Part 12

It is not however necessary to this theory, that the same individual particles of electric matter which were thrown upon one side of the plate, should make the whole circuit of the intervening conductors, especially in very great distances, so as actually to arrive at the exhausted side. It may be sufficient to suppose, that the additional quantity of fluid displaces and occupies the place of an equal portion of the natural quantity of fluid, belonging to those conductors in the circuit which lay contiguous to the charged side of the glass. This displaced fluid may drive forwards an equal quantity of the same matter in the next conductor; and thus the progress may continue, till the exhausted side of the glass is supplied by the fluid naturally existing in the conductors contiguous to it.

To account for the velocity with which electricity passes through good conductors, Dr. Franklin compares the electricity in the conductors, to a wire in the bore of a tube, which it exactly fills.—If one end of this wire be moved forward, every other part of it will move in the same direction, and at the same instant.

Dr. Priestley says, it may be thought a difficulty upon this hypothesis, that one of the sides of a glass plate cannot be exhausted, without the other receiving more than its natural share; particularly as the particles of this fluid are supposed to be repulsive of one another. But it must be considered, that the attraction of the glass is sufficient to retain even the large quantity of electric fluid which is natural to it, against all attempts to withdraw it, unless that eager attraction can be satisfied by the admission of an equal quantity from some other quarter. When this opportunity of a supply is given by connecting one of the coatings with the rubber and the other with the conductor, the two attempts, to introduce more of the fluid into one of the sides, and to subtract some from the other, are made, in a manner, at the same instant. The action of the rubber tends to disturb the equilibrium of the fluid in the glass; and no sooner has a spark quitted one of the sides to go to the rubber, than it is supplied by the conductor on the other; and the difficulty with which these additional particles move in the substance of the glass, effectually prevents its reaching the opposite exhausted side. It is not said, however, but that either side of the glass may give or receive a small quantity of the electric fluid, without altering the quantity on the opposite side. It is only a very considerable part of the charge that is meant, when one side is said to be filled while the other is exhausted.

The above is the substance of the theory most generally received. It depends upon the following principles.

1. All terrestrial substances, as well as the atmosphere which surrounds the earth, are full of electric matter.

2. Glass, and other electric substances, though they contain a great deal of electric matter, are nevertheless impermeable by it.

3. This electric matter violently repels itself, and attracts all other matter.

4. By the excitation of an electric, the equilibrium of the fluid contained in it is disturbed, and one part of it is overloaded with electricity, while the other contains too little.

5. Conducting substances are permeable to the electric matter through their whole substance, and do not conduct it merely over their surface.

6. Positive electricity is when a body has too much of the electric fluid, and negative electricity, when it has too little.

Of these positions we shall now adduce those proofs, drawn from different facts, which seem in the strongest manner to confirm them.

I. “All terrestrial substances, as well as the atmosphere which surrounds the earth, are full of electric matter.” The proofs of this are very easy. There is no place of the earth or sea where the electric fire may not be collected, by making a communication between it and the rubber of an electric machine. Therefore, considering that the whole earth is moist, and that moisture is a conductor of electricity, and that every part of the earth must thus communicate with another, it is certain that the electric matter must diffuse itself as far as the moisture of the earth reaches; and this may reasonably be supposed to be to the very centre.

The case is equally clear with regard to the atmosphere. The extract from Mr. Cavallo’s journal, given in the chapter upon atmospheric electricity, is a sufficient proof that the atmosphere is full of electric matter.

II. “Glass, and other electric substances, though they contain a great deal of electric matter, are nevertheless impermeable by it.” The principal arguments for the impermeability of glass by the electric fluid are drawn from the phenomena of the Leyden phial. It is very plain that there is, in charging this phial, an expulsion of fire from the outside, at the same time that it is thrown upon the inside. This appears from numberless experiments, but is most readily observable in the following. Let a coated phial be set upon an insulating stand, and the knob of another phial brought near its coating. As soon as sparks are discharged from the prime-conductor to the knob of the first, an equal number will be observed to proceed from its coating to the knob of the second. This is very remarkable, and an unphilosophical observer will scarce ever fail to conclude, that the fire runs directly through the substance of the glass. Dr. Franklin however concludes that it does not, because there is a very great accumulation of electricity on the inside of the glass, which discovers itself by a violent flash and explosion, when a communication is made between the outside and inside coatings. But it must be confessed, there is here no other reason for concluding the glass to be impermeable, than the _probability_ that the electric matter is accumulated on one side of the glass and deficient on the other.

Another argument against the permeability of glass and other electrics is, that coated phials can receive but a very slight charge when their outside coating is insulated, and this can be effected only with a very powerful machine.

III. “The electric fluid violently repels itself, and attracts all other matter.” The proofs of this position have been so abundantly given in the course of this work, particularly in the chapter on electric attraction and repulsion, that we think it entirely superfluous to repeat them here.

IV. “By the excitation of an electric, the equilibrium of the fluid is disturbed, and one part of it is overloaded with electricity, while the other contains too little.” This position must be considered as entirely hypothetical, as the manner in which the electric fluid is collected by the excitation of glass, or any other electric substance, has not yet been satisfactorily explained.

V. “Conducting bodies are permeable by the electric fluid, through the whole of their substance, and do not conduct it merely over their surface.” Take a wire of any kind of metal, and cover part of it with some electric substance, as rosin, sealing-wax, &c. then discharge a jar through it, and it will be found that it conducts as well as without the electric coating. This, says Mr. Cavallo, proves that the electric matter passes through the substance of the metal, and not over the surface. A wire, adds he, continued through a vacuum is also a convincing proof of this assertion.

VI. “Positive electricity is an accumulation, or too great a quantity of the electric matter contained in a body; and negative electricity is when there is too little.” This position, like the fourth, must be considered as hypothetical—the peculiar nature of the electric fluid not admitting of experiments to prove, or to disprove it.

APPENDIX.

NUMBER I.

_A description of the Cement used for electrical purposes._

The best cement for electrical purposes is made by melting two parts of rosin, two of bee’s-wax, and one of brick-dust, or red ochre, together. This method of making cement is much preferable to that of rosin alone, as it is not so brittle, and at the same time it insulates equally well.

NUMBER II.

_A Composition for Coating Cylinders or Globes._

The most approved composition for lining glass cylinders or globes, is made with four parts of Venice turpentine, one part of rosin, and one of bee’s-wax.—They must be boiled together for about two hours over a slow fire, and stirred very frequently; afterwards the composition is left to cool, when it is fit for use.

If a cylinder or globe is to be lined with this mixture, a sufficient quantity must be pulverised, and introduced into the glass; then by holding the glass near the fire, the composition is melted, and by a little skill may be spread over all its internal surface, to about the thickness of a wafer.—The glass, however, must be heated very gradually, otherwise, there is danger of its breaking in the operation.

NUMBER III.

_To make the best kind of Amalgam for exciting Electrics.—_

Any metal dissolved in mercury or quick-silver will answer the purpose very well, thus two parts of quick-silver with one of tin-foil, or Aurum Mosaicum, have been used to advantage. But the most powerful composition for an amalgam, is zinc and mercury, in the proportion of one part of the latter, with five of the former, to which may be added a little bee’s-wax or tallow, the proper way of preparing this amalgam is the following.—Let the quick-silver be heated, to about the degree of boiling water, and let the zinc also be melted in an iron ladle. Pour the heated quick-silver into a wooden box, and immediately afterwards pour the melted zinc into it likewise. Then shut the box, and shake it about for some time. You must now wait till the amalgam is cool, or nearly so, and then mix a little bee’s-wax, or mutton-suet with it, by trituration.

NUMBER IV.

_The preparation of electrical Paint._

The electrician will very frequently have occasion to make use of paint, both for ornament, and convenience. We shall therefore describe a pigment, which, while it looks very well, insulates the instrument, and answers a variety of other purposes.—If a red colour is wished, let a piece of red sealing-wax be dissolved in a sufficient quantity of highly rectified spirits of wine, then let the substance which you intend to colour be warmed, after which the paint may be laid on by means of a hair pencil. Care should be taken to render the instrument clean and dry, especially if it be a glass one.—Two or three coats of this paint, will generally answer every purpose. If a black colour is preferred, black sealing-wax may be used.

If the outside coating of a jar is desired to be coloured, common oil paint will do much better than that above described, for here insulation is not required; a covering of some paint or other is always necessary, in order to prevent the amalgam, which is often scattered about the table where the apparatus is placed, from corroding the tin-foil with which the jar is covered.

NUMBER V.

_To make the Artificial Bolognian Stone._

“Calcine some common oyster-shells, by keeping them in a good coal fire, for half an hour; let the purest part of the calx be pulverised and sifted. Mix with three parts of this powder, one part of flowers of sulphur. Let this mixture be rammed into a crucible of about an inch and a half in depth, till it be almost full; and let it be placed in the middle of the fire, where it must be kept red hot for an hour at least, and then set it by to cool: when cold turn it out of the crucible; and cutting or breaking it to pieces, scrape off, upon trial, the brightest parts; which, if good phosphorus, will be a white powder.”

EPITOME OF GALVANISM.

CHAP. I. _A Short account of the discovery of Galvanism._

This part of our subject has been called _animal electricity_, by the greater part of those persons who have written upon it;—but this name seems to be improper; for, as an author of reputation on the subject, remarks, “it has by no means been proved that these phenomena depend either upon electricity or animal life.” While this is the case, it is certainly best to distinguish this science by the name of its inventor Louis Galvani. He was an Italian, and professor of anatomy at Bologna, when he made the discovery of Galvanism, which was entirely accidental, as will appear in the following account.

Whilst Galvani was one day employed in dissecting a frog, in a room where some of his friends were amusing themselves with electrical experiments, one of them happened to draw a spark from the conductor, at the same time that the professor touched one of the nerves of the animal. The consequence was, that the animal’s whole body was instantly shaken by a violent convulsion. Astonished at the phenomenon, and at first imagining that it might be owing to his having wounded the nerve, the professor pricked it with the point of his knife, to assure himself whether or not this was the case; but no motion of the frog’s body was produced. He now touched the nerve with the instrument as at first, and directed a spark to be taken at the same time from the machine, on which the contractions were renewed. Upon a third trial the animal remained motionless; but observing that he held his knife by the handle, which was made of ivory, he changed it for a metallic one, and immediately the movements took place, which never was the case when he used an electric, or non-conducting substance.

After having made a great many similar experiments with the electrical machine, he resolved to prosecute the subject with atmospheric electricity. With this view he raised a conductor on the roof of his house, from which he brought an iron wire into his room.—To this he attached metal conductors, connected with the nerves of the animals, destined to be the subjects of his experiments: and to their legs he fastened wires which reached the floor. These experiments were not confined to frogs alone. Different animals, both of cold and warm blood, were subjected to them; and in all of them considerable movements were excited whenever it lightened. These movements preceded thunder, and corresponded with its intensity and repetition; and even when no lightning appeared, the movements took place when any strong cloud passed over the apparatus.—That all these appearances were produced by the electric fluid was obvious.

Having soon after this suspended some frogs, from the iron palisades which surrounded his garden, by means of metallic hooks fixed in the spines of their backs, he observed that their muscles contracted frequently and involuntarily, as if from a shock of electricity. Not doubting that the contractions depended on the electric fluid, he at first suspected that they were connected with changes in the state of the atmosphere. He soon found, however, that this was not the case; and having varied, in many different ways, the circumstances in which the frogs were placed, he at length discovered that he could produce the movements at pleasure, by touching the animals with two different metals, which at the same time touched one another, either immediately, or by the intervention of some other substance capable of conducting electricity.

CHAP. II. _Of the Animals best fitted for Galvanic Experiments, of the Metals best calculated for making these Experiments, and of Conductors._

Almost every animal can be made to produce these muscular contractions by the Galvanic power, but those called _cold blooded_ are the best. Thus _frogs_ have been found the most convenient, both on account of their size and abundance. They also retain their muscular irritability to the Galvanic influence longer than most other animals, and it is asserted that strong convulsions can be produced in them many hours after the brain and spinal marrow have been destroyed; and also that when pretty far advanced in the process of putrefaction they are capable of Galvanic excitement.

No contractions have been produced in animals killed by _corrosive sublimate_, nor in those which have been _starved_ to death: but a very slight motion can be made to appear in those killed by _opium_, the _electric shock_, or _azotic gas_.

With regard to the metals used to effect these motions, almost any two will answer the purpose; but the most powerful are the following, viz.

│ │Gold. Zinc│ │Silver. Tin │in conjunction with│Molybdena. Lead│ │Steel. │ │Copper.

Those which have the most power are placed first; that is zinc and gold, will produce greater muscular contractions than tin and silver, or tin and gold, and so of the rest.

The process by which these wonderful appearances are produced consists in effecting, by means of the Galvanic apparatus, a communication between a nerve and a muscle, in any part of an animal body. The part of the animal upon which the experiment is to be performed is denominated the _animal arc_: and the Galvanic instruments which form the communication between the muscle and the nerve, are called the _excitatory arc_. This latter generally consists of three pieces; one fixed to the muscle, another to the nerve, and a third forming a communication between both. This last, called the _communicator_, may be made of the same metal with either of the others, or be different from both. The best communicators or conductors, are the following.—The list begins with the most perfect.

Malleable platina. Silver. Gold. Quicksilver. Copper. Brass. Tin. Lead. Iron. The human body. Salt water. Fresh water.

The metallic ores are not so good conductors as the purified metals, and their conducting power varies, according to the nature of the ores.

The metallic salts are tolerably good conductors.

Dr. Valli observed that human bodies are not all equally good conductors. Out of four persons in a company, he found that when two of them formed the circuit of communication between the nerve and muscles of a frog, the motions took place very readily. When the third person formed the circuit, the motions were very weak; but that, when the fourth person formed the communication, no motion took place. This experiment, he adds, was often repeated with the same success. The effect however may be owing to the different dryness of the skin.

Vitriolic acid, and even alcohol, appear to conduct the Galvanic influence rather better than water.

The veins and arteries are not so good conductors as the nerves; for when a blood-vessel forms part of the circuit of communication, the contractions will take place only when ramifications of the nerves are adhering to it, and if these be carefully separated, the motion will not take place. The same thing may be said of the tendons, the bones, and the membranes; for when either of those parts is separated from the body, and is introduced into the circle of communication between the muscles and nerves of a prepared frog, no motion will ensue; excepting, indeed, when those parts are full of moisture and in immediate contact with the nerve.

CHAP. III. _A Description of the Galvanic Trough and Pile._

Professor Volta’s first contrivance for manifesting Galvanism in a more vigorous manner than had hitherto been done, was what he called a _couronne de tasses_. This consisted of tumblers of glass, half filled with water, or salt and water. These glasses or tumblers, were so placed that a metallic arc, in form of a C, could be fixed with one leg in one glass, and the other in the next glass. On one end of each arc, was fastened a small plate of silver or copper, and on the other end, a similar plate of zinc or tin. These plates were immersed in the fluid contained in the tumblers.—Thus in the water of every glass there was a plate of silver or copper, and another plate of zinc or tin. The metallic arcs were formed of any good conductor. When thirty or forty of these glasses were prepared, the experimenter put one of his hands into the fluid contained in the first glass, and the other hand into that in the last: when this was done a shock, something like the electrical one, was experienced, and would recur as often as the circuit was interrupted and completed.

Mr. Volta remarks, that _alkaline solutions_ are used to the most advantage when one of the metals is tin and the other silver or copper; but that where zinc is substituted for tin, salt water is preferable.

After this discovery, Volta invented a much more convenient instrument, which, besides other advantages over the former, was more powerful and less expensive. The instrument is called the _Galvanic pile_, and very often the _Voltaic pile_, from its inventor. It is made in the following manner—Take a number of circular plates of copper, or silver and an equal number of tin or zinc of the same dimensions. Next provide a like number of round pieces of paste-board, leather, or any other substance capable of retaining moisture for a considerable time. This leather, cloth or other substance, must be rather smaller than the metal plates and, when used, well moistened with salt and water. Now form a pile, by laying alternately the zinc over the silver, and, the cloth or other moistened substance, over the zinc; and so on successively.—By thus continuing the series to forty or fifty plates, a Galvanic pile will be constructed. If the pile is intended to be of any considerable height, it ought to be secured by pillars of varnished baked wood; or strong glass tubes.

To get the shock, one hand must touch the bottom, and the other the top plate.—The hands should be wet, as the cuticle or external part of the skin is a bad conductor.

Shocks may be received by applying the hands in this manner, as long as the leather, or other substance interposed between the zinc and silver, continues moist; but as soon as it becomes dry the operation closes.

The drying of the substance was a great inconvenience in the Voltaic pile, and the inventor proposed, as a remedy for this, to station the metallic plates at a greater distance from one another, and to fill up the cells or intervals between them with a saline solution. Mr. Cruickshank, an English chemist at Woolwich, improved this construction.—His _trough_ as it is called, is made thus.—

Get a wooden trough, made of hard baked mahogany, about thirty inches long, and four or five wide and deep.—On the inside let there be cut in the sides and bottom, and at equal distances from one another, as many grooves, as the number of plates required to be put into the trough;—the grooves of a size to admit the plates. The plates are to be cemented[18] separately to each of the grooves, so that no fluid can pass from one cell to another. In this instrument the plates are constructed by soldering a plate of zinc to one of copper. The zinc, or which is the same thing, the spelter of the shops, should be melted in a vessel which exposes but a small surface to the action of the air, otherwise it would absorb oxygen so rapidly as to be converted into the flowers of zinc.—The melted metal should be poured as soon as possible into a mould of the proper size, made of stone or brass.—It is not necessary that the plates of copper should be more than one tenth of the thickness of those of zinc.