SELENIUM.

Fig. 152.Fig. 152.

a.Air and bisulphide of carbon.b.Nitric oxide and ditto.c.Oxygen and ditto.d d.Stout cylinder of double wire gauze, open top and bottom.

Selenium (σεληνη, the Moon[B]); symbol, Se; combining proportion, 39.5.

This new metallic element is allied to sulphur, and is a species of chemical curiosity, being found in minute quantities in various minerals; it may be melted and cast into any form. Medallions of the discoverer (Berzelius) of selenium, in little cases, are imported from Germany, for the cabinets of the curious.

[B]Called selenium on account of its strong analogy to the metal tellurium (tellus, the earth).

Phosphorus (φως, light; φερειν, to bear; symbol, P; combining proportion, 32.)

Monsieur Salverte, in his work on the Occult Sciences of the Ancients, quotes a remarkable story respecting the probable discovery of the nature of phosphorus in 1761:—"A Prince San Severo, at Naples, cultivated chemistry with some success; he had, for example, the secret of penetrating marble with colour, so that each slab sawed from the block presented a repetition of the figure imprinted on its external surface. In 1761, he exposed some human skulls to the action of different reagents, and then to the heat of a glass furnace, but paying so little attention to his manner of proceeding, that he acknowledged he did not expect to arrive a second time at the same result. From the product he obtained a vapour, or rather a gas was evolved, which kindling at the approach of a light, burned for several months without the matter appearing to die or diminish in weight. San Severo thought he had found, the impossible secret of the inextinguishable lamp, but he would not divulge his process, for fear that the vault in which were interred the princes of his family should lose the unique privilege with which he expected to enrich it, of being illuminated with aperpetual lamp." Had he acted like a philosopher of the present day, San Severo would have attached his name to the important discovery of the existence ofphosphorusin thebones, and made public the process by which it might be obtained.

This element, formerly sold at four or five shillings the ounce, has now fallen so much in price, from the greater demand and larger production, that it may be bought for a few shillings the pound, and is imported in tin cases in large quantities from Germany. It was discovered about two hundred years ago by Brandt, a merchant of Hamburg, and may be prepared on a small scale by distilling at a red heat phosphoric acid previously fused with one-fourth of its weight of powdered charcoal.

Phosphorus, when pure, is without taste or colour, but generally of a very pale buff-colour, and semi-transparent; it is extremely combustible, and is usually preserved under the surface of water; when perfectly dry, a thin slice will take fire at 60° Fah., and burns with great brilliancy. Considering the heat produced during the combustion of phosphorus, it might be thought that it would infallibly set fire to any ordinary combustible, such as paper or wood, but this is not the case when phosphorus is employed by itself, as may be proved by the following experiment.

Cut five very small pieces of phosphorus, and place them like the five of diamonds on a sheet of cartridge-paper laid upon the table, set the bits of phosphorus on fire, when they will be rapidly burnt awayleaving only five black spots, but not firing the paper, as would be the case if some red-hot coals or charcoal were placed in the same position. The cause is very simple. Phosphorus in burning produces phosphoric acid, which is an anti-combustible, and coats the surface of the paper round the spot where the combustion occurs, and acting as a kind of glaze or glass, excludes the oxygen of the air, and prevents the fire spreading.

If some powdered sulphur is sprinkled round the spot where the bit of phosphorus is to be burnt, the case is very different; the heat melts and sets fire to the sulphur, which being uncoated with the phosphoric acid, communicates to the paper; and it is on this principle that lucifer-matches can be used as instantaneous lights. The tip of the wood of which they are composed is first dipped in sulphur, and then the phosphorus composition made of gum, chlorate of potash, vermilion, and phosphorus, is placed over it; and if the latter were used alone without the sulphur, not one match in a hundred would take fire properly.

Common phosphorus is perfectly and rapidly dissolved by bisulphide of carbon. The solution must be carefully preserved, as it is a liquid combustible, which takes fire spontaneously after the bisulphide of carbon evaporates; so that wherever it is dropped, a flame, arising from the spontaneous combustion of the finely-divided phosphorus, is sure to be produced. This liquid was recommended many years ago to the Government for the purpose of setting sails of ships or other combustible matter on fire. The solution of phosphorus alone did not answer the purpose, as already explained in the first experiment; but when wax was dissolved with the phosphorus, it then became a most dangerous fluid, which it was recommended should be used in shells, and discharged from a mortar or howitzer in the ordinary manner. Dr. Lyon Playfair was the first to make this proposed application of the solution, and it has since, we believe, been recommended by Captain Norton in his liquid-fire shells.

One of the most curious facts in connexion with phosphorus, is its assumption of the allotropic state in what is termedamorphous(shapeless) or red phosphorus. This substance, when handled for the first time, might be mistaken for a lump of badly-made Venetian red. There is no risk of its taking fire like the common phosphorus, and it does not (according to Schrötter, of Berlin, who discovered this peculiar condition) exhale those fumes which are so prejudicial to the lucifer-match makers. When the vapour of common phosphorus is continually inhaled, it is said to cause a peculiar and disgusting disease, which terminates in the destruction of the jaw-bone; whilst the bones in other parts of the body become brittle, and arm-bones thus affected are fractured with the slightest blow.

The difference between common and red phosphorus is well shownfirst, by placing a few small pieces of both kinds in separate bottles or vials containing bisulphide of carbon; the common phosphorus, as already explained, quickly dissolves in the liquid, and if poured on a sheet of paper, and hung up, is soon on fire; whilst the red variety is wholly unaffected, and if the bisulphide of carbon is poured off on to paper, it merely evaporates, and no combustion occurs.

The similarity in composition, though not in outward form, is further shown by filling two jars with oxygen gas, and having provided two deflagrating spoons, some common phosphorus is placed in one, and red phosphorus in the other; a wire, gently heated by dipping it into some boiling water, is now applied to the former, which immediately takes fire, and may be plunged into the jar of oxygen gas, when it burns with the usual brilliancy. The red phosphorus, however, must be brought to a much higher temperature (500° Fah.) before it will even shine in the dark, and then with a still further increase of heat it takes fire, and on being placed in the other jar of oxygen burns up much more slowly than the yellow phosphorus, but at last exhibits that brilliant flash of light which is so characteristic of the combustion of phosphorus in oxygen.

The amorphous or red phosphorus is employed in the manufacture ofsafety chemical matches, and M. A. Meunons has secured a patent in England for an improvement in lucifer matches, with a view to obviate the risks of accidental ignition. To attain this end the matches are first cut by a machine from cubes of wood, the cut being stopped at a short distance from the end of each cube, so as to leave the lower extremities adherent. The upper or free extremity of each packet of splints thus formed being coated with wax or sulphur, is dipped in one of the following preparations:—Chlorate of potash, two parts; pulverized charcoal, one part; umber, one part; or, chlorate of potash, sulphur, and umber, in equal parts, thoroughly mixed with glue. The opposite extremity or "cut" of each packet is then painted over with amorphous phosphorus blended with size, so that on separating the matches the phosphorus is only found on the top of each. The matches thus prepared are ignited by breaking off a small piece of the phosphorised end and rubbing it on the opposite extremity covered with the inflammable preparation.

Loud exploding and dangerous lucifers were formerly made by dipping bundles of matches, previously coated with sulphur at the tips, into a thick solution of gum, at a temperature of 104° Fahr., coloured with smalt or red lead, in which was dissolved a certain proportion of chlorate of potash, and also containing finely divided particles of phosphorus obtained by the constant stirring and rubbing of the materials in a mortar. When dry the matches exploded if rubbed against a gritty surface, and there was always a risk of a fragment flying off and entering the eye. To obviate this danger,silentornoiseless lucifer matcheswere invented, and the composition used (according to Böttger) is as follows:—Gum arabic, 16 parts by weight; phosphorus, 9 parts; nitre, 14 parts; powdered black oxide of manganese, 16 parts. The above ingredients are worked up in a mortar with water, at 104° Fahr., and the matches previously tipped with sulphur are dipped therein and afterwards dried.

The combustion of phosphorus under water is easily demonstrated by placing some ordinary stick phosphorus in a metallic cup, and then plunging it rapidly under the surface of boiling water. If a jet of oxygen gas is now directed upon the liquid phosphorus, it burns with great brilliancy. When the oxygen escapes too rapidly from the jet, it causes some small particles to be thrown out of the water, so that it is advisable to defend the face with a sheet of wire gauze held a few inches above the glass whilst the experiment is being conducted. (Fig. 153.)

Fig. 153.Fig. 153.

a a.Finger-glass of boiling water containing a metallic cup with melted phosphorus.c.Jet of oxygen gas.d d.Sheet of wire gauze.

Phosphorus burns and emits beautiful flashes of light in the presence of the gas called peroxide of chlorine (ClO4), which must be very carefully generated under the surface of water by first placing some cut phosphorus and chlorate of potash at the bottom of a long and stout cylindrical glass nearly full of water; sulphuric acid is then conveyed to the chlorate of potash by means of a syphon, the end of which must be drawn out to a small opening, or else the oil of vitriol will descend too rapidly, and the glass will be cracked by the heat. Immediately the peroxide of chlorine comes in contact with the phosphorus it explodes, and passes again to its original elements, oxygen and chlorine. These bubbles envelope minute particles of phosphorus, which rapidly ascend, like water-spiders, to the surface, and burn as they pass upwards, producing a continual series of sparks of fire, which have an extremely pretty effect. (Fig. 154.) The syphon is of course first filled with water, and as that is displaced, the oil of vitriol takes its place.

Fig. 154.Fig. 154.

a a.Tall glass nearly full of water; at the bottom are the chlorate of potash and phosphorus.b.Wolfe's bottle and syphon, conveying the oil of vitriol to bottom ofa a.

If a little phosphorus is placed in a small copper boiler, and the steam allowed to escape from a jet, it is observed to be luminous, in consequence of a minute portion of phosphorus being carried up mechanically with the steam. The same fact is shown very prettily by boiling water in a flask containing some phosphorus.

Phosphorus explodes violently when rubbed with a little chlorate of potash, and in order to perform this experiment safely, it should be made in a strong iron mortar, the pestle of which must be surrounded with a large circle of cardboard and wire gauze; so that when it is brought down upon the phosphorus and chlorate of potash, any particles that may fly out are detained by the shield. Without this precaution the experiment is one of the most dangerous that can be made. (Fig. 155.)

Fig. 155.Fig. 155.

a.The iron mortar containing the phosphorus and chlorate of potash.b.The pestle, with the shield,c C, composed of a circle of wire gauze, covered with one of cardboard.

Phosphuretted hydrogen owes its property of spontaneous combustion to the presence of the vapour of a liquid, phosphide of hydrogen (PH2), which may be prepared by placing some phosphide of calcium into a flask with water heated to a temperature of 140° Fah., and conveying the gas into a U-shaped tube surrounded with a mixture of ice and salt. The liquid obtained is colourless, and must be preserved from contact with air, as it takes fire spontaneously directly it is exposed to the atmosphere. (Fig. 156.)

Fig. 156.Fig. 156.

a.The flask containing the phosphide of calcium and water, and placed in a water-bath heated to 140° Fah.b.Bent tube conveying the gas toc c, the U-shaped tube, to which it is attached by india-rubber tubing,c c.The U-shaped tube, surrounded with a freezing mixture.d d.Bent tube, passing into a cup of water to prevent contact with air.

Phosphide of calcium is quickly prepared by placing some small pieces of lime in a crucible and making them red-hot; if lumps of dry phosphorus are thrown into the crucible, and the cover placed on quickly, and immediately after the phosphorus, the latter unites with the calcium, and forms a brown substance which produces gaseous phosphide of hydrogen (PH3) when placed in water, and the gas takes fire spontaneously when it comes in contact with the air.

Phosphorus placed in a retort with a tolerably strong solution of potash, and a small quantity of ether, affords a large quantity of phosphide of hydrogen (commonly called phosphuretted hydrogen) when boiled. The neck of the retort must dip into a basin of water, and the object of the ether is to prevent the combustion of the first bubbles of gasinsidethe retort, which by their explosion would probably break the glass. If the neck of the retort is kept under water in which potash is dissolved, the gas may be generated for many days at pleasure, although it is not a desirable experiment to renew too often, on account of the disagreeable odour produced. (Fig. 157.)

Fig. 157.Fig. 157.

Aretort containing the phosphorus, water, potash, and ether.b.Neck dipping into a basin of water.c.The gas burning, and producing beautiful rings of smoke.

When a jar of oxygen is held over the neck of the retort generating the phosphuretted hydrogen, a bright flash of light and explosion are observed; and if the experiment is performed in a darkened room, it is just like a sudden flash of lightning. A bottle of chlorine held over the neckof the retort, and dipping of course in the water of the basin, produces a green flame every time the bubble of gas passes into it. That curious appearance of light, sometimes seen in marshy districts, called will-o'-the-wisp, is supposed to be due to the escape, from decomposing matter, of bubbles of hydrogen, nitrogen, &c., through which the spontaneously inflammable phosphide of hydrogen is diffused.

At a place called Dead Man's Island, near Sheerness, magnificent effects of this kind are sometimes apparent when the mud banks are accidentally stirred at night by a boat-hook. A credible observer says, he once saw there a flash of yellowish-green light, accompanied with noise, about thirty feet in height. The apparent height might be due to the duration of the impression of the flash on the eye, as the light from the burning phosphuretted hydrogen ascended rapidly upwards. The source of this gas appears to be due to the fact, that during the time some Russian ships were watched by the Brest fleet, a number of the sailors died of cholera, and were buried in the banks; the decomposition of the bone containing phosphorus would account for the appearance of light already described.

With the discussion of some of the most interesting properties of the thirteen non-metallic elements we take leave of the subject of chemistry, reserving the consideration of the metals for another popular juvenile work, of which they will form the subject.

In answer to the oft-repeated question, "Where can I get thethingsfor the experiments?" it may be stated that every kind of glass vessel and the chemicals mentioned in this chapter, can be procured either of Messrs. Simpson, Maule, and Co., Kennington, or of Griffin and Co., Bunhill-row, or Bolton and Co., High Holborn.

Fig. 158.Fig. 158.

Will-o'-the-wisp.

Fig. 159.Fig. 159.

Franklin and his kite.

Of all the agents with which man is acquainted, not one can afford a greater source of wonderment to the ignorant, of meditation to the learned, than the effects of that marvellous force pervading all matter called electricity. We look at matter endowed with life, and matter wanting this divine gift, with some degree of interest, depending on our various tastes and occupations; we know at a glance a bird, a beast, or a fish; we observe with pleasure and admiration the wonderful changes of nature, and know that a few seeds thrown into the broken clods and well-tilled earth may become either the waving, golden corn-field or in time may grow from the tender little shrub to the stately forest-tree; we know all these things because they belong to the visible world, and are continually passing before our eyes: but in looking at the visible, we must not forget and ignore the invisible. It may with truth bestated that the greatest powers of nature are all concealed, and if any truth would lead us from Nature to Nature's God, it is the fact that no visible, solid, tangible agent can work with so much force and power as invisible electricity. Many centuries passed away since the commencement of the Christian era, before the human mind was prepared to appreciate this great power of nature; other forces had claimed attention, and the difference in the presence or absence of two of the imponderable agents, heat and light, as derived from the sun, in the effects of the change of the seasons, and other common facts, had led philosophers to speculate early upon their nature; but electricity, from its peculiar properties, long escaped observation, and it was not until the beginning of the eighteenth century (about 1730) that any material facts had been discovered in this science, when Mr. Stephen Grey, a pensioner of the Charterhouse, discovered what he termedelectricsandnon-electrics, and also the use of insulating materials, such as silk, resin, glass, hair, &c.; and it is obvious that, until the latter fact was discovered, the science would remain in abeyance, because there would be no mode of preserving the electrical excitement in the absence of non-conductors of this force.

The year 1750 was remarkable for Volta's discoveries and Dr. Franklin's identification of the electricity of the machine with the stupendous effects of the thunderstorm. Sir Humphry Davy, in 1800, with his commanding genius, threw fresh light upon the already numerous electrical effects discovered. In 1821, Faraday commenced his studies in this branch of philosophy; which he has since so diligently followed up, that he has been for some years, and is still the first electrician of the age. From the commencement of the present century, discoveries have succeeded each other in regular order and with the most amazing results; and now electricity is regularly employed as a money-getting agent in the process of the electrotype and electro-silvering and gilding; also in the electric telegraph; and in a few years we may possibly see it commonly employed as a source of artificial light.

The nature of electricity, says Turner, like that of heat, is at present involved in obscurity. Both these principles, if really material, are so light, subtle, and diffuse, that it has hitherto been found impossible to recognise in them the ordinary characteristics of matter; and therefore electric phenomena may be referred, not to the agency of a specific substance, but to some property or state of common matter, just as sound and light are produced by a vibrating medium. But the effects of electricity are so similar to those of a mechanical agent, it appears so distinctly to emanate from substances which contain it in excess, and rends asunder all obstacles in its course so exactly like a body in rapid motion, that the impression of its existence as a distinct material substancesui generisforces itself irresistibly on the mind. All nations, accordingly, have spontaneously concurred in regarding electricity as a material principle; and scientific men give a preference to the same view, because it offers an easy explanation of phenomena, and suggests a natural language intelligible to all.

There are five well-ascertained sources of electricity, and three which are considered to be uncertain. The five sources are friction, chemical action, heat, magnetism, peculiar animal organisms. The three uncertain sources are contact, evaporation, and the solar rays.

A stick of sealing-wax or a bit of glass tube, perfectly dry, rubbed against a warm piece of flannel, has elicited upon its surface a new power, which will attract bits of paper, straw, or other light materials; and after these substances are endowed with the same force, a repellent action takes place, and they fly off. One of the most convenient arrangements for making experiments with the attractive and repellent powers of electricity is to fix with shell-lac varnish round discs of gilt paper, of the size of a half-crown, at each end of a long straw that is supported about the centre with a silk thread, which may hang from the ceiling or any other convenient support. (Fig. 160.)

The varnish is easily prepared by placing four or eight ounces of shell-lac in a bottle, and pouring enough pyroxylic spirit (commonly termed wood naphtha) upon the lac to cover it. After a short time, and by agitation, solution takes place. In a variety of ways friction is proved to be a source of electricity, and forms a distinct branch of the science, under the name offrictionalelectricity.

Fig. 160.Fig. 160.

a.The glass pillar support.b.Straw with discs, hanging by a silk thread.

The nature of chemical action has been already explained, and is alluded to here as a source of electricity of which the proof is very simple. A piece of copper and a similar-sized plate of zinc have attached to them copper wires; these plates are placed opposite to, but do not touch each other, in a vessel containing water acidulated with a small quantity of sulphuric acid. When the wires are brought in contact, a current of electricity circulates through the arrangement, but has no power to attract bits of paper, straw, &c. In order to ascertain whether the current of electricity passes or not, a piece of covered copper wire is bent several times round a magnetic needle, so that it has freedom of motion inside the core or hollow formed by twisting the copper wire. This arrangement, properly constructed, is called the galvanometerneedle, and is invaluable as a means of ascertaining the passage of electricity derived from chemical action. (Fig. 161.)

When the wires leading from the metal plates are connected with the extremities of the coil in the galvanometer, the needle is deflected or pushed aside to the right hand or to the left, according to the direction of the current.

Fig. 161.Fig. 161.

a.The galvanometer needle.b.Vessel containing weak acid and the zinc and copper plates. The arrows show the path of the electric current.

The third source of electricity is heat, and the effect of this agent is well shown by twisting together a piece of platinum and silver wire, so as to form one length. If the silver end is attached to any screw of the galvanometer, and the platinum end to the second screw, no movement of the magnetic needle takes place until the heat of a spirit-lamp is applied for a moment to the point of juncture between the silver and platinum wires, when the magnetic needle is immediately deflected.

Fig. 162.Fig. 162.

a.The galvanometer needle, with wires attached.s, s.Silver wire joined top, p, the platinum wire. The heat of the spirit-lamp is applied at the point of juncture, +.

The fourth source of electricity—viz., magnetism—requires a somewhat more complicated arrangement; and a most delicate galvanometer needle must be provided, to which is attached the extremities of a long spiral coil of copper wire covered with cotton or silk. Every time a bar magnet is introduced inside the coil, so that the conducting wire cuts the magnetic curves, a deflection of the galvanometer needle takes place,and the same effect is produced on the withdrawal of the magnet, the needle being deflected in the opposite direction.

The magnetic spark can be obtained by employing a magnet of sufficient power; and the arrangement for this purpose is very simple. A cylinder of soft iron is provided, and round its centre are wound a few feet of covered thin copper wire, one end of which is terminated with a copper disc well amalgamated, and the other end, after being properly cleaned and coated with mercury, is brought into contact with the disc. Directly this cylinder is laid across the poles of the magnet, and as quickly removed, the point and disc, from the elasticity of the former, separate for the moment, the contact is broken between the point and disc, and a brilliant but tiny spark is apparent.

Fig. 163.Fig. 163.

a b.Horse-shoe magnet.c.Cylinder of soft iron.d.Coil of copper wire and contact breaker.

The fifth mode of procuring electricity would require the assistance of an electrical eel, a fine specimen of which (forty inches in length) was exhibited at the Adelaide Gallery some years ago. Various experiments were made with this animal, and the author had the pleasure of witnessing all the ordinary phenomena of frictional electricity, illustrated by Dr. Faraday, with the assistance of the animal electricity derived from this curious creature. Recent experiments have, however, proved that the electric current is induced through the agency of the nervoussystem. This important fact has been communicated by M. Dubois-Raymond, whose experiment is thus recorded:—A cylinder of wood is firmly fixed against the edge of a table; two vessels filled with salt and water are placed on the table, in such a position that a person grasping the cylinder may, at the same time, insert the fore-finger of each hand in the water. Each vessel contains a metallic plate, and communicates, by two wires, with an extremely sensitive galvanometer. In the instrument employed by M. Dubois-Raymond, the wire is about 3¼ miles in length. The apparatus being thus arranged, the experimenter grasps the cylinder of wood firmly with both hands, at the same time dipping the fore-finger of each hand in the saline water. The needle of the galvanometer remains undisturbed; the electric currents passing by the nerves of each arm, and being of the same force, neutralize each other. Now, if the experimenter grasp with energy the cylinder of wood with the right hand, the left hand remaining relaxed and free, immediately the needle will move from west to south, and describe an angle of 30°, 40°, and even 50°; on relaxing the grasp, the needle will return to its original position. The experiment may be reversed by employing the left arm, and leaving the right arm free: the needle will, in this case, be deflected from west to north. The reversing of the action of the needle proves the influence of the nervous force. The conditions, it may be added, essential to the success of the experiment are: 1st, Great muscular and nervous energy; 2nd, The contraction of only one arm at a time; 3rd, Dryness and cleanliness of skin; and 4th, Freedom from any kind of wound on the immersed part.

In making electrical experiments of the simplest kind, it soon becomes apparent that certain substances, such as glass, sealing-wax, &c., retain the condition of electrical excitement; whilst other bodies, and especially the metals, seem wholly incapable of electrical excitation: hence the classification of bodies into conductors and non-conductors of electricity. This arrangement is not strictly correct, because no substance can be regarded as absolutely a conductor, orvice versâ. It is better to consider these terms as meaning the two extremes of a long chain of intermediate links, which pass by insensible gradations the one into the other. In the manufacture of electrical apparatus, glass is of course largely employed, and this substance, with brass and wood, constitute the usual materials. One of the most instructive pieces of apparatus is the electroscope, which can be made with a gas jar, a cork, a piece of glass tube, brass wire and ball, or a flat disc of brass, with some Dutch metal, or still better, gold leaf. The latter is first cut into strips by retaining the leaf between a sheet of well-glazed paper and cutting through the paper and the copper or gold leaf, otherwise it would be impossible to cut the metal, on account of its excessive thinness, except with a gilder's knife and cushion. The cork is next fitted to the gas jar, and perforated with a hole to admit the glass tube, which must be thoroughly dry, andis best coated both inside and out with the shell-lac varnish described atpage 175. Some dry silk is wound round the brass wire, so that it remains fixed and upright in the glass tube, the end outside the jar having a ball, or still better, a flat disc of brass attached, and the other extremity being split so as to act like a pair of forceps, to retain a piece of card to which the gold leaves are attached. By removing the cork, tube, and brass wire bodily from the neck of the gas jar, and then in a perfectly still atmosphere carefully bringing the card, slightly wetted with gum at the extremity, on two of the cut gold leaves, they may be stuck on, and the whole is again arranged inside the dry gas jar, and forms the important instrument called the electroscope. (Fig. 164.) With the help of this arrangement, a number of highly instructive experiments are performed.

Fig. 164.Fig. 164.

a.The brass wire, with flat disc outside, and forceps holding gold leafbinside the jar.c c.The glass tube.

First, the difference between conductors and non-conductors is admirably shown by rubbing a bit of sealing-wax against a piece of woollen cloth or flannel; on bringing the wax to the brass disc of the electroscope the gold leaves no longer hang quietly side by side, but stand out and repel each other, in obedience to the law "that bodies similarly electrified repel each other." If the brass cap is touched whilst the leaves are in this electrical state, they fall again to their original position, showing that sealing-wax, after being excited, retains its electrical condition, as also the gold leaves, because they are supported on glass, or what is termedinsulated—i.e., cut off from conducting communication with surrounding objects. When, however, the sealing-wax is passed through a damp hand, or the brass disc of the electroscope touched, the electricity is conveyed away to the earth, because the human body is a conductor of electricity.

When a brass wire is rubbed and brought to the electroscope, the leaves do not move, in consequence of the electricity passing away to the earth through the body as fast as it is generated: it is just like pouring water into a leaky cistern; but if the brass wire is tied to a long stick of sealing-wax, and this latter held in the hand whilst the wire is rubbed with a bit of flannel, then the gold leaves of the electroscope are affected, on account of the insulation of the metal, as every substance which can be rubbed (even fluids, as water) produces electricity.

An insulating stool is merely a piece of strong square board, supported on glass legs, which should be well varnished. If the assistant stands on this stool and touches the disc of the electroscope, no movement of the leaves takes place until his coat is briskly struck with a piece of dry silk or skin, when the usual repulsion occurs.

Fig. 165.Fig. 165.

Assistant standing on the insulating stool and touching the disc of the electroscope whilst being struck with a dry handkerchief.

If a little powdered chalk is placed inside a pair of bellows, and then forcibly ejected on to the disc of the electroscope, the friction of the particles of chalk against the inside of the nozzle of the bellows and against the disc of the instrument soon liberates sufficient electricity to cause the gold leaves to stand out and repel each other.

Whilst the leaves of the electroscope are repelled from each other by the application of a bit of rubbed sealing-wax, they may be again caused to approach each other on bringing a dry glass tube previously rubbed with a silk-handkerchief; because the electricity obtained from sealing-wax is different from that procured from glass: the former is calledresinousornegativeelectricity, the latterpositiveorvitreous electricity. Either, separately, isrepulsiveof its own particles, butattractiveof theother. No electrical excitation can occur without the separation of these two curious states of electricity, and electrical quiescence takes place when the two electricities are brought together; hence the fall of the gold leaves repelled by rubbed wax when the excited glass is brought towards the disc of the electroscope. This experiment may be reversed by repelling the leaves first with the excited glass, and then bringing the rubbed wax, when the same effect takes place.

To show the important elementary truth, that in all cases of electrical excitation the two kinds of electricity are generated, take a dry roll of flannel, and holding it as lightly as possible, rub it against a bit of wax. If the flannel is brought to the electroscope, the leaves repel each other, and they immediately fall when the wax is now approached, because the flannel is in the positive or vitreous state of electricity, whilst the sealing-wax is in the negative or resinous condition.

Any kind of friction generates electricity. A little roll brimstone placed in a dry mortar and powdered, and then thrown on to the electroscope, quickly causes the repulsion of the leaves.

A sheet of dry brown paper laid on a flat surface, and vigorously rubbed with a piece of india-rubber, produces so much electricity that sparks and flashes of light are apparent in a dark room when it is lifted from the table; and it affects the leaves of the electroscope very powerfully, so much so that care must be taken to apply it very carefully to the disc, or the violence of the repulsion may cause the fracture of the gold leaves, and then a great deal of time is wasted before they can be put on again.

A dry wig or bunch of horse-hair when combed becomes electrical, and likewise affects the leaves of the electroscope.

Two dry silk ribbons, the one white and the other black, passed rapidly together through the fingers, exhibit sparks and flashes of light when drawn asunder, and also cause the gold leaves to repel each other.

Much instructive amusement is afforded by testing the gold leaves when separated from each other during either of the former experiments,with an excited piece of sealing-wax. If the electricity produced is negative, they repel each other further when the excited wax is approached; if positive, they fall when the excited wax is brought near them.

When fresh, dry, ground coffee is received on to the disc of the electroscope, as it falls from the mill, powerful electrical excitation is displayed, and this is sometimes so apparent, that the particles cling around the lower part of the mill or to the sides of the cup or basin held to catch it.

After playing a tune on a violin, hold the bow (well rosined) to the electroscope, when the usual divergence of the leaves will be apparent.

Cut some chips from a piece of wood with a knife attached to a glass handle, and as they fall on to the electroscope the leaves are repelled.

Warm a piece of bombazine by the fire and then draw out some of the threads (which are of two kinds—viz., silk and wool), and place them on the electroscope, when divergence of the leaves immediately takes place.

Put upon the same leg a worsted stocking and over that a silk one, if the latter is now quickly rubbed all over with a dry hand and near the fire, and then suddenly slipped off, the sides repel each other, and the silk stocking retains very much the same shape as if the leg still remained in it, and of course collapses as the electricity passes away.

Electrical machines consist only in the better arrangement of larger pieces of glass and a more convenient mechanical contrivance for rubbing them, and are of two kinds—viz., the cylinder and plate machines; it is usual to give directions for the manufacture of an electrical machine from a common bottle, and doubtless such rude instruments have been made, but as Messrs. Elliott Brothers, of 30, Strand, now supply excellent small machines at a very low cost, it is hardly worth while to incur even a small expense for an instrument that must at the best be a very imperfect one and frequently out of order. (Fig. 166.)

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