effects, and the importance of light, we may see with what reason
the great epic poet of our language has apostrophized it in the
" Hail, holy Light ! offspring of He aven, first born,
Bright effluence sf bright essence increate ;"
<itd why the author of the "Seasons" has in a similar manntu
addressed it in the terms :
" Prime cheerer, Light !
Of all material beings first aud best !
Efflux divine ! Nature's resplendent robe !
Without whose vesting beauty all were wrapt
In unessential gloom ; and thou, Sun !
Soul of surrounding worlds, in whom best seen
Shines out thy Maker ! may I sing of thee 1 "
950. ELECTRICITY. Electricity is the
What is Eleo . . , ,. J . .
name given to an imponderable agent which
pervades the material world, and which is
visible only in its effects.
951. It is quite imponderable, susceptible of
high degrees Of intensitv > with a tendency to
equilibrium unlike tha-l; of any other known
agent. Its simplest exhibition is seen in the form of attraction
952. If a piece of amber, sealing-wax, or smooth glass, perfectly
olean and dry, be briskly rubbed with a dry woollen cloth, and im-
mediately afterwards held over small and light bodies, such as
pieces of paper, thread, cork, straw, feathers, or fragments of gold-
leaf, strewed upon a table, these bodies will be attracted, and fly
towards the surface that has been rubbed, and adhere to it for a
953. The surfaces that have acquired this power of attraction
are said to be excited; and the substances thus susceptible of being
excited are called electrics, while those which cannot be excited in a
similar manner are called non-electrics.
954. The science of Electricity, therefore,
What are the ,. . , _. -,*'
Metrical divis- divides all substances into two kinds, namely,
ions of all sub- Electrics, or those suostances which can be
excited, and Non-electrics, or those sub
stances which cannot be excited.
SIOWPI where tha heating power is feeblest, and tnut the optical
Bovver is the strongest between the other two.
940. The chemical properties of light are shown in this, that the
light of the sun, and in an inferior degree that of day when the sun
is hidden from view, is a means of accelerating chemical combina-
tions and decompositions. The following experiment exhibits the
chemical effects of light :
Place a mixture of equal parts (by measure J of chlorine and hy-
drogen gas in a glass vessel, and no change will happen so long as
the vessel be kept in the dark and at an ordinary temperature ; but,
on exposing it to the daylight, the elements will slowly combine
and form hydrochloric acid ; if the glass be set in the sun's rays,
the union will be accompanied with an instantaneous detonation.
The report may also be produced by transmitting ordinary daylight
through violet or blue glass to the mixture, but by interposing a re
glass between the vessel and the light all combination of the elements
947. The chemical effects of light have recently
What is , .
meant by Pho- been employed to render permanent the images ob-
tography., or tained by means of convex lenses. The art of thua
Heliography? fo in ^ them ig termed p hotograpnVj or Heliography.
These words are Greek derivatives ; the former meaning " writing
or drawing by means of light" the latter " writing or draw-
ing by the aid of the sun." (See par. 1491.)
Who is the ^" ^ he mO( * e * n wn * cn tne process is performed
, f pi * s essentially as follows: The picture, formed by a
^o- T h i l ~ camera obscura, is received on a plate, the surface of
grap y . w hich has been previously prepared so as to make it
as susceptible as possible of the chemical influence of light. After
the lapse of a longer or shorter time, the light will have so acted on
the plate that the various objects the images of which were pro-
jected upon it will appear, with all their gradations of light and
shade, most exactly depicted in black and white, no color being
present. This is the process commonly known by the name of
Daguerreotype, from M. Daguerre, the author of the discovery
Since his original discovery, he has ascertained that by isolating and
electrifying the plate it acquires such a sensibility to the chemical
influence of light that one-tenth of a second is a sufficient time to
obtain the requisite luminous impression for the formation of the
949. The chemical effects of light are seen in the varied colors of
the vegetable world. Vegetables which grow in dark places are either
vhite or of a palish-yellow. The sunny side of fruits is of a richer
tinge than that which grows in the shade. Persons whose daily
employment keeps them much within doors are pale, and more or
less aickly, in consequence of such confinement.
260 NATURAL PHILOSOPHY.
Tlier n no-Ehctricity ; 4thly, by Magnetism. Frictiona^
Electricity forms the subject of that branch of Electricitj
usually treated under the head of Natural Philosophy;
Electricity excited by chemical action forms the subject
of Galvanism ; and Electricity produced by the agency
of heat, or by Magnetism, is usually considered in connec-
tion with the subject of Electro-Magnetism. The intimate
connection between these several subjects shows ho'/r close
arc the links of the chain by which all the departments of
physical science are united.
95U. The electric fluid is readily coinmu-
by a Conductor n ^ cate ^ fr m one substance to another. Some
and a Non-con- substances, however, will not allow it to pass
tricitl i tbrough or over them, while others give it a
free passage. Those substances through
which it pa?EC& without obstruction are called Conductors
while those through which it cannot readily pass are called
Non-conduct?) s ; and it is found, by experiment, that all
electrics* are non-conductors, and all non-electrics ars
good conductors of electricity.
960. The following substances are electrics, or non-conductors
vF electricity ; namely,
Atmospheric air (when dry), Feathers,
All pvccious stones, Silk,
All *roms and resins, Wool,
The r&ides of all metals, Hair,
Soal ing-wax, Cotton.
All these substances must be dry, or they will beccran mor*
<w less conductors.
* Tbe terms "electrics" and " uou -electrics"' huve fallec into disuse
ELEU'JKICITY. 26 1
^61. The following substances are non-electrics, or conOuctora
of electricity ; namely,
All metals, Living animals,
Charcoal, Vapor, or steam.
962. The following are imperfect conductors (that is, they
<unduct the electric fluid, but not so readily as the substances
above mentioned^ ; namely,
Water, Common wood,
Green vegetables, Dead animals
Damp air, Bone,
Wet wood, Horn, &c.
All substances containing moisture.
When is a con- 963. When a conductor is surrounded on
tTinsulated? a11 si(ies ^J non-conducting substances, it is
said to be insulated.
964. As glass is a non-conducting substance, any conducting
mibstance surrounded with glass, or standing on a table or stool
with glass legs, will be insulated.
965. As the air is a non-conductor when dry, a substance
which rests on any non-conducting substance will be insulated,
unless it communicate with the ground, the floor, a table, &c.
966. When a communication is made be-
H iuctor S ckar^edJ tween a conductor and an excited surface,
the electricity from the excited surface is
Immediately conveyed by the conductor to the ground ; but,
if the conductor be insulated, its whole surface will become
electrified, and it is said to be charged.
What is the 967 '. The earth may be considered as the
yrand reservoir principal reservoir of elec tricity ; and when a
communication exists, by means of any con-
ducting substance, between a body containing more than its
natural share of the fluid and the earth, the body will imme
liately lose its: redundant quantity, and the fluid will escape to
iJO*2 NATURAL PHILOSOPHY.
the earth. Thus, when a person holds a metallic tuoe to afc
excited surface, the electricity escapes from the surface 30 the
tube, and passes from the tube through the person to the floor ;
and the floor being connected with the earth by conducting sub-
stances, such as the timbers, &c., which support the building,
ihe electricity will finally pass off, by a regular succession of
conducting substances, from the excited surface to the earth.
But, if the chain of conducting substances be interrupted, that
is, if any non-conducting substance occur between the excited
surface and the course which the fluid takes in its progress to
the earth, the conducting substances will be insulated, and be-
come charged with electricity. Thus, if an excited surface be
connected by a long chain to a metallic tube, and the metallic
tube be held by a person who is standing on a stool with glass
legs, or on a cake of sealing-wax, resin, or any other non-con-
ducting substance, the electricity cannot pass to the ground, and
the person, the chain and the tube, will all become electrified.
What is the sim- A ,, m . . . , p .
pkst mode of "68. The simplest mode of exciting elec-
exciting electric- tricity is by friction.
Thus, if a thick cylinder of sealing-wax, or sulphur, or a
glass tube, be rubbed with a silk handkerchief, a piece of clean
flannel, or the fur of a quadruped, the electric fluid will be
excited, and may be communicated to other substances from the
electric thus excited.
Whatever substance is used, it must be perfectly dry. It,
therefore, a glass tube be used, it should previously be held o
the fire, and gently warmed, in order to remove all moisture
from its surface.
What is meant 969 ' The electri <% excit e<* in glass ift
by Vitreous and called the Vitreous or positive electricity j
/ridf T deC ~ and that obtained from sealing-wax, or other
resinous substances, is called Resmous t o/
970. The vitreous and lesinous or, in
other words, the positive and negative eleo-
lofy is charged tricities, always accompany each other ; for,
0/ electricity f ^ an j surface become positive, the surface
with which it is rubbed will become nega-
tive, and if any surface be made positive, the nearest con-
ducting surface will become negative ; and, if positive
electricity be communicated to one side of an electric, (as
a pane of glass, or a glass vial), the opposite side will be-
jome negatively electrified, and the plate or the glass if
then said to be charged.
971. When one side of a metallic, or other conductoi
receives the electric fluid, its whole surface is instantly per-
vaded ; but when an electric is presented to an electrified body
it becomes electrified in a small spot only.
What is the 972. When two surfaces oppositely electrified are
effect when united, their powers are destroyed; and, if their
oppositely union be made through the human body, it pro-
electrified are duces an affection of the nerves, called an electric
What is the law of 973 - Similar states of electricity repe
electrical attraction each other ; and dissimilar states attract
and repulse! each other .
Thus, if two pith-balls, suspended by a silk thread, are both
positively or both negatively electrified, they will repel each
other ; but if one be positively and the other negatively electri-
fied, they will attract each other.
What is the 974. The Leyden jar is a glass vessel used
Leyden jar? f or the purpose of accumulating the electric
tiuid, procured from excited surfaces.
Ei plain 97^. Fig. 143 represents a Leyden jar. It
is a glass jar, coated both on the inside and the
wutside with tin -foil, with a cork, or wooden stopper, through
264 NATURAL 1'HILOSOI'lll
which a metallic rod passes, terminating upwards in a bia
knob, and connected by means of a wire, at the other Fig. us
end, with the inside coating of the jar. The coating
extends both on the inside and outside only to within
two or three inches of the top of the jar. Thus pre-
pared, when an excited surface is applied to the
brass knob, or connected with it by any conducting
surface, it parts with its electricity, the fluid enters
the jar, and the jar is said to be charged.
When a jar is 976. When the Leyden jar
charged where h , h fl id contained Qn the
is the electric-
ity? surface of the glass. The coating
serves only as a conductor to the fluid ; and, as this conductor
within the glass is insulated, the fluid will remain in the jar ujtil
a communication be made, by means of some conducting sub
stance, between the inside and the outside coating of the jar.
If then a person apply one hand or finger to the brass knob, and
the other to the outside coating of the jar, a communication will
be formed by means of the brass knob with the inside and out-
side of the jar, and the jar will be discharged. A vial or jai
that is insulated cannot be charged.
What if an Eke- 977. Anelectrical battery is composed of
incai Battery? a num ^ er O f Leyden jars connected together
The inner coatings of the jars are connected together by
chains or metallic bars attached to the brass knobs of each jar;
and the outer coatings have a similar connection established by
placing the vials on a sheet of tin-foil. The whole battery may
then be charged like a single jar. For the sake of convenience
in discharging the battery, a knob connected with the tin-foil on
which the jars stand projects from the bottom of the box which
3ontai.is the jars.
What is th* joint- 978. The jointed discharger is an instru-
ct discharger ? ment used to discharge a jar or battery.
Explain Fig. 144 represents the jointed discharger. It
riff. j. 44. (VQppjgt? of two rods, generally of brass, terminating
at one ei'd in brass balls, and connected p 's- T44
together at the other end by a joint, like
that of a pair of tongs, allowing them
to be opened or closed. It is furnished
with a glass handle, to secure the person
who holds it from the effects of a shock.
When opened, one of the balls is made to touch the outside
coating of the jar, or the knob connected with the bottom of the
battery, and the other is applied to the knob of the jar or jars.
A. communication being thus formed between the inside and the
outside of the jar, a discharge of the fluid will be produced.
Where must ^^' ^ ien a charge of electricity is to be
i body be sent through any particular substance, the
olaced, in or- gu lb gtanco must f orm a part O f fa e circuit of
uT 10 TCC61VC A "^
a charge of electricity ; that is, it must be placed in such
electricity ? a manner ^hat the fluid cannot pass from the
inside to the outside surface of the jar, or battery, without
passing through the substance in its passage.
What, effect have sharp 9 ^0. Metallic rods, with sharp points
metallic points ? silently attract the electric fluid.
If the balls be removed from the jointed discharger, and the
two rods terminate in sharp points, the electricity will pass off
silently, and produce but little effect.
How may a ^^^ 981. A Ley den jar, or a battery, may be silently
batt 1 ' Discharged by presenting a metallic point, even that
lenity dis- of the finest needle, to the knob ; but the point must
charged? i e i r<ni ght slowly towards the jar.
982. It is on this principle that lightning-rods
riple ate tight-* constructed. The electric fluid is silently
ning-rods drawn from the cloud by the sharp points on the
c instructed ? rQ ^ ^ ^ ^^ p revente( j f rom su ddcrily exploding
on high buildings.
. 983. Electricity of one kind or the other is gen-
>nea-/it by erally induced in surrounding bodies by the vie-in-
ity of a highly-excited electric. This mode of com-
raunicating electricity by approach is styled induc-
984. A body, on approaching another body powerfully elec-
trified, will be thrown into a contrary state of electricity. Thus,
a feather, brought near to a glass tube excited by friction, will
be attracted to it ; and, therefore, previously to its touching the
tube, negative electricity must have been induced in it. On the
contrary, if a feather be brought near to excited sealing-wax, it
will be attracted, and, consequently, positive electricity must
have been induced in it before contact.
What is 985- When electricity is communicated from
Electricity by one body to another in contact with it, it is
Transfer* ^^ electricity by transfer.
W/,attsan 986 The e i ectr i ca i mac hine is a machine
Maeki we, and constructed for the purpose of accumulating or
Ir^it'con collectia S electricity, and transferring it to other
987. Electrical Machines are made in various forms, but all
on the same principle, namely, the attraction of metallic points.
The electricity is excited by the friction of silk on a glass sur-
face, assisted by a mixture or preparation called an amalgam,
composed of mercury, tin, and zinc. That recommended by
Singer is made by melting together one ounce of tin and two
ounces of zinc, which are to be mixed, while fluid, with six
ounces of mercury, and agitated in an iron or thick w r ooden box,
until cold. It is then to be reduced to a very fine powder in a
mortar, and mixed with a sufficient quantity of lard to form it
into a paste.
The glass surface is macTe either in the form of a cylinder or
a circular plate, and the machine is called a cylinder or a plata
maohine, according as it is made with a cylinder or with a plate.
Explain 988. Fig, 145 represents a plate electrical m.v
Fig. 145. chine. A D is the stand of the machine, L L L L
are the four glass legs, or posts, which support and insulate the
parts of the machine. P is the glass plate (which in some ma-
chines is a hollow cylinder) from wh ; ch the electricity is excited,
and H is the handle by which the plate (or cylinder) is turned.
R is a leather cushion, or rubber, held closely to both sides of
the glass plate by a brass clasp, supported by the post G L
which is called the rubber-post. S is a silk bag, embraced by
the same clasp that holds the leather cushion or rubber ; and it
is connected by strings S S S attached to its three other corners,
and to the legs L L and the fork F of the prime conductor. G
IP the prime conductor, terminating at one end with a movable
brass ball. B, and at the other by the fork F, which has one
prong on each side of the glass plate. On each prong of the
fork there are several sharp points projecting towards the plate,
to collect the electricity as it is generated by the friction of the
plate against the rubber. V is a chain or wire, attached to the
brass ball on the rubber-post, and resting on the table or the
fioor, designed to convey the fluid from the ground to the plate
When negative electricity is to be obtained, this chain is re
moved from the rubber-post and attached to the prime conductor
and the electricity is to be gathered from the ball on the rubber
Explain the ^89. OPERATION OF THE MADHINE. By turning
operation of the handle H, the glass plate is pressed by the rub-
Ii68 NATURAL PHILOSOPHY.
the Electri- ber. The friction of the rubber against the glas*
cal Machine. pl a ^ e ( or cylinder) produces a transfer of the elec-
tric fluid from the rubber to the plate; that is, the cushion be-
comes negatively and the glass positively electrified. The fluid
which thus adheres to the glass, is carried round by the revolu-
tion of the cylinder ; and, its escape being prevented by the silk
oag, or flap, which covers the plate (or cylinder) until it comes
to the immediate vicinity of the metallic points on the fork F,
it is attracted by the points, and carried by them to the prime
conductor. Positive electricity is thus accumulated on the prime
conductor, while the conductor on the rubber-post, being deprived
of this electricity, is negatively electrified. The fluid may then
be collected by a Leyden jar from the prime conductor, or con-
veyed, by means of a chain attached to the prime conductor, to
any substance which is to be electrified. If both of the conduc-
tors be insulated, but a small portion of the electric fluid can be
excited ; for this reason, the chain must in all cases be attached
to the rubier-post, when positive electricity is required, and to
the prime conductor when negative electricity is wanted.
What is an ^90. ^ n ^ e P r ^ me conductor is placed an
Electron.- Electrometer, or measurer of electricity. It ia
*what*mincl ma( ^ e ^ n various forms, but always on the prin-
ple is it con- ciple that similar states of electricity repel each
< /rucied - other.
It sometimes consists of- a single pith-ball, attached to a light
rod in the manner of a pendulum, and behind is a graduated arc,
or circle, to measure the repulsive force by degrees. Sometimes
it is more simply made (as in the figure), consisting of a wooden
ball mounted on a metallic stick, or wire, having two pith-balls,
suspended by silk, hair, or lineif threads. When the machine
is worked, the pith-balls, being both similarly electrified, repel
each other ; and this caus is them to fly apart, as is represented
in the figure;- and they will continue elevated until the electric-
ity is drawn off. But, if an uninsulated conducting substance
tou".h the prime conductor, the pith-balls will fall. The height
k which the balls rise, and the quickness with which they are
elevated, afford some test of the power of the machine. This
simple apparatus may be attached to any body the electricity
of which we wish to measure.
The balls of the electrometer, when elevated, are attracted by
any resinous substance, and repelled by any vitreous substance
that has been previously excited by friction.
991. If an electric, or a non-conductor, be presented to the prime
conductor, when charged, it will produce no effect on the balls ;
but if a non-electric, or any conducting substance, be presented
to the conductor, the balls of the electrometer will fall. This
shows that the conductor has parted with its electricity, and
that the fluid has passed off to the earth through the substance,
and the hand of the person presenting it.
~ ., 992. An Electroscope is an instrument, of more
Bennett's delicate construction, to detect the presence of
Electroscope, electricity. The most sensitive of this kind of
apparatus is that called Bennett's Gold-leaf Electroscope, im-
proved by Singer. It consists of two strips of gold-leaf suspended
under a glass covering, which completely insulates them. Strip?
of tin-foil are attached to the sides of the glass, opposite the
gold-leaf, and when the strips of gold-leaf diverge, they will touch
the tin-foil, and be discharged. A pointed wire surmounts the
instrument, by which the electricity of the atmosphere may be
993. An Electrophorus is a simple apparatus by which small
portions of electricity may be generated by induction. It con-
sists of a disc, or circular cake of resinous substance,^ on which
is laid a smaller circular disc of metal, with a glass handle. Rub
the resinous disc with hair or the fur of some animal, and the
metallic disc, being pressed down on the resii by the finger,
may then be raised by the glass handle. It will contain a small
portion of electricity, which may be communicated to the Leyden
jar, and thus the jar may slowly be charged.
A mixture of Shell-lac resin and Venice- turpentine, aast in a tin mcuH
Z70 NATURAL PHILOSOPHY.
994. EXPERIMENTS WITH THE ELECTRICAL MACHINE In
peforming experiments with the Electrical Machine, great ear*
inurt be taken that all its parts be perfectly dry and clean
Moisture arid dust, by carrying off the electricity as fast as it is
generated, prevent successful action. Clear and cold weather
should be chosen, if possible, as the machine will always perform
its work better then.
995. When the machine is turned, if a person touch the prime
conductor, the fluid passes off through the person to the floor
without his feeling it. But if he present his finger, his knuckle,
or any part of the body, near to the conductor, without touching
it, a spark will pass from the conductor to the knuckle, which
will produce a sensation similar to the pricking of a pin or
996. If a person stand on a stool with glass legs, or any other
non-conductor, he will be insulated. If in this situation he
touch the prime conductor, or a chain connected with it, when
the machine is worked, sparks may be drawn from any part of