coils are fixed. The front end of the spindle is, for the purpose of insulation, made of ivory or hard wood, and the lancetshaped blades F are mounted on a copper wire, which passes through the centre of the spindle, and to which one end of each coil of wire is soldered. E is the copper disc which always remains in contact with the mercury in the cup below, and is by a simple contrivance brought into contact with either of the other ends of the two coils. This contrivance is shown in fig. 5, which is an enlarged and side view of the front end of the spindle: at G, in the socket of the copper wheel, is a notch, in which terminate one end of each of the wires A B and C D; one side of the notch is represented in contact with A B, or the long wire for giving the shock, but by twisting the socket partly round the other side of the notch it will be brought in contact with CD, or the short wires for showing the brilliant spark, and producing the strongest heating effect. The points F in fig. 4, are in the proper position to take the spark from the coils CD, provided the socket of the disc is in contact with C D. To obtain the spark from the coil A B, which is however far less bright than the former, the notch must be brought into contact with A B, and the points twisted round one quarter of a revolution, or to that position that they will leave the surface of the mercury at the instant when the coils from which the spark is to be taken arrive at their greatest distance from the poles of the magnet.

For obtaining the shock, igniting wires, decomposing water, &c., the points should be removed, and the two ends of the wire forming the circuit should be connected, one with the mercury in the cup, and the other with the termination of the wire which passes through the insulating end of the spindle.

The action of the machine will be more readily understood by confining the attention to a single circuit; for this purpose we must suppose two of the cylinders (those opposite each other) with their coils of wire to be removed. Each of the soft iron cylinders becomes, from the known laws of induction, a temporary magnet when it is opposite one of the poles of the permanent magnet: as each cylinder passes successively both poles of the magnet, its poles are changed twice during each revolution, and the cylinders cease to be magnetic when they are at equal distances between the two poles. Electric currents are induced in the coils round each cylinder, and on account of the alternate change of the poles, these currents are alternately in opposite directions. The part of the coil round one cylinder being, as above described, connected with the copper disc, and that round the other cylinder con

nected with the dipping points, so that the current in both parts of the coil is continuous in the same direction, it is obvious that by the rotation of the spindle the circuit is alternately broken and renewed, and a spark occurs every time either of the copper points leaves the surface of the mercury, into which the copper disc also dips, thus completing the metallic communication twice during the revolution of the spindle. In the arrangement here described the successive transient currents are in opposite directions; to obtain a series of currents in the same direction, the double must be replaced by a single point, but in this case one half of the effect is lost.

The first electro-magnetic machine, that is, an instrument by which a continuous and rapid succession of sparks could be obtained from a magnet, was invented by M. Hypolite Pixii of Paris, and was first made public at the meeting of the Académie des Sciences on September 3, 1832. A description of this invention will be found in the Annales de Chimie for July 1832 (that journal is always published several months after its date), and a representation of it in Becquerel's Traité de l'Electricité, vol. iii. It differs from mine principally in two respects: first, in M. Pixii's instrument the magnet itself revolves and not the armature; and secondly, the interruptions, instead of being produced by the revolution of points, were made by bringing one of the ends of the wire over a cup of mercury, and depending on the jerks given to the instrument by its rotation for making and breaking the contact with the mercury. With this machine, furnished with a coil about 3000 feet in length, sparks and strong shocks were obtained, a gold-leaf electrometer was made to diverge, a Leyden jar was weakly charged, and water was decomposed.

My first complete magneto-electrical machine was exhibited at the meeting of the British Association at Cambridge in June 1833, and that now in the Gallery of Practical Science in Adelaide-street was placed there in August of the same year. The effects first shown by my machine were the following: 1st, the ignition and fusion of platina wire; 2ndly, the excitation of an electro-magnet of soft iron (these were first shown August 25th, 1833); and 3rdly, those of the double armature, producing at pleasure, either the most brilliant sparks and strongest heating power, or the most violent shocks and effective chemical decompositions; this was added to the instrument in December 1835.

I was led to furnish my magnet with the double armature from the following circumstances. In November 1833, Count di Predevalli brought from Paris one of Mr. Pixii's

machines, and it was sent to the Adelaide-street Gallery in order that its effects might be compared with those of mine *. Mine was found to excel in the brilliancy of the spark, while M. Pixii's machine was more effective in giving the shock and affecting the electrometer. M. Pixii's machine had a larger keeper and a much greater extent of copper wire. Shortly after, Mr. Newman of Regent-street made a smaller instrument on my construction, which gave the shock more powerfully than my large one did: this also had a greater length of coil, but the effect was at that time partly attributed to the better insulation of the wire. I then convinced myself by some experiments that the increased shock solely depended on the length of the wire. The cause of the difference of effect in the two cases admitted no longer of dispute after the publication of the experiments of Dr. Henry of Philadelphia, Mr. Jennings, and Dr. Faraday; as their investigations fully proved that the spark is best obtained from a magnetoelectric coil when short, and the shock when it is long. Mr. Clarke has no more claim to the application of the double armature to the magnet than he has to the discovery of the facts which suggested that application.

In conclusion: I think it will be evident from the preceding statement, that the magneto-electrical machine which Mr. Clarke has brought forward, "after much anxious thought, labour, and expense," is a piracy of mine; the piracy consisting not in manufacturing the instrument,-for every one is at full liberty to do so, but in calling it an invention of his own and suppressing all mention of my name as connected with it. I do not presume that Mr. Clarke is so ignorant as not to know the meaning of the word "invention," but he has strangely misapplied it by calling several other well-known pieces of apparatus his inventions. Thus he has appropriated to himself Ampère's Bascule Electrique, and calls it the Electrepeter. Among other uses of this simple contrivance of the French philosopher, it was employed in Pixii's magnet for the pose of changing the direction of the current in the wire. JOSEPH SAXTON.

24, Sussex-street, London University.

* See the Literary Gazette, No. 878.

pur

LXXI. Reply to Dr. Boase's "Remarks on Mr. Hopkins's "Researches in Physical Geology'," in the Number for July. By W. HOPKINS, Esq., M.A., F.G.S., of St. Peter's College, Cambridge.

THE

[Continued from p. 175, and concluded.]

HE two theories of the formation of veins of which I have spoken, equally depend on some process of infiltration or segregation into previously existing fissures, and differ only in the manner in which those fissures are supposed to have been formed, in the one case by dislocation, in the other by the mass becoming jointed. There is no reason why both should not be true as applied not merely to veins of different districts, but also to different veins of the same district, and both will enable us to account for nearly all the phænomena referrible to mechanical causes which veins present to us. Let us now proceed to analyse the rival theory of the contemporaneous formation of veins, of which Dr. Boase has been one of the ablest advocates.

In the first place, then, What are the physical causes which this theory assigns for the observed phænomena? We cannot of course do better than answer this question by a quotation from Dr. Boase's "Primary Geology*." "Is it not within the bounds of probability that the chemical union of the elements of the fused mass (of the earth's crust) whence resulted such a vast body of definite minerals, should be accompanied by the evolution of numerous currents of electricity, or of analogous fluids? for we know that the oscillations of the particles of matter, whether produced mechanically or during chemical combinations, will elicit streams both of common and galvanic electricity. If, then, it be acceded that the primary rocks may have been traversed by such currents during their formation, we have an explanation of the regular disposition of the granitic rocks, of veins, and other crystalline substances; and indeed not only of the subordinate parts but of the entire mass.

"This idea will remind the reader of Mr. R. W. Fox's experiments, from which he has concluded that the Cornish metalliferous veins were formed by electro-magnetism. By such imaginary currents, crossing each other in different directions, we also fancy that the phænomena of intersecting veins might be accounted for, the more powerful ones having uninterruptedly continued their course, whilst the weaker ones ex* p. 385.

perienced various degrees of diversion, being either partially or altogether involved in the impetus of their stronger oppo

nents.

Now it appears to me that all we can conclude from the above reasoning (and I am not aware that it has been put by any one in a better form) is this,—that it is not impossible that veins may have been the effect of certain electric currents which, it is possible, may have existed. The theory rests, not on our knowledge, but entirely on our ignorance. We know not that these electric currents could be produced as above supposed, and we know not whether if they did exist they could produce the effects assigned to them. Let any one consider whether by any reasoning like the above he could give any rational account of such phænomena as the following:

1. The approximate rectilinearity and parallelism of veins. 2. The relations which their directions usually bear in stratified masses to the dip and strike of the beds. 3. Their division into two principal systems, approximately perpendicular to each other.

4. The irregularity of the cross courses in width, as compared with the bearing veins.

5. The throw of a vein, or the difference of level of the same stratified bed in the opposite walls of the vein. 6. The general relation between the throw and the hade, or inclination of the vein to the vertical.

7. The numerous appearances of heaves and shifts in veins. These are some of the most obvious and general characters which mineral veins present to us; and yet I am not aware that the advocates of contemporaneous formation have made even an attempt (for as such we cannot regard the second paragraph of the above quotation) to account for one of these phænomena as a necessary or probable consequence of any definite physical cause connected with their theory, while all of them are, I conceive, perfectly accounted for on the hypothesis of an elevatory force, considered either as the original cause of fissures, or as modifying them when previously produced by joints. In the present imperfect state of our knowledge of geological causation, I would not positively reject any hypothesis carrying with it the most remote plausibility, provided it could be received without giving up others of stronger claims to our notice; and therefore I would not absolutely reject this hypothesis of contemporaneous formation as possibly applicable to certain veins, though I must still regard the process as an inconceivable one; but that we should adopt it with reference at least to the veins of our