nitric acid diluted with its own volume of water. Into the nitric acid were poured alternately small quantities of a solution of nitrate of silver and of hydrochloric acid, the object being to cause the chloride of silver to form in a minutely divided state, so as to produce a milky liquid, into the interior of which the brilliant converging cone of light might pass, and the currents generated in the flask by the heat, might drift all the chloride successively through the light. The chloride, if otherwise exposed to the sun, merely blackens upon the surface, the interior parts undergoing no change; this difficulty I hoped therefore to avoid. The burning-glass promptly brings on a decomposition of the salt, evolving on the one hand chlorine, and disengaging a metal on the other. In one experiment the exposure lasted from 11 A.M. to 1 P.M.; it was therefore equal to a continuous midday sun of seventy-two hours. The metal was disengaged very well. But what is it? It cannot be silver, since nitric acid has no action upon it. It burnished in an agate mortar, but its reflexion is not like the reflexion of silver: it is yellower. The light must therefore have so transmuted the original silver as to enable it to exist in the presence of nitric acid. In 1837 I published some experiments on the nature of this decomposition in the Journal of the Franklin Institute.

Though this experiment, and several modifications of it which I might relate, fail to establish any permanent change in the metal under trial in the sense of an actual transmutation, it does not follow that we should despair of a final success. It is not likely that Nature has made fifty elementary substances of a metallic form, many of them so closely resembling one another as to be with difficulty distinguished; moreover, chlorine and other elementary substances can be changed by the sunlight in some respects permanently; and if silver has not thus far been transmuted into a more noble metal, as platinum or gold, it has at all events been made transiently into a something which is not silver. Those who will reflect a little on the matter, cannot fail to observe that the sun-rays really possess many of the powers once fabulously imputed to the powder of projection and the philosopher's stone.

XXXVIII. On the Induction Apparatus. By the Rev. N. J. CALLAN, D.D., Professor of Natural Philosophy in the Roman Catholic College, Maynooth*.

IT

T is now more than twenty years since I discovered the method of making the induction coil, or a coil by which an electric current of enormous intensity may be produced with the *Communicated by the Author.

aid of a single galvanic cell,—a coil which is now to be used for working the Atlantic Telegraph. Mr. Faraday was the first who developed the laws of electrical induction; but he did not discover the method of making a coil by which a current of very great intensity may be obtained by means of a very small battery. This was first discovered in Maynooth College in 1836. In the summer of 1837, I sent the late Mr. Sturgeon a small coil which he exhibited at a meeting of the Electrical Society in London, and from which he gave shocks to several of the members. After the meeting, I received a letter of thanks from him, in which he described the astonishment of those who experienced the extraordinary power of the coil. This was the first induction coil of great power ever seen outside the College of Maynooth. The first notice of the discovery of the coil is found in a paper of mine published in the London Philosophical Magazine for December 1836. In 1836 and 1837 I also discovered that the intensity of the current induced in the coil increased with the number of cells employed, and that a shock may be got from the coil at the moment of making as well as of breaking connexion with the battery. In April 1837 I published, in Sturgeon's 'Annals of Electricity,' a description of an instrument which I devised for producing a rapid succession of electrical currents in the coil by rapidly making and breaking communication with the battery. This, as Mr. Bachhoffner says in one of his papers published in Sturgeon's 'Annals,' was the first contact-breaker ever made. Thus, before April 1837 I had completed the coil as a machine for producing a regular supply of electricity. From 1837 till the end of 1854 my attention was directed to other matters. Since the beginning of 1855, I made a long series of experiments on the various parts of the induction coil and apparatus. Although my experiments are not yet finished, I thought it better to lay the results already obtained before the British Association*.

The following are the results of my experiments:-First, a method of getting a shock directly from the armature of a magnet at the moment of its demagnetization; secondly, the discovery of what I believe to be a new fact or law connected with the action of iron on a battery by which it is magnetized, viz. that if iron be put into a coil of covered wire, the ends of which are connected with a battery, the quantity of electricity flowing from the same battery through another coil connected with it will be considerably greater when the first coil is nearly filled with iron than when there is little or no iron in it; thirdly, a

*This paper was read in Section A. (on Mathematics and Physics) at the late meeting of the British Association in Dublin. The paper being hastily written, some things were omitted which are here supplied.

form of core which has five advantages over the cores in common use, which will enable us to get intensity and quantity currents, and may therefore answer for the Atlantic Telegraph and for the electric light; fourthly, an improved method of insulating the secondary coil; fifthly, a contact-breaker in which the striking parts are copper, and which acts as well as if they were platina; sixthly, an explanation of the action of the condenser, which appears to me more satisfactory than any other I have seen; lastly, some new facts regarding the condenser, and an improved method of making it.

The first result is a means of obtaining, not from a coil surrounding the armature of a magnet, but from the armature itself, a voltaic current capable of giving a shock. This result is obtained by making a coil of fine insulated iron wire, and an electro-magnet of such a form that the coil will fit between its poles. The iron coil is then the armature of the magnet. If the helix of the electro-magnet be connected with a battery, the iron becomes magnetized; and on account of its proximity to the magnetized iron, the coil of iron wire, or the armature of the electro-magnet, will be also magnetized, and will lose its magnetism when the connexion between the battery and electro-magnet is broken, or when the electro-magnet is demagnetized. If, at the moment the iron coil loses its magnetism, the ends of it be held in the hands, a shock will be felt. If the ends of the iron coil be connected with a delicate galvanometer, the needle will be deflected. at the moment the coil is magnetized by the electro-magnet. Hence at the moment of magnetization or demagnetization, an electric current is produced in each section of the iron at right angles to its magnetic axis. From this, two inferences may be drawn,-first, that if for the copper coils used in magnetic telegraphs, coils of iron wire were substituted, electrical currents of greater intensity might be obtained; secondly, that if iron wire were used in the secondary coil of induction coils, the intensity of the secondary currents would be increased.

Here I shall take occasion to explain the causes which produce the secondary current in the induction coil. I believe that this current is the result of the combined action of three inductive forces; one arising from the sudden cessation or destruction of the magnetism of the core, the second from the cessation of the magnetism of the primary coil, and the third from the destruction of the magnetism of the secondary coil at the moment the connexion between the battery and primary coil is broken. This supposes, first, that as long as the primary coil is connected with the battery, magnetic power is given, not only to the iron core, but also to the primary and secondary coils; and secondly, that in each of them, at the moment of losing its magnetism, an electrie

current is produced in each of them as well as in all contiguous conductors. Both, I think, may be satisfactorily proved. First, every one knows that the iron core is magnetized by the primary current. Secondly, the primary coil itself is a magnet as long as it is connected with the battery; for every wire or conductor through which a voltaic current flows has magnetic properties : one of its sides will attract the north pole of a magnetic needle, and the opposite will attraet the south pole; so that if the wire be placed over the needle at rest, the latter will be deflected from the magnetic meridian. The wire, or conductor of a galvanic current has its magnetic poles, not at its extremities, but at its opposite sides; so that were the wire divided into two halves along its length, one half would be a north and the other a south magnetic pole. The magnetic axis of such a wire is one of its diameters, or a line joining its opposite sides. Thirdly, the secondary coil is a magnet when the primary coil is connected with the battery. This is evident when the secondary coil is made of iron wire; for the primary current magnetizes iron by which it is surrounded as well as iron enclosed within it: it induces in each section of the surrounding as well as of the enclosed iron, an electrical current which magnetizes the iron. I have found by experiment that iron outside the primary coil is not so strongly magnetized as iron enclosed within it. When, as is commonly the case, the secondary coil is made of copper wire, it is also a magnet; for the primary current induces an electrical current in each spiral of the secondary coil of copper, as well as in each section of the iron core. This current magnetizes each spiral of the copper coil, and makes the whole coil a magnet at the moment the primary coil is connected with the battery. Now we must suppose, that as the primary current, whilst it continues to flow, maintains in the iron core the magnetic power produced by the currents induced in each section of the iron at the moment the primary coil is connected with the battery, although these currents last but an instant, so also the same primary current will maintain in each of the spirals of the copper coil the magnetism given to them by the currents induced in them at the moment the battery connexion is made. There is no reason why the continuance of the primary current should not maintain its first effect in the copper spirals as well as in the iron, since the first effect is the same in both, viz. the magnetization of both. Hence, when the primary wire of an induction coil is connected with a battery, the secondary coil is always a magnet, as well as the core and primary coil; and therefore in every induction coil we have three magnets so long as its primary coil is connected with a voltaic battery; and the three lose their magnetism the moment the battery communication is

broken. Now in every magnet, at the moment of the cessation of its magnetism, an electric current is produced in a direction at right angles to the magnetic axis, in the magnet itself and in all contiguous bodies. First, it has been already shown that at the moment iron loses its magnetic power, an electric current is produced in each section of it in a direction perpendicular to its magnetic axis. By the laws of induction, these currents induce parallel ones in every contiguous conductor. Secondly, when a current flowing from a battery through a copper wire ceases, the wire loses its magnetism; and it is found by experiment, that at the moment of losing its magnetism, an opposite electrical current is produced in the whole length of the wire, or in a direction at right angles to its magnetic axis. Hence, because in every induction coil excited by a battery there are three magnets, viz. the core, the primary and secondary coils, having a common axis, and because at the moment the connexion with the battery is broken the three lose their magnetic power, an electrical current is produced in each section of each of the magnets in a direction perpendicular to their common axis; and these currents in each magnet induce electrical currents in the other two. Therefore, when the connexion with the battery is broken, a current is produced in the secondary coil, which is the result of the combined action of three inductive forces arising from the suspension of the magnetism of the core, of the primary and of the secondary coil. When the secondary coil is made of iron wire, the magnetic power it will receive from the primary current, and from the magnetic inductive force of the core, will be far greater than if it be made of copper wire; and therefore the intensity of the secondary current in a coil of iron wire must be much greater than that of the secondary current in a coil of copper wire. I showed, at the late meeting of the British Association in Dublin, an induction coil in which the secondary wire was of iron: its length was about 21,000 feet, and its thickness about the Toodth of an inch. With a single cell, 6 inches by 4, and without a condenser, this coil gave sparks half an inch long. Should a condenser of the proper size increase the length of the sparks, as it does in Mr. Gassiot's great coil, in a thirtyfold ratio, my coil ought to give sparks 15 inches long with a single cell. I have not yet tried it with a condenser: I made two large condensers, in which, when both were united, the acting surface of each plate exceeded 600 square feet. After being used for some time, the insulation of the plates gave way, and the action of the condenser became feeble, and once ceased altogether. I intend to reconstruct both condensers as soon as possible, and to try their effect on the coil, on which I have, since the meeting of the Association, coiled about 28,000 feet more of fine iron wire, so