Before dismissing the important subject of specific gravity (or, as it is termed by the French _savants_, "density"), it may be as well to state that astronomers have been enabled, by taking the density of the earth and by astronomical observations, to calculate the gravity of the planets belonging to our solar system; and it is interesting to observe that the density of the planet Venus is the only one approaching the gravity of the earth:--

The Earth 1.000 The Sun .254 The Moon .742 Mercury 2.583 Venus 1.037 Mars .650 Jupiter .258 Saturn .104 Herschel .220

CHAPTER VI.

ATTRACTION OF COHESION.

In previous chapters one kind of attraction--viz., that of gravitation, has been discussed and ill.u.s.trated 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 (_a 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 a.s.sumes 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 quant.i.ty 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 _c.u.m_, together, and _haereo_, 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 sc.r.a.ped, 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 ma.s.s 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.)

[Ill.u.s.tration: Fig. 75. A A. Two pieces of lead, sc.r.a.ped 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.)

[Ill.u.s.tration: Fig. 76. A. Whitworth's planes, with film of air between them.

B. Film of air excluded when cohesion occurs.]

A gla.s.s 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 gla.s.s 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 pota.s.sium 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 ill.u.s.tration 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 a.s.sumes 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.)

[Ill.u.s.tration: 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 sh.e.l.l, 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.)

[Ill.u.s.tration: 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 s.p.a.ce between the sh.e.l.ls. In the second place, if there should be any difficulty in procuring ballast, the s.p.a.ce 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 ill.u.s.trated 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.)

[Ill.u.s.tration: 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 ma.s.sive 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.

[Ill.u.s.tration: Fig. 80. A. Ordinary gla.s.s 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 ill.u.s.trations 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 ill.u.s.trated 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 gla.s.s is not fractured; whilst the same sand united by any glutinous material, would break its way through, and soon fracture the brittle gla.s.s. 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 ill.u.s.trated in the experiment of the water hammer. The little instrument is generally made of a gla.s.s 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 ma.s.se_, 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 gla.s.s 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-c.o.c.k 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 c.o.c.k, 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 c.o.c.k 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 ma.s.s, 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 ill.u.s.tration, 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 ill.u.s.tration 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 c.o.c.k 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."

CHAPTER VII.

ADHESIVE ATTRACTION.

The term cohesion must not be confounded with that of adhesion, which refers to the clinging to or attraction of bodies of a dissimilar kind.

The late Professor Daniell defines cohesion to be an attraction of h.o.m.ogeneous ([Greek: _h.o.m.os_], like, and [Greek: _genos_], kind) or similar particles; adhesion to be an attraction subsisting between particles of a heterogeneous, [Greek: _eteros_], different, and [Greek: _genos_], kind.

There are numerous ill.u.s.trations of adhesion, such as mending china, and the use of glue, or paste, in uniting different surfaces, or mortar, in building with bricks; it is also well shown at the lecture table by means of a pair of scales, one scale-pan of which being well cleaned with alkali at the bottom, may then be rested on the surface of water contained in a plate; the adhesion between the water and the metal is so perfect, that many grain weights may be placed in the other pan before the adhesion is broken; and after breakage, if the pan be again placed on the water, and a few grains removed from the other, so as to adjust the two pans, and make them nearly equal, a drop of oil of turpentine being added, instantly spreads itself over the water, and breaking the adhesion between the latter and the metal, the scale-pan is immediately and again broken away, as the adhesion between the turpentine and the metal is not so great as that of water and metal. The adhesion of air and water is well displayed in an apparatus recommended for ventilating mines, in which a constant descending stream of water carries with it a quant.i.ty of air, which being disengaged, is then forced out of a proper orifice. The same kind of adhesion between air and water is displayed in the ancient [Page 68] Spanish Catalan forge, where the blast is supplied to the iron furnace on a similar principle, only, a natural cascade is taken advantage of instead of an artificial fall of water through a pipe.

The adhesion of air and water becomes of some value when a river flows through a large and crowded city, because the water in its pa.s.sage to and fro, must necessarily drag with it, a continuous column of air, and a.s.sist in maintaining that constant agitation of the air which is desirable as a preventive to any acc.u.mulation of noxious air charged with foetid odours, arising from mud banks or from other causes. The fact of adhesion, existing between water and air, is readily shown, by resting one end of a long gla.s.s tube, of at least one inch diameter, on a block of wood one foot high. If water is allowed to flow down the tube, so as to leave a sufficient s.p.a.ce of air above it, the adhesion between the two ancient elements becomes apparent, directly a little smoke is produced, near the top end of the gla.s.s tube resting on the block of wood. The smoke, which has a greater tendency to rise than to fall, is dragged down the gla.s.s tube, and accompanies the water as it flows from the higher to the lower level. The same truth is also ill.u.s.trated in horizontal troughs or tubes through which water is caused to flow.

The adhesion between air and gla.s.s is so great, that it is absolutely necessary to boil the mercury in the tubes of the best barometers; and if this is not carefully attended to, the adhering air between the gla.s.s and mercury gradually ascends to, and destroys, the Torricellian vacuum at the top of the barometer tube. Even after the mercury is boiled, the air will creep up in course of years; and in order to prevent its pa.s.sage between the gla.s.s and quicksilver, it has been recommended, that a platinum ring should be welded on to the end of the gla.s.s tube, because mercury has the power of wetting or enfilming the metal platinum, and the two being in close contact, would, as it were, shut the only door by which the air could enter the barometer tube.

[Ill.u.s.tration: Fig. 81. Model of the apparatus for drawing down air. A, cistern of water, supplied by ball-c.o.c.k, and kept at one level, so that the water just runs down the sides of the tube, and draws down the air in the centre, B C. The vessel to which the air and water are conveyed by a gutta-percha tube, T. There is another ball-c.o.c.k to permit the waste water to run away when it reaches a certain level; the end of the pipe always dips some inches into this water, whilst the air escapes from the jet, D.]

[Page 69]

CHAPTER VIII.