ATTRACTION OF COHESION.
In previous chapters one kind of attraction--viz., that of gravitation,
has been discussed and illustrated in a popular manner, and pursuing the
examination of the invisible, active, and real forces of nature, the
attraction of cohesion will next engage our attention. There is a
peculiar satisfaction in pursuing such investigations, because every
step is attended by a reasonable proof; there is no ghostly mystery in
philosophic studies; the mind is not suddenly startled at one moment
with that which seems more than natural; it is not carried away in an
ecstasy of wonder and awe, as in the so-called _spirit-rapping_
experiments, to be again rudely brought back to the material by the
disclosure of trickeries of the most ludicrous kind, such as those
lately exposed by M. Jobert de Lamballe, at the Academy of Sciences at
Paris. This gentleman has unmasked the effrontery of the spirit-rappers
by merely stripping the stocking from the heel of a young girl of
fourteen. M. Velpeau declares that the rapping is produced by the
muscles of the heel and knee acting in concert, and quotes the case of a
lady once celebrated as a medium, who has the power of producing the
most curious and interesting music with the tendons of the thigh. This
music is said to be loud enough to be heard from one end of a long room
to the other, and has often played a conspicuous part in the revelations
made by the medium. M. Jules Clocquet also explained the method by which
the famous girl pendulum had so long abused the credulity of the Paris
public. This girl, whose self-styled faculty is that of striking the
hour at any time of the day or night, was attended at the Hospital St.
Louis by M. Clocquet, who states that the vibrations in [Page 60] this
case were produced by a rotatory motion in the lumbar regions of the
vertebral column. The sound of these (_à la_ rattlesnake) was so
powerful, that they might be distinctly heard at a distance of
twenty-five feet.
In studying the powers of nature, which the most sceptical mind allows
must exist, there is an abundant field for experiment without attempting
the exploits of Macbeth's witches, or the fanciful powers of Manfred;
and, returning to the theme of our present chapter, it may be asked, how
is cohesion defined? and the answer may be given, by directing attention
to the three physical conditions of water, which assumes the form of
ice, water, or steam.
In the Polar regions, and also in the Alpine and other mountains where
glaciers exist, there the traveller speaks of ice twenty, thirty, forty,
nay, three hundred feet in thickness. Here the withdrawal of a certain
quantity of heat from the water evidently allows a new force to come
into full play. We may call it what we like; but cohesion, from the
Latin _cum_, together, and _hæreo_, I stick or cleave, appears to be the
best and most rational term for this power which tends to make the atoms
or particles of the same kind of matter move towards each other, and to
prevent them being separated or moved asunder. That it is not merely
hypothetical is shown by the following experiments.
If two pieces of lead are cast, and the ends nicely scraped, taking care
not to touch the surfaces with the fingers, they may by simple pressure
be made to cohere, and in that state of attraction may be lifted from
the table by the ring which is usually inserted for convenience in the
upper piece of lead; they may be hung for some time from a proper
support, and the lower bit of lead will not break away from the upper
one; they may even be suspended, as demonstrated by Morveau, in the
vacuum of an air-pump, to show that the cohesion is not mistaken for the
pressure of the atmosphere, and no separation occurs. And when the union
is broken by physical force, it is surprising to notice the limited
number of points, like pin points, where the cohesion has occurred;
whilst the weight of the lump of lead upheld against the force of
gravitation reminds one forcibly of the attraction of a mass of soft
iron by a powerful magnet, and leads the philosophic inquirer to
speculate on the principle of cohesion being only some masked form of
magnetic or electrical attraction. (Fig. 75.)
[Illustration: Fig. 75. A A. Two pieces of lead, scraped clean at the
surfaces B B. C. Stand, supporting the two pieces of lead attached to
each other by cohesion.]
[Page 61]
A fine example of the same force is shown in the use of a pair of flat
iron surfaces, planed by the celebrated Whitworth, of Manchester. These
surfaces are so true, that when placed upon each other, the upper one
will freely rotate when pushed round, in consequence of the thin film of
air remaining between the surfaces, which acts like a cushion, and
prevents the metallic cohesion. When, however, the upper plate is slid
over the lower one gradually, so as to exclude the air, then the two may
be lifted together, because cohesion has taken place. (Fig. 76.)
[Illustration: Fig. 76. A. Whitworth's planes, with film of air between
them.
B. Film of air excluded when cohesion occurs.]
A glass vessel is a good example of cohesion. The materials of which it
is composed have been soft and liquid when melted in the fire, and on
the removal of the excess of heat it has become hard and solid, in
consequence of the attractive force of cohesion binding the particles
together; in the absence of such a power, of course, the material would
fall into the condition of dust, and a mere shapeless heap of silicates
of potash and lead would indicate the place where the moulded and
coherent glass would otherwise stand.
A lump of lead, six inches long by four broad, and half an inch thick,
may be supported by dexterously taking off a thick shaving with a proper
plane, and after pressing an inch or more of the strip on the planed
surface of the large lump of lead, the cohesion is so powerful that the
latter may be lifted from the table by the strip of metal.
The bullets projected from Perkins' steam-gun, at the rate of three
hundred per minute, are thrown with such violence, that, when received
on a thick plate of lead backed up with sheet iron, a cold welding takes
place between the two surfaces of metal in the most perfect manner, just
as two soft pieces of the metal potassium may be squeezed and welded
together. The surfaces of an apple torn asunder will not readily cohere,
but if cut with a sharp knife, cohesion easily occurs; so with a wound
produced by a jagged surface, it is difficult to make the parts
[Page 62] heal, whereas some of the most desperate sabre-cuts have been
healed, the cohesion of the surfaces of cut flesh being very rapid;
hence, if the top of a finger is cut off, it may be replaced, and will
grow, in consequence of the natural cohesion of the parts.
The art of plating copper with silver, which is afterwards gilt, and
then drawn out into flattened wire for the manufacture of gold lace and
epaulets, usually termed bullion, is another example of the wonderful
cohesion of the particles of gold, of which a single grain may be
extended over the finest plate wire measuring 345 feet in length.
The process of making wax candles is a good illustration of the
attraction of cohesion; they are not generally cast in moulds, as most
persons suppose, but are made by the successive applications of melted
wax around the central plaited wick. Other examples of cohesion are
shown by icicles, and also stalagmites; which latter are produced by the
gradual dropping of water containing chalk (carbonate of lime) held in
solution by the excess of carbonic acid gas; the solvent gradually
evaporates, and leaves a series of calcareous films, and these cohere in
succession, producing the most fantastic forms, as shown in various
remarkable caverns, and especially in the cave of Arta, in the island of
Majorca.
In metallic substances the cohesion of the particles assumes an
important bearing in the question of relative toughness and power of
resisting a strain; hence the term cohesion is modified into that of the
property of "tenacity."
The tenacity of the different metals is determined by ascertaining the
weight required to break wires of the same length and gauge. Iron
appears to possess the property of tenacity in the greatest, and lead in
the least degree. (Fig. 77.)
[Illustration: Fig. 77. B. Pan supported by leaden wire broken by a
weight which the iron wire at A easily supports.]
[Page 63]
The tenacity of iron is taken advantage of in the most scientific manner
by the great engineers who have constructed the Britannia Tube, and that
eighth wonder of the world, the _Leviathan_, or _Great Eastern_
steam-ship. In both of these sublime embodiments of the genius and
industrial skill of Great Britain the advantage of the cellular
principle is fully recognised. The magnitude of this colossal ship is
better realized when it is remembered that the _Great Eastern_ is six
times the size of the _Duke of Wellington_ line-of-battle ship, that her
length is more than three times that of the height of the Monument,
while in breadth it is equal to the width of Pall Mall, and that a
promenade round the deck will afford a walk of more than a quarter of a
mile. Up to the water-mark the hull is constructed with an inner and
outer shell, two feet ten inches apart, each of three-quarter-inch
plate; and between them, at intervals of six feet, run horizontal webs
of iron plates, which convert the whole into a series of continuous
cells or iron boxes. (Fig. 78.)
[Illustration: Fig. 78. Transverse section of _Great Eastern_, showing
the cellular construction from keel to water-line, A A.]
This double ship is useful in various ways; in the first place, the
danger arising from collision is diminished, as it is supposed that the
outer web only would be broken through or damaged; so that the water
would not then rush into the steam-ship, but merely fill the space
between the shells. In the second place, if there should be any
difficulty in procuring ballast, the space can be filled with 2500 tons
of water, or again pumped out, according to the requirements of the
vessel. The strength of a continued cellular construction can be easily
imagined, and may be well illustrated by a thin sheet of common tin
plate. If the ends be rested on blocks of wood, so as to lap over the
wood about one inch, they are easily displaced, and the mimic bridge
broken down from its [Page 64] supports by the addition to the centre
of a few ounce weights; whilst the same tin plate rolled up in the
figure of a tube, and again rested on the same blocks, will now support
many pounds weight without bending or breaking down. (Fig. 79.)
[Illustration: Fig. 79. A. Flat tin plate, breaking down with a few
ounce weights.
B. Same tin plate rolled up supports a very heavy weight.]
The deck of the ship is double or cellular, after the plan of Stephenson
in the Britannia Tubular Bridge, and is 692 feet in length. The tonnage
register is 18,200 tons, and 22,500 tons builder's measure; the hull of
the _Great Eastern_ is considered to be of such enormous tenacity, that,
if it were supported by massive blocks of stone six feet square, placed
at each end, at stem and stern, it would not deflect, curve, or bend
down in the middle more than _six inches_ even with all her machinery,
coals, cargo, and living freight.
In adducing remarkable instances of the adhesive power and tenacity of
inorganic matter, it may not be altogether out of place to allude to the
strength and force of living matter, or muscular power. It is stated
that Dr. George B. Winship, of Roxbury in America, a young physician,
twenty-five years old, and weighing 143 pounds, is the strongest man
alive; in fact, quite the Samson of the nineteenth century. He can raise
a barrel of flour from the floor to his shoulders; can raise himself
with either _little_ finger till his chin is half a foot above it; can
raise 200 pounds with either little finger; can put up a church bell of
141 pounds; can lift with his hands 926 pounds dead weight without the
aid of straps or belts of any kind. As compared with Topham, the Cornish
strong man, who could raise 800 pounds, or the Belgic one, his power is
greater; and as the use of straps and belts increases the power of
lifting by about four times, it is stated that Winship could lift at
least 2500 pounds weight.
[Illustration: Fig. 80. A. Ordinary glass water hammer. B. Copper tube
ditto, showing exhausting syringe at D, the height of the water at B,
and the end to be placed in the fire at C.]
With these illustrations of cohesion we may return again to the abstract
consideration of this power with reference to water, in which we have
noticed that the antagonist to this kind of attraction is the force or
power termed caloric or heat. The latter influence removes the frozen
bands of winter and converts the ice to the next condition, water. In
this state cohesion is almost concealed, although there is just a
slight [Page 65] excess to hold even the particles of water in a state
of unity, and this fact is beautifully illustrated by the formation of
the brilliant diamond drops of dew on the surfaces of various leaves, as
also in the force and power exercised by great volumes of water, which
exert their mighty strength in the shape of breaker-waves, dashing
against rocks and lighthouses, and making them tremble to their very
base by the violence of the shock; here there must be some unity of
particles, or the collective strength could not be exerted, it would be
like throwing a handful of sand against a window--a certain amount of
noise is produced, but the glass is not fractured; whilst the same sand
united by any glutinous material, would break its way through, and soon
fracture the brittle glass. It is so usual to see the particles of water
easily separated, that it becomes difficult to recognise the presence of
cohesion; but this bond of union is well illustrated in the experiment
of the water hammer. The little instrument is generally made of a glass
tube with a bulb at one end; in this bulb the water which it contains is
boiled, and as the steam issues from the other extremity, drawn out to a
capillary tube, the opening is closed by fusion with the heat of a
blowpipe flame. As the water cools the steam condenses, and a vacuum, so
far as air is concerned, is produced; if now the tube is suddenly
inverted, the whole of the water falls _en masse_, collectively, and
striking against the bottom of the tube, produces a metallic ring, just
as if a piece of wood or metal were contained within the tube. If the
end to which the water falls is not well cushioned by the palm of the
hand, the water hammers itself through and breaks away that part of the
glass tube. Hence it is better to construct the water hammer of copper
tube, about three-quarters of an inch in diameter and three feet long;
at one end a female screw-piece is inserted, into which a stop-cock is
fitted; when the tube is filled to the height of about six inches with
water, and shaken, the air divides the descending volume of water, and
the ordinary splashing sound is heard; there is no unity or cohesion of
the parts; if, however, the end of the copper tube is thrust into a fire
and the water boiled so that steam issues from the cock, which is then
closed, and the tube removed and cooled, a smart blow is given, and
distinctly heard when the copper tube is rapidly inverted or shaken so
as to cause the water to rise [Page 66] and fall. The experiment may be
rendered still more instructive by turning the cock and admitting the
air, which rushes in with a whizzing sound, and on shaking the tube the
metallic ring is no longer heard, but it may be again restored by
attaching a small air syringe or hand pump, and removing the air by
exhaustion. (Fig. 80.)
In the fluid condition water still possesses a surplus of cohesion over
the antagonistic force of heat; when, however, the latter is applied in
excess, then the quasi-struggle terminates; the heat overpowers the
cohesive attraction, and converts the water into the most willing slave
which has ever lent itself to the caprices of man--viz., into
steam--glorious, useful steam: and now the other end of the chain is
reached, where heat triumphs; whilst in solids, such as ice, cohesion is
the conqueror, and the intermediate link is displayed in the fluid state
of water. If any fact could give an idea of the gigantic size of the
_Great Eastern_, it is the force of the steam which will be employed to
move it at the rate of about eighteen miles per hour with a power
estimated at the nominal rate of 2600 horses, but absolutely of at least
12,000 horses. This steam power, coupled with the fact that she has been
enormously strengthened in her sharp, powerful bows, by laying down
three complete iron decks forward, extending from the bows backward for
120 feet, will demonstrate that in case of war the _Great Eastern_ may
prove to be a powerful auxiliary to the Government. These decks will be
occupied by the crew of 300 or 400 men, and with this large increase of
strength forward, the _Great Eastern_, steaming full power, could
overtake and cut in two the largest wooden line-of-battle ship that ever
floated. Should war unhappily spread to peaceful England, and the
enormous power of this vessel be realized, her name would not
inappropriately be changed from the _Great Eastern_ to the _Great
Terror_ of the ocean. The _Times_ very properly inquires, "What fleet
could stand in the way of such a mass, weighing some 30,000 tons, and
driven through the water by 12,000 horse-power, at the rate of
twenty-two or twenty-three miles per hour. To produce the steam, 250
tons of coal per diem will be required, and great will be the honourable
pride of the projectors when they see her fairly afloat, and gliding
through the ocean to the Far West."
A good and striking experiment, displaying the change from the liquid to
the vapour state, is shown by tying a piece of sheet caoutchouc over a
tin vessel containing an ounce or two of water. When this boils, the
india-rubber is distended, and breaks with a loud noise; or in another
illustration, by pouring some ether through a funnel carefully into a
flask placed in a ring stand. If flame is applied to the orifice, no
vapour issues that will ignite, provided the neck of the flask has not
been wetted with the ether. When, however, the heat of a spirit-lamp is
applied, the ether soon boils, and now on the application of a lighted
taper, a flame some feet in length is produced, which is regulated by
the spirit-lamp below, and when this is removed, the length of the flame
diminishes immediately, and is totally extinguished if the bottom of the
flask is plunged into cold water; the withdrawal of the heat restores
the power of cohesion. Another illustration of the vast power of steam
[Page 67] will be shortly displayed in the Steam Ram; and, "Supposing,"
says the _Times_, "the new steam ram to prove a successful design, the
finest specimens of modern men-of-war will be reduced by comparison to
the helplessness of cock boats. Conceive a monstrous fabric floating in
mid-channel, fire proof and ball proof, capable of hurling broadsides of
100 shot to a distance of six miles; or of clapping on steam at pleasure
and running down everything on the surface of the sea with a momentum
utterly irresistible.
"This terrible engine of destruction is expected to be itself
indestructible. We are told that she may be riddled with shot (supposing
any shot could pierce her sides), that she may have her stem and her
stern cut to pieces, and be reduced apparently to a shapeless wreck,
without losing her buoyancy or power. Supposing that she relies upon the
shock of her impact instead of fighting her guns, it is calculated that
she would sink a line-of-battle ship in three minutes, so that a
squadron as large as our whole fleet now in commission would be
destroyed in about one hour and a quarter."