CHAPTER XIV.

Fig. 166.Fig. 166.

A cylinder electrical machine.

Plate machines are somewhat more expensive than cylinder ones, but at the same time are more quickly prepared for experiments, and Mr. Hearder, of Plymouth, states, that the secret in obtaining the greatest amount of electricity from a cylinder machine, is to keep the inside of the glass absolutely clean, dry, and free from dust. Sometimes the glass of which electrical machines are made is wholly unfit for electrical purposes, in consequence of the decomposition of the surface from imperfect manufacture and the liberation of the alkali. (Figs. 167, 168.)

Fig. 167.Fig. 167.

Fig. 168.Fig. 168.

Cylinder and plate machines are furnished with proper rubbers, and before using the instrument it is usual to remove them, and after carefully cleaning the glass with a dry silk handkerchief before a fire, the rubbers are scraped with a paper-knife to remove the old amalgam, and fresh applied by first melting the end of a tallow candle slightly, and after passing this over the rubber, the finely powdered amalgam is now dusted on to it. Electrical amalgam is prepared by fusing one part of zinc with one of tin, and then agitating the liquid mass with two parts of hot mercury placed in a wooden box; when cold it should be carefully powdered and kept in a well-stoppered bottle for use. When the amalgam has been applied, the rubbers are again screwed in their places, and the machine when turned (if the atmosphere is tolerably dry) will emit an abundance of bright sparks.

Attraction and repulsion are shown on a larger scale, with the assistance of electrical machines, by placing a fishing rod (the last joint ofwhich is made of glass) in an erect position, and attaching to the extremity a long tassel of paper from which a thin wire passes to the prime conductor of the electrical machine; on turning the instrument, the strips of paper all stand out and repel each other. (Fig. 169.)

Fig. 169.Fig. 169.

a a.The glass joint of the fishing-rod, from which the last joint, carrying the paper tassel,b, projects.c.The electrical machine.

Suspend from the prime conductor by a chain a circular brass plate and under this place another supported by a brass adjusting stand. If pith figures of men and women are placed on the lower plate, they rise directly the machine is turned, although sometimes, in consequence of irregularity in the adjustment of the centre of gravity, they perversely dance on their heads instead of the usual position; out of half a dozen figures, one only perhaps will be found to dance well, by alternately jumping to the upper plate and falling to the lower one to discharge the excess of electricity; and indeed the experiment will be found to succeed better with one or two only on the plate instead of a number, as they cling together and impede each other's movements. (Fig. 170.)

Fig. 170.Fig. 170.

a.Prime conductor.b.Upper brass-plate.c.Lower ditto. The figures are seen betweenbandc.

An assistant provided with a wig of well-combed hair presents a most ridiculous appearance when standing on the insulating stool and connected by a wire with the prime conductor of the electrical machine, every hair, when not matted together, standing out in the most absurd manner, when the machine is put in motion.

Whilst standing on the stool, sparks may be obtained from his body, and if some tow is tied over a brass ball, and moistened with a little ether, and presented to the tip of his finger, a spark flies off which quickly sets fire to the inflammable liquid.

If small discs of tinfoil, cut out with a proper stamp, are pasted in continuous lines over plate glass, or spirally round glass tubes, a verypretty effect is produced when they receive the sparks from the electrical machine, and the passage of the electricity from one disc to the other produces a vivid spiral or other line of light. When the tube is mounted in a proper apparatus, so as to revolve whilst the sparks pass down the spiral tube, the effect of the continuous electric sparks is much heightened. (Fig. 171.)

Fig. 171.Fig. 171.

a a a.A ring of brass wire supported on a glass pillar inside which the spiral tube,b, revolves, and produces beautiful and ever-changing circles of light, when connected with the conductor,c, of the electrical machine.

A great variety of experiments, depending on the proper arrangement of discs of tinfoil on various tubes of coloured glass are manufactured, and some in the form of windmills, the sails being made luminous by the passage of the electricity. The names of illustrious electricians, beautiful crescents, stars, and even profile portraits, have been produced in continuous streams of electric sparks.

When an electrified body is brought towards another which is not electrical, the latter is thrown into the opposite state of electricity as long as the excited body remains in its neighbourhood; and this condition of electrical disturbance, set up without any contact or supply of electricity, is calledinduction, and involves a vast number of interesting facts, which are thoroughly discussed in Dr. Noad's excellent work on electricity, but can only be briefly alluded to here.

If a number of lengths of brass wire, supplied with balls at the extremities, are supported on glass legs and arranged in a line, with a little pith ball attached to a thread hanging from each end of the length of brass wire, the effect of induction is shown very nicely; and when an excited glass rod is brought towards one end of the series, the rising of the pith balls to each other betrays the change which has occurred in the electrical state of the brass wires by the mere neighbourhood of the excited glass tube. The glass tube is electrified positively, and attracts the negative electricity from the brass wire towards the end nearest toit; the other extremity of the brass wire is found to be in the positive state, and this re-acting on the next, and so on throughout the lengths, completes the electrical disturbance in the whole series. (Fig. 172.)

Fig. 172.Fig. 172.

The lengths of brass wire supported on glass rod pillars indented by blowpipe, so as to retain the brass wires with the pith balls hanging from each series, the letterspandnmean Positive and Negative, and the signs for these terms are placed above. The letterspandnare painted on the blocks which support the glass rods.

If an insulated brass rod (such as has been described in the last experiment) is touched by the finger whilst under induction, it remains permanently electrified on the removal of the disturbing electrified body; and it is on this principle that the useful electrical machine called the Electrophorus is constructed. Thisconstantelectrical machine—for it will remain in action during weeks and months if kept sufficiently dry—was invented by Volta in the year 1774, and has been brought to great perfection by Mr. Lewis M. Stuart, of the City of London School; so that with a little additional apparatus the whole of the fundamental principles of electricity can be demonstrated. It consists of a flat brass or tin circular dish about two feet in diameter and half an inch deep, which is filled with a composition of equal parts of black rosin, shell-lac, and Venice turpentine; the rosin and the Venice turpentine being first melted together, and the shell-lac added afterwards, care of course being taken that the materials do not boil over and catch fire, in which case the pot must be removed from the heat, and a piece of wet baize or other woollen material thrown over it. Another tin or brass circular plate of twelve inches diameter, and supported in the centre with a varnished glass handle nine inches long, is also provided, and the resinous plate being first excited by several smart blows with a warm roll of flannel, the plate held by the glass handle is now laid upon the centre of the resinous one, and if removed directly afterwards, does not afford the electric spark; but if, whilst standing upon the excited resinous plate, it is touched, and then removed by the glass handle, a powerful electric spark is obtained; and this may be repeated over and over again with the like results, provided the plate with the glass handle is touched with the finger just before lifting it from the resinous plate. (Fig. 173.)The electricity excited on the resinous plate is not lost, and by induction sets up the opposite condition in the plate with the glass handle. The resinous plate, being excited with negative electricity, disturbs the electrical quiescence of the upper plate, and positive electricity is found on the surface touching the resinous plate, and negative electricity on the upper one, so that when it is removed without being touched, the two electricities come together again, and no spark is obtained; but if, as already described, the upper plate is touched whilst under induction, then positive electricity appears to pass from the finger to the negative electricity on the upper side of the plate, when the two temporarily neutralize each other, and then, when the plate is removed, the excess of electricity derived from the earth through the finger becomes apparent. Induction requires no sensible thickness in the conductors, and can be just as well produced on a leaf of gold as on the thickest plate of metal; and it should be remembered that non-conductors do not retain their state of electrical excitation when the disturbing cause is removed, whereas conductors possess this power, and this fact brings us to the consideration of the Leyden jar.

Fig. 173.Fig. 173.

a a.Large circular tin or brass disc with turned-up edge half an inch deep, and containing the resinous mixtureb, which is rubbed with the warm flannel.c c.The upper plate supported by the glass handled, a pith ball attached to a wire shows the electrical excitation, and the spark is supposed to be passing to the hande.

If one side of a dry glass plate is held before and touches a brass ball proceeding from the prime conductor of an electrical machine whilst in action, the other side is soon found to be electrical; this does not arise from the conduction of the electricity through the particles of the glass, but is produced by induction, the side nearest the ball being in the positive state, and the other side negative: as glass is a non-conductor of electricity, the effect is much increased by coating each side with tinfoil, leaving a margin of about two inches of uncovered glass round the covered portion, then, if one side of such a plate is held to the prime conductor of the electrical machine, and the other connected with the ground, a powerful charge is accumulated; and if the opposite sides are brought in contact with a bent brass wire, a loud snapping noise is heard, and the two electricities resident on either side of the glass come together with the production of a brilliant spark, or if the hands are substituted for the bent brass wire, that most disagreeable result is obtained—viz., anelectric shock; hence these glass plates are sometimes fitted up as pictures, and when charged and handed to the unsuspecting recipient, he or she receives the electric discharge to the great discomfort of their nervous system.

Mica is sometimes substituted for glass, and the late Mr. Crosse, the celebrated electrician, constructed a powerful combination of coated plates of this mineral. It consisted of seventeen plates of thin mica, each five inches by four, coated on both sides with tinfoil within half an inch of the edge. They were arranged in a box with a glass plate between each mica plate, all the upper sides were connected by strips of tinfoil to one side of the box, and all the under surfaces in the same manner with the opposite extremity of the box. They were charged like an ordinary Leyden battery.

If the glass plate coated with tinfoil is charged, and then placed upright on a stand, it may be slowly discharged by placing a bent wire on the edge with the extremities covered with pith balls. The wire balances itself, and continues to oscillate with noise until the electricities of the two surfaces neutralize each other. (Fig. 174.)

Fig. 174.Fig. 174.

a a.Glass plate or stand coated with tinfoil on each side,b.c.Wire with pith balls oscillating during the discharge of the glass plate.

It is easy to imagine the glass plate of the last experiment rolled up into the more convenient form of the Leyden jar, which consists of a glass vessel lined both inside and out with tinfoil, leaving some two or three inches of the glass round the mouth uncovered and varnished with shell-lac; a piece of dry wood is fitted into the mouth of the jar, through which a brass wire and chain are passed, and the end outside is fitted with a ball. The Leyden jar is charged by holding the ball to the prime conductor of the electrical machine until a sort of whizzing noise is heard, caused by the excess of electricity passing round the uncovered part of the jar and not through it, as the smallest crack in the glass of the Leyden jar would render it useless. Electricity is sometimes called a fluid, and the fact of collecting it like water in a jar, helps us to understand this analogy. The noise, the bright spark, or the shock are obtained by grasping the outside with one hand and touching the ball with a brass wire held in the other. (Fig. 175.)

Fig. 175.Fig. 175.

The Leyden jar and brass wire discharger.

The jar is silently discharged if the balls are removed from the discharger and points used instead; so, also, the whole of the electricity produced by an electrical machine in full action may be readily drawn off by a pointed conductor, such as a needle, placed at the end of a brass wire. Electricity passes much more rapidly through points than rounded surfaces, hence the reason why all parts of electrical apparatus are free from sharp points and rough asperities.

Extremely thin wires may be burnt by passing the charge of a large Leyden jar through them. The show jars, called specie jars, usually decorated and placed in the windows of chemists' shops, make excellent Leyden jars, when not too thick; and with two of the largest, all the interesting effects produced by accumulated electricity may be displayed. To pass the discharge through wires, nothing more is required than to strain them across a dry mahogany board, between two brass wires and balls, and if a sheet of white paper is placed under them, most curious markings are produced by the fine particles of the deflagrated metal blown into the surface of the paper. An arrangement of two or more Leyden jars is usually called a Leyden Battery, just as a single cannon is spoken of as a gun, whilst two or more constitute a battery. (Fig. 176.)

Fig. 176.Fig. 176.

a.Mahogany board with a sheet of white paper and three pairs of brass wires and balls fixed in the wire, three on each side. The thin wires are stretched between the balls, and the lower one is in course of deflagration.b b.Charged large Leyden battery of two jars; the arrows indicate the path of the electricity.

Little models of houses, masts of ships, trees, and towers are sold by the instrument makers, and by placing a long balanced wire on the top of the pointed wire of a large Leyden jar, having one end furnished with wool to represent a cloud, a most excellent imitation of the effects of a charged thunder-cloud is produced. The mechanical effect of a flash of lightning has been analysed, and it has been stated, in one instance, that the power developed through fifty feet was equal to a 12,220 horse-power engine, or about the power of the engines of theGreat Eastern, and that the explosive power was equal to a pressure of three hundred millions of tons. (Fig. 177.)

Fig. 177.Fig. 177.

A. Charged Leyden jar with balanced wire and wool at B, representing a thunder-cloud. C. The obelisk overturned with the discharge. D. Another model of the gable end of a house; the square pieces of wood fly out when the continuity of the conductor is broken.

It was the learned but humble minded Dr. Franklin who established the identity between the mimic effects of the electrical machines (such as have been described), and the awe-inspiring thunder and lightning of nature. A copper rod, half an inch thick, pointed and gilt at the extremity, and carried to the highest point of a building, will protect a circle with a radius of twice its length. The bottom of the rod must be passed into the earth till it touches a damp stratum.

Fig. 178.Fig. 178.

A storm.

In describing the various means by which electricity may be obtained, it was stated that "Chemical Action" was a most important source of this remarkable agent; at the same time it must be understood that it is not every kind of chemical action which is adapted for the purpose; there are certain principles to be rigidly adhered to—first, in the generation of the force; and secondly, in carrying it by wires so as to be applicable either for telegraphic purposes, or for the highly valuable processes of electrotyping and electro-silvering, plating, and gilding.

A lighted candle, or an intense combustion of coal, coke, or charcoal, no doubt involves the production of electricity, but there are no means at present known by which it may be collected and conducted; when that problem is solved, the cheapest voltaic battery will have been constructed, in which the element decomposed is charcoal, and not a metal, such as iron or zinc. The first and most simple experiment that can be adduced in proof of electrical excitation by chemical means, is to take a bit of clean zinc and a clean half-crown, and placing one on the tongue and the other below it, as long as they remain separate no effect is observed, but directly they are made to touch each other, whilst in that position, a peculiar thrill is rendered evident by the nerves of the tongue, which in this case answers the same purpose as the electroscope already described, and in a short time a peculiar metallic taste is perceptible.

It has been stated over and over again that it was to a somewhat similar circumstance we owe the discovery of voltaic electricity, and the story of the skinned frogs agitated and convulsed by an accidental communication with two different metals, or, as some say, with the electricity from an ordinary machine, has been repeated in nearly every work on the science. Professor Silliman, however, asserts that the galvanic story is doubtful, and is a fabrication of Alibert, an Italian writer of no repute, and that greater merit is due to Galvani than that of being merely the accidental discoverer of this kind of electricity, because he had been engaged forelevenyears in electro-physiological experiments, using frogs' legs as electroscopes. It was whilst experimenting on animal irritability, Galvani noticed the important fact that when the nerve of a dead frog, recently killed, was touched with a steel needle, and the muscle with a silver one, no convulsions of the limb were produced until the two different metals were brought in contact, and he explained the cause of these singular after-death contortions by supposing that the nerves and muscles of all animals were in opposite states of electricity, and that these nervous contractions were caused by the annihilation, for the time, of this condition, by the interposition of a good conductor between them.

This theory of Galvani had several opponents, one of whom, the celebratedVolta, succeeded in pointing out its fallacy; he maintained that the electrical excitement was due entirely to the metals, and that the muscular contractions were caused by the electricity thus developed passing along the nerves and muscles of the dead animal.

To Volta we are indebted for the first voltaic battery, and the distinguished philosopher may truly be said to have laid the foundation of this nowcommerciallyvaluable branch of science.

If a plate of clean bright zinc is placed in a vessel containing some dilute sulphuric acid, energetic action occurs from the oxidation of the metal, and its union as an oxide with the acid, and the escape of a multitude of bubbles of hydrogen gas. After the action has proceeded some time, the zinc may be removed, and if a little quicksilver is now rubbed over the surface with a woollen rag tied on the end of a stick, it unites with the metal, and the surface of the zinc assumes a brilliant silvery appearance, and is said to be amalgamated. In that condition it is no longer acted upon by dilute sulphuric acid, and for the sake of economy this is the only form in which zinc should be employed in the construction of voltaic batteries or single circles. If a clean plate of copper, with a wire attached, is now placed in the dilute acid opposite to and not touching the amalgamated zinc plate, which may also be furnished with a conducting wire, no bubbles of hydrogen escape until the wires from the two metals are brought in contact, and then, singular to relate, the hydrogen escapes from the copper plate, whilst the oxygen is rapidly absorbed by the zinc, and a current of electricity will now be found to pass from the zinc through the fluid to the copper, and back again through the wire to the starting point, and if the wires are disconnected, the chemical action ceases, and no more electricity is produced. (Fig. 179.)

Fig. 179.Fig. 179.

A single voltaic circle, consisting of a zinc and copper plate (markedzandc) in dilute acid. The arrows show the direction of the current.

The passage of the current of electricity is not discoverable by the electroscope, because it is adapted only to indicate electricity of high tension or intensity, such as that produced from the electrical machine, which will pass rapidly through a certain thickness of air, and cause pith balls to stand out and repel each other; such effects are not producible by a single voltaic circle, or even an ordinary voltaic battery, although one comprising some hundreds of alternations would producean effect on a delicate electrometer; hence voltaic electricity is said to be of low intensity, and this property makes it much more useful to mankind, because it has no desire to leave a metallic path prepared for it, and does not seize the first opportunity, like the electricity from the electrical machine, to run away to the earth through the best and shortest conductor offered for it. If electricity had only been producible by friction, we should never have heard of electrotyping, and the other useful applications of electrical force of low intensity.

To ascertain the passage of a current of voltaic electricity, the instrument called the galvanometer needle is provided, which consists of a coil of copper wire surrounding a magnetic needle, so as to leave the latter freedom of motion from right to left, orvice versá. When this coil is made part of the voltaic circle it becomes magnetic, and reacting on the magnetized needle, deflects it to one side or the other, according to the direction of the current. (Fig. 180.)

Fig. 180.Fig. 180.

A galvanometer needle, consisting of a coil of covered copper wire, the ends of which terminate at the binding screws. The magnetic needle is suspended on a point in the centre, and the coil is surrounded with a graduated circle.

If a number of simple voltaic circles, such as the one described in the first experiment, are connected together, they form a voltaic battery, in which of course the quantity of electricity is greatly increased. Batteries of all kinds, from the original Volta's pile, consisting of round zinc and copper plates soldered together with interposed cloth moistened with dilute sulphuric acid, or hiscouronne des tasses, consisting of zinc and silver wires soldered together in pairs, and placed in glass cups containing dilute acid, to the improved batteries of Cruikshank, Wilkinson, Babington, Wollaston, and the still more perfect arrangements of Daniell, Mullins, Shillibeer, and Grove, have been from time to time recommended for their own peculiar features.

Amongst these several inventions, none will be found more useful than theconstantbattery of Daniell for electrotyping, silvering, gilding, and other purposes, and Grove's battery for all the more brilliant results, such as the deflagration of the metals or the production of the electric light. The construction of the Daniell and Grove batteries will therefore be described. The former consists of a cylindrical vessel made of copper, in which is suspended or placed (as it is open at the top) a membranous, brown-paper, canvas, or porous earthenware tube, containing an amalgamated rod of zinc. To charge this arrangement, a strong solution of sulphate of copper, with some sulphuric acid, is poured into the copper vessel, which is provided usually with a sort ofcolander at the top to hold crystals of sulphate of copper, and in the porous tube containing the zinc rod is poured dilute sulphuric acid. A number of these cylinders of copper, twenty inches high and three inches and a half in diameter, arranged in wooden frames to the number of twenty, afford a quantity of electricity sufficient to demonstrate all the usual phenomena. (Fig. 181.)

Fig. 181.Fig. 181.

a a.Copper cylindrical vessel with colander to hold the crystals of sulphate of copper.b.The amalgamated zinc rod inside the porous cellc c.d.A series of single cells forming a Daniell's battery.

Professor Grove's battery consists of a flat glazed earthenware vessel containing a flat porous cell. An amalgamated zinc plate is placed outside the porous cell, and a platinum plate inside the latter. The arrangement is put in action by pouring dilute sulphuric acid round the zinc and strong nitric acid inside the porous cell. A set of Grove's nitric acid battery, as manufactured by Messrs. Elliott, Brothers, of 30, Strand, with fifty pairs of sheet platinum, five inches by two inches and a quarter, and double amalgamated zinc plates, flat porous cells, and separate earthenware troughs for each pair, and stout mahogany stand, arranged in ten series of five pairs, will evolve with a proper voltameter one hundred cubic inches of the mixed gases per minute from the decomposition of water, and will exhibit a most brilliant electric light, when arranged as a single series of fifty pairs of plates. Even thirty pairs exhibit the most splendid effects, whilst forty may be regarded as the happy medium, giving all the results that can be desired. (Fig. 182.)

The advantage of employing amalgamated zinc is very prominently illustrated whilst using any powerful arrangements of either Daniell's or Grove's batteries, as they will remain for hours quiescent, like a giant asleep, until the terminal wires of the series are brought in contacteither through the intervention of some fluid under decomposition or by means of charcoal points. The author had the pleasure of witnessing at King's College some of the effects of an enormous battery, prepared by the late Professor Daniell, and consisting of seventy of his cells.

Fig. 182.Fig. 182.

a a.Amalgamated zinc plate in flat earthenware trough. Attached to a binding screw is the platinum plate in porous cell,c c.d.A series of single cells forming a Grove's battery.

A continuous arch of flame was produced between two charcoal points, when distant from each other three quarters of an inch, and the light and heat were so intense that the professor's face became scorched and inflamed, as if it had been exposed to a summer heat. The rays collected by a lens quickly fired paper held in the focus.[C]

[C]By the light from the same battery photogenic drawings were taken, and the heating power was so great as to fuse with the utmost readiness a bar of platinum one-eighth of an inch square; and all the more infusible metals, such as rhodium, iridium, titanium, &c., were melted like wax when placed in small cavities in hard graphite and exposed to the current of electricity.

It is by "chemical action" the electricity is produced, and as action and reaction are always equal, but contrary, we are not surprised to find that the electricity from the voltaic battery will in its turn decompose chemically many compound bodies, of which water is one of the most interesting examples. It was in the year 1800, and immediately after Volta's announcement to Sir Joseph Banks of his discovery of the pile, that Messrs. Nicholson and Carlisle constructed the first pile in England, consisting of thirty-six half-crowns, with as many discs of zinc and pasteboard soaked in salt water. These gentlemen, whilst experimenting with the pile, observed that bubbles of gas escaped from the platinum wires immersed in water and connected with the extremities of the Volta's pile, and covering the wires with a glass tube full of water, on the 2nd of May, 1800, they completed the splendid discovery of the fact that the Volta's current had the power to decompose water and other chemical compounds.

In 1801, Davy had succeeded to a vacant post in the Royal Institution, and on Oct. 6th, 1807, made his transcendent discovery of potassium with the aid of the voltaic battery, and from that and other experiments inferred that the whole crust of the globe was composed of the oxides of metals. To exhibit the decomposition of water, two platinum plates with proper connecting wires, passing to small metallic cups full of mercury, are cemented inside a glass vessel, which is then filled with dilute sulphuric acid. Just above the platinum plates and over them, stand two glass tubes also containing the same fluid in contact with the battery. Two measures of hydrogen are found in one tube, and one of oxygen in the other. (Fig. 183.)

Fig. 183.Fig. 183.

a a.A finger glass with two holes drilled to pass the wires through, which are imbedded in cement up to the platinum plates.b b.Glass tubes, closed at one end and open at the other, which are placed over the platinum plates to receive the liberated oxygen and hydrogen. The scale at the side shows the respective volumes of two of H to one of O.

To measure the quantity power of the voltaic battery, an important instrument invented by Faraday is used. It consists of separate platinum plates cemented in a wooden stand, and over which a capped air-jar with a bent pipe is also cemented. This apparatus contains dilute sulphuric acid of the same strength as that used in the battery under examination, and by taking the time, the quantity of the mixed oxygen and hydrogen gases producible by a battery per minute is accurately determined, the gases of course being collected in a graduated jar. (Fig. 184.)

Fig. 184.Fig. 184.

a.Gas jar with cap and bent tube passing to the graduated tubec; the jar is cemented in the same stand which carries the connecting cups, wires, and platinum plates, which are bent round each other to improve the action of the voltameter.

By grouping the simple circles forming a voltaic battery in various numerical relations, thequantityandintensityeffects are modified.

Thus, if a series of thirty pairs of Grove's battery are all connected together in consecutive order, the smallestquantityand the largestintensityeffect is produced.

If changed to two groups of fifteen each, the quantity is doubled—that is to say, it will produce double the quantity of the mixed gases from the voltameter with half the intensity.

If arranged in three groups of ten each, it is trebled with a proportional loss of intensity, until the grouping reaches six series of five each, when a maximum supply of the mixed gases is obtained from the voltameter.

In arranging the groups, all the zinc ends of each series are connected, and all the platinum ends are likewise joined by proper wires.

A plate-glass trough, containing a few grains of iodide of potassium dissolved in water with some starch, is quickly decomposed into its elements by placing in two platinum plates and connecting them with the wires of the voltaic battery. If the glass trough is divided in the centre with a bit of cardboard, the purple colour of the iodine and starch is shown very beautifully on one side, but not on the other, as iodine is liberated at one pole and the alkali at the other. (Fig. 185.)

Fig. 185.Fig. 185.

a a.A glass trough containing the salt dissolved in water, and divided temporarily with a bit of cardboard,b.c care the two platinum plates connected with the battery, and the shaded side is supposed to represent the liberation of the iodine.

Some solution of common salt coloured with sulphate of indigo and placed in the trough is decomposed into chlorine, which bleaches one side of the indigo solution, and the alkali liberated on the other does not affect it.

Some nitrate of potash dissolved in water and coloured with litmus placed in the glass trough, changes red on one side of the cardboard by the liberation of acid, and is not affected on the other.

In these experiments the oxygen, iodine, chlorine, and nitric acid are liberated at the electro-positive pole, and are hence termed electro-negative bodies, whilst hydrogen and the alkalies are set free at the electro-negative pole, and are therefore called electro-positive bodies.Faraday has modified these terms, and calls the two classes "anions" and "cathions," and the two poles "anodes" and "cathodes."

Anode, from ανα, up, and ὁδος, a way: the way which the sun rises. Anions, from ανα, up, and ειμι, to go: that which goes up; a substance which passes to the anode during the passage of a current of electricity. Cathode, from κατα, down, and ὁδος, a way: the way which the sun sets. Cathion, from κατα, down, and ειμι, to go: that which goes down; a substance which passes to the cathode during the passage of electricity from the anode to the cathode.

In the process of the electrotype is presented a valuable application of the chemical power of the voltaic circle or battery, and it may be conducted either as a single cell operation or by distinct batteries. In the former case the most simple arrangement will suffice; the only articles necessary are—a large mug or tumbler; some brown paper and a ruler; a bit of amalgamated zinc, four inches long and half an inch wide; a short length of copper wire; some black lead, blue vitriol, and oil of vitriol.

The mould from which the electrotype is to be taken can be made of common sealing wax, plaster of Paris, white wax, gutta percha, or fusible alloy. Supposing the first to be selected—viz., a common seal, it is first thoroughly black-leaded,[D]then one end of the copper wire is bent round the top of the amalgamated zinc, and the other is gently warmed and melted into the side of the seal, leaving a small portion uncovered by the wax, which is then well black-leaded. A few ounces of blue vitriol are dissolved in boiling water, and when cold the solution is poured into the tumbler, and the porous cell to contain the mixture of eight parts water to one of sulphuric acid is made by rolling the brown paper three or four times round the ruler and closing the end, and fixing the side with a little sealing wax. The porous cell of brown paper is now filled with the dilute acid, and placed in the tumbler containing the solution of blue vitriol, the amalgamated zinc being arranged in the paper cell, and the attached seal in the copper solution; in about twelve hours a good deposit of copper is produced, and a perfect cast in metal of the seal obtained. (Fig. 186.)

[D]The application of plumbago, or black lead, for electrotype purposes, was first made by the late lamented Mr. Robert Murray.

Fig. 186.Fig. 186.

a a.The tumbler containing the solution of sulphate of copper.b b.The brown paper cell containing the dilute sulphuric acid, inside which is the amalgamated zinc with wire attached to the seald.

Messrs. Elliott provide every kind of convenient vessel for the purpose, and in the picture below it will be noticed that the single cell apparatus, though not so economical as the simple tumbler arrangement already described, is perhaps more convenient for electrotyping. (Fig. 187.)

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