different kinds to each other is often striking and curious; as, for
instance, those of salt to those of water. The salt attracts the water,
and the water the salt, till at last, if there is a sufficient quantity
of water, all the salt is attracted particle by particle from itself,
and taken up and united to the water. The salt is no longer visible to
the eye, and is said to be dissolved or in solution; but this change of
form is due to its affinity for the water, and the resulting attraction
of the one to the other. The same phenomena are observed, and they are
due to the same cause, in other solutions; as when we infuse our tea or
sweeten it with sugar. The attraction of water, or one of its elements
rather, for other substances, sometimes shows itself in vehement forms.
When a piece of potassium, for example, is thrown into a vessel of
water, its attraction for the water is such, and of the water for it,
that it instantly takes fire, and the two blaze away, particle violently
seizing on particle until the elements of the water unite part for part
with the metal. It is the mutually attractive force that causes the heat
and flame which accompany the combination; and this force is most
violently active in the union of dissimilar substances. Unions of a
quieter kind, though not less thorough, occur even between solids when
placed in contact. For instance, sulphate of soda and sulphate of
ammonia, when placed side by side, will diliquesce, and in liquid form
unite into a new combination. Sulphuric acid, when we mix it with water,
generates great heat; and this is due to its attraction for the water.
Sometimes two fluids unite together, and, in doing so, pass from the
liquid into the solid form; as, _e.g._, sulphuric acid and chloride of
calcium. Attraction of this nature is called chemical: it takes effect
between dissimilar particles, and results in combinations with new
properties. It operates not only between solid and solid, solid and
liquid, and liquid and liquid, but between these and gases, and gases
with one another; and these as well as those combine into new
substances, and evince in the act not a little violent commotion. Thus,
phosphorus catches fire in the atmosphere at a temperature of 140
degrees, and it goes on rapidly combining with the oxygen, burning with
a dazzling white light, and producing phosphoric acid. Indeed, most
metals have an affinity for the oxygen in the air, and oxydise in it
with more or less facility; and a metal, as such, has more value than
another according as it has less affinity for that element, and is less
liable to oxydise or rust in it. This is one reason, among others, why
gold is the most precious metal, and the conventional representative of
highest worth in things.
There are some metals, such as lead, for instance, which oxydise
readily, but this process stops short at the surface in contact with the
air, and so forms a coating which prevents the metal from further
oxydation; so that here, as in so many things else, strength is
connected with weakness.
_Electricity_.--This, in the most elementary view of it, is a more or
less attractive or repellant force latent in bodies, and which is
capable of being roused into action by the application of friction. It
is excited in a rod of glass by rubbing it with silk, and in a piece of
sealing-wax by rubbing it with flannel, though the effect is different
when we apply first the one and then the other to the same body. Thus,
_e.g._, if we apply the excited sealing-wax to a paper ring, or a
pith-ball, hung by a silk thread from a horizontal glass rod, it will,
after contact, repel it; and if, thereafter, we apply to it the excited
glass rod, it will attract it; or if we first apply the excited glass
rod to the paper ring, or pith-ball, it will, after contact, repel it;
and if thereafter we apply to it the excited sealing-wax, it will
attract it. The reason is, that when we once charge a body by contact
with either kind, it repels that kind, and attracts the opposite; if we
charge it from the glass, _i.e._, with vitreous electricity, it refuses
to have more, and is attracted to the sealing-wax; and if we charge it
from the sealing-wax, _i.e._, with resinous electricity, it refuses to
have more, and is attracted to the glass-rod; only it is to be observed
that, till the body is charged by either, it has an equal attraction for
both. From all which it appears that kindred electricities repel, and
opposite attract, each other.
Two pieces of gold leaf suspended from a metal rod, inserted at the top
of a glass shade full of perfectly pure, dry air, will separate if we
rub our foot on the carpet, and touch the top of the rod with one of our
fingers; for the motion of the body, as in walking, always excites
electricity, and it is this which, as it passes through the finger,
causes the phenomenon; though the least sensation of damp in the glass
would, by instantly draining off the electricity, defeat the experiment.
What happens in this case is, that one kind of electricity passes from
the finger to the leaves, while another kind, to make room for it,
passes from the leaf to the finger; and the leaves separate because they
are both more or less charged with the same kind of electricity, and
kindred electricities repel each other. Ribbons, particularly of white
silk, when well washed, are similarly susceptible of electrical
excitation; and they behave very much as the gold leaf does when they
are rubbed sharply through a piece of flannel. Gutta-percha is another
substance which, when similarly treated, is similarly affected.
This power is a very mysterious one, and of a nature to perplex even the
philosophic observer. Certain bodies, such as the metals, convey it, and
are called conductors; certain others, such as glass and porcelain,
arrest it, and are called insulators. It is for this reason that the
wires of the telegraph are supported by a non-conductor, for if not, the
electric current would pass into the earth by the first post and never
reach its final destination. Glass being an insulator, it was found
that, if a glass bottle was filled with water, and then corked up with a
cork, through which a nail was passed so that the top of it touched the
water, it would receive and retain a charge as long as it was held in
the hand; and this observation led to an invention of some account in
the subsequent applications of electricity, known, from the place of its
conception, as the Leyden jar. This is a glass jar, the inside of which
is coated with tinfoil, and the outside as far as the neck, and into
which, so as to touch the inside coating, a brass rod with a knob at
the top is inserted through a cork, which closes its mouth. By means of
this, in consequence of the isolation of the coatings by the glass,
electricity can, in a dry atmosphere, be condensed, and stored up and
husbanded till wanted.
A series of eggs, arranged in contact and in line, give occasion to a
pretty experiment. In consequence of the shells being non-conductors,
and the inside conducting, it happens that a current of electricity,
applied to the first of the series, will pass from one to another in a
succession of crackling sparks, in this way forcing itself through the
obstructing walls. This effect of electricity in making its way through
non-conducting obstructions accounts for the explosion which ensues when
a current of it comes in contact with a quantity of gunpowder; as it
also does for the fatal consequences which result when, on its way from
the atmosphere to the earth, it rushes athwart any resisting organic or
inorganic body.
_Magnetism_.--Unlike electricity, which acts with a shock and then
expires, magnetism is a constant quantity, and constant in its action;
and it has this singular property, that it can impart itself as a
permanent force to bodies previously without it. Thus, there being
natural magnets and artificial, we can, by passing a piece of steel over
a magnet, turn it into a strong magnet itself; although we can also,
when it is in the form of a horse-shoe, by a half turn round and then
rubbing it on the magnet, take away what it has acquired, and bring it
back to its original state. The magnetic property is very readily
imparted (by induction, as it is called) to soft iron, but when the iron
is removed from the magnetising body, it parts with the virtue as fast
as it acquired it. To obtain a substance that will retain the power
induced, we must make some other election; and hard steel is most
serviceable for conversion into a permanent magnet.
The properties of the magnet are best observed in magnetised steel; and
when we proceed to test its magnetic power, it will be found that it is
most active at the extremities of the bar, which are hence called its
poles, and hardly, if at all, at the centre; that while both poles
attract certain substances and repel others, the one always points
nearly north and the other nearly south when the bar is horizontally
suspended; and that, when we break the bar into two or any number of
pieces, however small, each part forms into a complete magnet with its
virtue active at the poles, which, when suspended, preserves its
original direction; so that of two particles one is, in that case,
always north of the other; nay, it is probable that each of these has
its north pole and its south, as constant as those of the earth itself,
which, too, is a large magnet.
The magnet acts through media and at a distance, as well as in contact;
and it has an especial attraction for iron, the more so when the
conducting medium is solid, such as a table; and so when the magnet is
horizontally suspended, or poised, in the vicinity of iron, its tendency
to point north and south is seriously disturbed. The disturbance of the
bar, or needle, in such a case, is called its _deflection_; and it is
corrected by so placing a piece of soft iron or another magnet in its
neighbourhood as to neutralise the effect, and leave said bar, or
needle, free to obey the magnetism of the earth. The needle, it is to be
remarked, does not point due north and south, neither, when poised
freely on its centre, does it lie perfectly horizontal; in our latitude
it points at present 20° west of north, which is called its
_declination_, and its north pole slopes downwards at an angle of 68°,
which is called its _dip_.
By holding a rod of iron, or a poker, for a length of time parallel to
the direction of the needle, so as to have the same declination and the
same dip, it will gradually assume and display magnetic virtue, and this
will ere long become fixed and powerful under a succession of vibratory
shocks. There is a beautiful experiment in which a needle, when
magnetised, can be made to float on water, when it adjusts itself to the
magnetic meridian, and will incline north and south the same as the
needle of the compass.
_The Chemical Action of Electricity and Magnetism_.--These agents
possess powers which develop wonderfully in connection with chemical
combination. Thus, if we suspend a piece of iron in a vessel which
contains oxygen gas, and apply to the metal an electric current, it will
immediately begin to unite rapidly, and form an oxide with oxygen,
emitting, during the process, intense heat and a bright flame. Zinc,
too, when similarly acted on, will ignite in the common atmosphere and
burn away, though with less intensity, till it also is, under the
electric force, reduced to an oxide. It is presumed that many other
chemical combinations take place because of the simultaneous joint
development of electric agencies, as in copper, water, and aquafortis,
nitrate of copper, &c. So also it happens that, when a plate of iron is
for some time immersed in a copper solution, it comes out at length
covered over with a coating of copper. And it is because there is
electricity at work that a silver basin will be coated with copper when
we pour into it a copper solution, and at the same time place in it a
rod of zinc, so that it rests on the side and bottom, though no coating
will form at all when there is no rod present to excite the electric
current. The same phenomena will appear if we deposit a silver coin in
the solution in question: the coin will come out unaffected, unless we
excite affinity by means of a rod of iron. It is under the action of an
electric current that one metal is coated with another. The metal,
copper say, is steeped in a solution of the coating substance, and
connected by means of wires with a galvanic battery, under the action of
which the metal in solution unites with the surface of the plate
immersed in it. Heat also is developed under magnetic influence, and
that often of great intensity. Thus, if we connect the poles of a
voltaic battery by means of a platinum wire, heat will develop to such a
degree that the platinum will almost instantaneously become red hot and
emit a bright light, and that along a wire of some considerable length.
A similar effect is noticeable when we substitute other metals, such as
silver or iron, for platinum. And the _electric light_, which flashes
out rays of sunlike brilliance, is the result of placing a piece of
compact charcoal between the separated but confronting poles of a
powerful galvanic battery, light, developing more at the one pole and
heat more at the other of the incandescent substance.
Kindred, though much milder, results will show themselves under simpler,
though similar, contrivances. A flounder will jump and jerk about
uneasily if we lay it upon a piece of tinfoil and place over it a thin
plate of zinc, and then connect the two with a bent metal rod; which
will happen to an eel also, if we expose it to a gentle current from a
battery.
By means of electric or magnetic action we can separate bodies
chemically combined, as well as unite them into chemical compounds; as
will appear if we place a piece of blotting paper upon tinfoil, and this
upon wool; if we then spread above these two pieces of test-paper,
litmus and turmeric, the one the test of acids, and the other of
alkalis, and saturate both with Glauber salt (which is by itself neither
an acid nor an alkali, but a combination of the two), and, finally,
connect each by means of a piece of zinc with the poles of a battery,
the test-papers will immediately change colour, as they do the one in
the presence of an acid simply, and the other of an alkali simply, but
never in a compound where these are neutralised; thus proving that the
compound has in this case been decomposed, and its elements
disintegrated one from another.
A very powerful magnet can be produced by coiling a wire round a bar of
soft iron, and attaching its extremities to the poles of a galvanic
battery, when it will be found that its strength will be proportioned to
the strength of the current and the turns of the coil. This is
especially the case when the bar is bent into the form of a horse-shoe,
and the wires are insulated and coiled round its limbs. The force
communicated to a magnet of this kind, which is often immense, is the
product of the chemical action which goes on in the battery, and, in a
certain sense, the measure of it. How great that is we may judge when
we consider that, evanescent as it is in itself, it has imparted a
virtue which is both powerful and constant, and ever at our service.
_Summary_.--Thus, then, on a review of the whole, we find all things are
endowed with attractive power, and that there is no particle which is
not directly or indirectly related, in manifold ways, to the other
particles of the universe. There is, first, the universal attraction of
gravitation, under which every particle is, by a fixed law, drawn to
every other within the sphere of existence. There is, secondly, the
attraction of cohesion or aggregation, which acts at short distances,
and unites the otherwise loose atoms of bodies into coherent masses.
There is, thirdly, the power by which elements of different kinds
combine into compounds with new and useful qualities, known by the name
of chemical affinity. And, lastly, related to the action of affinity,
aiding in it and resulting from it, there are those strange negative and
positive, attractive and repellant polar forces which appear in the
phenomena of electricity and magnetism, agencies of such potency and
universal avail in modern civilisation.
On the permanency of such forces and their mutual play the universe
rests, and its wonderful history. With the collapse of any of them it
would cease to have any more a footing in space, and all its elements
would rush into instant confusion. What a Hand, therefore, that must be
which holds them up, and what a Wisdom which guides their movements!
Verily, He that sends them forth and bids them work His will is greater
than any one--greater than all of them together. How insignificant,
then, should we seem before Him who rules them on the wide scale by
commanding them, while we can only rule them on the small by obeying
them! And yet how benignant must we regard Him to be who both wields
them Himself for our benefit and subjects them to our intelligence and
control!
FOOTNOTES:
[B] This paper on "Attraction" is the substance of a lecture which I
composed on the basis of notes taken by me when. I had the honour of
attending the Prince of Wales at the course given, on the same subject
by the late Professor Faraday. The Professor, having seen the _resumé_ I
had written, warmly commended the execution, and generously accorded me
his sanction to make any use of it, whether for the purpose of a lecture
or otherwise, as might seem good to me. It is on the ground of this
sanction I feel warranted to print it here.
_THE OIL FROM LINSEED_.
Various processes have for a long time been in use for the purpose of
extracting the oils from different species of nuts and seeds, a few of
the more interesting of which are not unworthy of brief notice and
description.
In Ceylon, where cocoa-nuts and oil-producing seeds abound, the means
employed by the natives in the last century for extracting the oils were
of a most primitive character. A few poles were fixed upright in the
ground, two horizontal bars attached to them, between which a bag
containing the pulp of the seed or nut was placed. A lever was then
applied to the horizontal bars, which brought them together, thus
creating a pressure which, by squeezing the bag, gradually expressed the
oil from the pulpy substance. This rude machine was at that time of day
one of the most approved for the purpose.
The system of pestle and mortar was also in use, but as the process was
necessarily very slow, this method was seldom resorted to. An
improvement on this system was invented by a Mr. Herbert, whose design
it had been to construct a powerful and efficient machine which should
combine cheapness and simplicity. It consisted of three pieces of wood,
viz., an upright piece fixed in the ground, from the lower and upper
extremities of which there projected the two other pieces, the top one
attached to the joint of a long horizontal lever, and the lower one to
the joint of a vertical one. The fixed upright post and the horizontal
lever formed the press. The bag of pulp being put between the upright
one and the vertical, the pressure was obtained by suspending a negro or
a weight from the lever.
In another press of the same or a similar kind, the bags were placed in
a horizontal frame, and a loose beam of wood pressed down on it by a
lever.
Another form of press had cambs and wedges; also a modification of it by
Mr. Hall of Dartford, who applied the pressure by means of a
steam-cylinder. The cambs are arranged alternately, so that one is
filled while the other is being pressed. This brief notice will suffice
to give an idea of such machines as are wrought by lever pressure.
We pass on, therefore, to later inventions and improvements.
First, The Dutch or _stamper_ press, invented in Holland; second, the
_screw_; and, third, the _hydraulic_:--
(1.) _The stamper press_ is something like a beetling-machine, in which
wedges are driven in between the bags, containing, of course in a
bruised condition, the seed to be pressed.
(2.) _The screw press_ has an ordinary square-threaded screw, and it
acts in the same way as press for making cider or cheese.
(3.) _The hydraulic press_. Here the pressure is produced by means of a
piston driven up by the force of water, the immense power of which is,
in great part, due to its almost total incompressibility. This is by far
the most perfect form of press. Its power must be familiar to all who
remember the lifting of the tubes of the Britannia Bridge, and the
_launching of the Great Eastern_.
An oil-mill is in form something like a flour-mill. The operation
begins at the top, where the seed is passed through a flat screw or
shaker and then through a pair of rollers, which crush it. These rollers
are of unequal diameter, the one being 4 feet, and the other 1 foot; but
they are both of the same length, 1 foot 4 inches, and make fifty-six
revolutions in a minute. By this arrangement it is found the seed is
both better bruised and faster than when, as was formerly the case, the
rollers were of the same diameter. A pair of rollers will crush 4-1/2
tons of seed in eleven hours, a quantity enough to keep two sets of
hydraulic presses going.
After the seed is crushed in this way, it is passed under a pair of edge
stones. These stones weigh about seven tons, are 7 feet 6 inches in
diameter and 17 inches broad, and make seventeen revolutions a minute.
If of good quality, they will not require to be faced more than once in
three years, and they will last from fifteen to twenty. They are fitted
with two scrapers, one for raking the seed between the stones, the other
for raking it off at the proper period. One pair of stones will grind
seed sufficient for two double hydraulic presses, and the operation
occupies about twenty-five minutes. The seed is now crushed and ground,
but before it is passed on to the press it is transferred to the
heating-kettle.
The heating-kettle is composed of two cylindrical castings, one fitting
loosely into the other, so that a space is left between them for a free
circulation of steam all round both the sides and bottom of the interior
vessel. The internal casting is again divided horizontally into two
partitions, one above the other therefore, by two plates, between which
also there is a space left for the admission and circulation of steam;
and a communication is kept up between the upper compartment and the
under by means of a stripping valve. Besides this, there is a
communication from the internal kettle through the external one, and
also a shaft passes between the two horizontal parts to give motion to
the stirrer, which revolves thirty-six times a minute. A cover encloses
the top, and it is through this the vessel is charged. The upper portion
is filled first, where the contents introduced are allowed to remain ten
or fifteen minutes, after which the valve is opened and the whole falls
into the lower kettle, where it is kept till wanted. The seed is then
taken away from the lower kettle by an opening, and bestowed in bags of
sufficient size to make a cake of 8 lbs. weight after the oil is pressed
out of it. Indeed, the compartments of the heating-kettle are of a size
to contain enough to charge one side of a hydraulic press. These,
therefore, are so constructed as to render the operation continuous, the
upper one being discharged into the under as soon as its contents are
withdrawn to the press. The seed is heated to the temperature of 170
degrees Fahr., when it is drawn off and placed in the bags.
In another form of kettle the seed is heated on a hot hearth, and on the
top of the hearth is a loose ring, within which a spindle revolves to
stir the seed. After the requisite temperature has been reached, the
ring is raised and the seed swept into the bags, which are made of
horse-hair. There is great loss of heat in this method, however, as the
seed is exposed to the atmosphere, which of course cools it.