conductor between the clouds and the earth. When two clouds
charged with opposite kinds, but of equal tension, approach within a
certain distance, the intensity increases on the sides of the clouds
that are nearest to one another; and when the tension is great enough to
overcome the coercive pressure of the atmosphere, a discharge takes
place which causes a flash of lightning, the stroke being given either
by the cloud or the rain. The actual quantity of electricity in any part
of a cloud is extremely small. The intensity of the flash arises from
the great extent of surface over which it is spread, so that clouds may
be compared to enormous Leyden jars thinly coated with electricity,
which only acquires its intensity by its instantaneous condensation. The
rapid and irregular motions of thunder clouds are probably more owing to
strong electrical attractions and repulsions among themselves than to
currents of air, though both are no doubt concerned in these hostile
movements. The atmosphere becomes intensely electric on the approach of
rain, hail, snow, sleet, and wind; but it varies afterwards, and the
transitions are very rapid on the approach of a thunderstorm.
Since air is a non-conductor, it does not convey the electricity from
the clouds to the earth, but it acquires from them an opposite kind, and
when the tension is very great the force of the electricity becomes
irresistible, and an interchange takes place between the clouds and the
earth; but so rapid is the motion of lightning, that it is difficult to
ascertain whether it goes from the clouds to the earth or shoots upwards
from the earth to the clouds, though there can be no doubt that it does
both. In a storm that occurred at Manchester in June 1835, the lightning
was observed to issue from various points of a road, attended by
explosions as if pistols had been fired out of the ground, and a man
seems to have been killed by one of these explosions taking place under
his foot. M. Gay Lussac ascertained that a flash of lightning sometimes
darts more than three miles in a straight line. A person may be killed
by lightning, although the explosion takes place at a distance of twenty
miles, by what is called the back stroke. Suppose that the two
extremities of a highly charged cloud hang down towards the earth, they
will repel the electricity from the earth’s surface if it be of the same
kind with their own, and will attract the other kind; and if a discharge
should suddenly take place at one end of the cloud, the equilibrium will
be instantly restored by a flash at that point of the earth which is
under the other. Though the back stroke is often sufficiently powerful
to destroy life, it is never so terrible in its effects as the direct
stroke, which is often of inconceivable intensity. Instances have
occurred when large masses of iron and stone, and even many feet of a
stone wall, have been carried to a considerable distance by a stroke of
lightning. Rocks and the tops of mountains often bear the marks of
fusion from its intense heat; and occasionally vitreous tubes descending
many feet into banks of sand mark its path. Dr. Fiedler exhibited
several of these fulgorites in London of considerable length, which had
been dug out of the sandy plains of Silesia and Eastern Prussia. One
found at Paderborn was forty feet long. Their ramifications generally
terminate in pools or springs of water below the sand, which are
supposed to determine the course of the lightning. No doubt the soil and
substrata must influence its direction, since it is found by experience
that places which have been once struck by lightning are often struck
again. An insulated conductor on the approach of a storm gives out such
quantities of sparks that it is dangerous to approach it, as was fatally
experienced by Professor Richman at Petersburg, who was struck dead by a
globe of fire from the extremity of a conductor, while making
experiments on atmospheric electricity. Copper conductors afford the
best protection, especially if they expose a broad surface, since
electricity is conveyed along the surface of bodies. There is no
instance of an electric cloud of high tension being dispelled by a
conductor, yet those invented by Sir William Snow Harris, and
universally employed in the navy, afford a complete protection in the
most imminent danger. The Shannon, a 50-gun frigate, commanded by the
brave and lamented Sir William Peel, was enveloped in a thunder-storm
when about 90 miles to the north-west of Java. It began at fifty minutes
past four in the afternoon; the ship was driven before the storm, in a
high sea, amid streams of vivid lightning, deafening thunder, hail, and
rain. At five o’clock an immense ball of fire covered the maintopgallant
mast, ran up the royal pole, and exploded in the air with a terrific
concussion, covering all the surrounding space with sparks of electric
light, which were driven rapidly to leeward by the wind. Fifteen minutes
later an immense mass of lightning struck the mainmast, attended by a
violent gust of wind; and another heavy discharge fell on it a quarter
of an hour afterwards. From that time till six o’clock the ship was
continually encompassed by sharp forked lightning, accompanied by
incessant peals of thunder. Though actually enveloped in electricity,
and struck three times, neither the hull nor the rigging sustained the
slightest injury.
When the air is rarefied by heat, its coercive power is diminished, so
that the electricity escapes from the clouds in those lambent diffuse
flashes without thunder so frequent in warm summer evenings; and when
the atmosphere is highly charged with electricity, it not unfrequently
happens that electric light, in the form of a star, is seen on the
topmasts and yard-arms of ships. In 1831 the French officers at Algiers
were surprised to see brushes of light on the heads of their comrades,
and at the points of their fingers when they held up their hands. This
phenomenon was well known to the ancients, who reckoned it a lucky omen.
Many substances, in decaying, emit light, which is attributed to
electricity, such as fish and rotten wood. Oyster-shells, and a variety
of minerals, become phosphorescent at certain temperatures when exposed
to electric shocks or friction: indeed, most of the causes which disturb
molecular equilibrium give rise to phosphoric phenomena. The minerals
possessing this property are generally coloured or imperfectly
transparent; and, though the colour of this light varies in different
substances, it has no fixed relation to the colour of the mineral. An
intense heat entirely destroys this property, and the phosphorescent
light developed by heat has no connexion with light produced by
friction; for Sir David Brewster observed that bodies deprived of the
faculty of emitting the one are still capable of giving out the other.
Among the bodies which generally become phosphorescent when exposed to
heat, there are some specimens which do not possess this property;
wherefore phosphorescence cannot be regarded as an essential character
of the minerals possessing it. Sulphuret of calcium, known as Canton’s
phosphorus, and the sulphuret of barium, or Bologna stone, possess the
phosphorescent property in an eminent degree.
Multitudes of fish are endowed with the power of emitting light at
pleasure, no doubt to enable them to pursue their prey at depths where
the sunbeams cannot penetrate. Flashes of light are frequently seen to
dart along a shoal of herrings or pilchards; and the Medusa tribes are
noted for their phosphorescent brilliancy, many of which are extremely
small, and so numerous as to make the wake of a vessel look like a
stream of silver. Nevertheless, the luminous appearance which is
frequently observed in the sea during the summer months cannot always be
attributed to marine animalculæ, as the following narrative will show:—
Captain Bonnycastle, coming up the Gulf of St. Lawrence on the 7th of
September, 1826, was roused by the mate of the vessel in great alarm
from an unusual appearance. It was a starlight night, when suddenly the
sky became overcast in the direction of the high land, and an
instantaneous and intensely vivid light, resembling the aurora, shot out
of the hitherto gloomy and dark sea on the lee bow, which was so
brilliant that it lighted everything distinctly even to the mast-head.
The light spread over the whole sea between the two shores, and the
waves, which before had been tranquil, now began to be agitated. Captain
Bonnycastle describes the scene as that of a blazing sheet of awful and
most brilliant light. A long and vivid line of light, superior in
brightness to the parts of the sea not immediately near the vessel,
showed the base of the high, frowning, and dark land abreast; the sky
became lowering and more intensely obscure. Long tortuous lines of light
showed immense numbers of very large fish darting about as if in
consternation. The sprit-sail yard and mizen-boom were lighted by the
glare, as if gaslights had been burning directly below them; and until
just before daybreak, at four o’clock, the most minute objects were
distinctly visible. Day broke very slowly, and the sun rose of a fiery
and threatening aspect. Rain followed. Captain Bonnycastle caused a
bucket of this fiery water to be drawn up; it was one mass of light when
stirred by the hand, and not in sparks as usual, but in actual
coruscations. A portion of the water preserved its luminosity for seven
nights. On the third night, the scintillations of the sea reappeared;
the sun went down very singularly, exhibiting in its descent a double
sun; and, when only a few degrees high, its spherical figure changed
into that of a long cylinder, which reached the horizon. In the night
the sea became nearly as luminous as before, but on the fifth night the
appearance entirely ceased. Captain Bonnycastle did not think it
proceeded from animalculæ, but imagined it might be some compound of
phosphorus, suddenly evolved and disposed over the surface of the sea.
It had probably been that peculiar form of electricity known as the glow
discharge, of which the author once saw a very remarkable instance.
M. E. Becquerel assures us that almost all substances are phosphorescent
after being exposed to the sun if instantly withdrawn into darkness, and
that it depends upon the arrangement of the particles and not upon
chemical action. The salts of uranium give the same kind of
phosphorescent light as that produced by the violet rays of the solar
spectrum. A solution of the bisulphate of quinine emits a yellow
phosphorescent light, whereas the fluorescent light of that liquid is
blue. The colours of these two kinds of light are generally
complementary to one another.
Phosphorescence is probably more or less concerned in some, at least, of
a series of very curious experiments made by M. Niepcé de Saint-Victor,
on what he calls the saturation of substances with light. It has long
been known that, if a person in an intensely dark room should expose his
arm to the sun through a hole in a window-shutter, it will shine on
being drawn into the darkness. Now, M. de Saint-Victor found that if an
engraving be exposed for a certain time to the sun, and instantly
brought into darkness, it will make a photographic impression on a
collodion or argentine surface, and that anything written or drawn with
tartaric acid, or a solution of the salts of uranium, in large
characters, is reproduced even at a small distance from a sensitive
surface. It may be presumed that the light communicates its vibrations
to the surfaces exposed to it with sufficient force to enable them to
disturb the unstable equilibrium of such sensitive substances as
collodion or the argentine salts. M. de Saint-Victor has shown that
tartaric acid, which is readily impressed by sunlight, is neither
fluorescent nor phosphorescent, whence he concludes that his experiments
are independent of both of these modes of action. Uranium appears to
have very peculiar properties: its salts are strongly luminous when
exposed to the sun; they are very fluorescent; and the crystallized
azitote of uranium becomes phosphorescent by percussion.