All the tests from which

time, however, remaining the same. these curves are drawn were made with 0.075 volt electromotive force. The curves a aa, bbb, ccc, fig. 6, are the discharge-curves obtained in the three above experiments for 15° C., 57° C., and 14° C., observations of the discharge in the experiment for 61° C. not having been taken. The scale both for conductivity and for time is the same exactly as that employed in the curves A A A, CCC, fig. 5. All the curves show a regular increase of resistance with electrification, the increase being far more rapid at a high than at a low temperature. As was seen from the curve S T U, fig. 4, so also from fig. 6 we learn that the conductivity is much greater at a high than at a low temperature; and we also see from the curves AAA, BBB, CCC, D D D, that the general effect of testing day by day appears to lower the conductivity.

Electrification-curves E E E, F FF, G GG, HH H, fig. 7, were obtained from four successive tests with the copper-box condenser, and correspond with the temperatures 15° C., 60° C., 13° C., 61° C. respectively. Time is measured parallel to O X, the points O and X corresponding to the moment of applying the battery; and 50 minutes afterwards conductivity is measured parallel to OY from a zero as far below OX as the point Y is above. Curve FFF is on a scale for vertical distances one twentieth of that employed for the curves EEE GGG, and HHH on a scale one fifth of that used with EEE, GGG: that is to say, if the same scale were employed for vertical distances for all four curves, the two for the higher temperatures would be far above those for the lower temperatures-in fact, would be off the paper altogether. For horizontal distances (that is, for time) the same scale is employed for all the curves. An electromotive force of 7.5 volts was employed with all the experiments from which these four curves were drawn. Although the curves are irregular, still, on the whole, there is an increase of conductivity or diminution of resistance with electrification; and that this is probably due to the chemical action of the current referred to above is shown from the irregularity of the discharge-curve ggg obtained after removing the battery in the test at 15° C. Curve hhh, however, which is the discharge-curve for the test at 61° C., does not show any such irregularity; but then it must be noticed that HHH, the charge-curve for this temperature, indicates on the whole rather an increase than a diminution of resistance by electrification.

April 1st, 1878.

XIX. Theory of Voltaic Action. By J. BROWN, Esq.*

THE

HE production of a difference of electric potential by voltaic action is attributed by some primarily to the difference of chemical attraction between the two elements of a voltaic couple for one of the components (ions) of some compound body (electrolyte) in contact with both, that element which has the greater affinity being the positive one. It is said by others to be due to the simple "contact" of the two elements without the intervention of any third substance or combination, and has been attributed, in the case of two metals such as copper and zinc, to their mutual chemical attractionf. Faraday could not, however, discover any current during the combination of two metals (tin and platinum), through great heat was evolved; and he considered that though the source of energy in a voltaic pair was the combination of the active ion with the positive plate, decomposition was necessary to its development in the form of electricity. Numerous old experiments may be cited which show how in various ways alterations in the electric relations of metals in voltaic pairs may be produced without altering their contact.

The following experiments seem to go far towards establishing the truth of the first-mentioned (chemical) theory.

If a potential series (A) be formed by immersing couples of various metals &c. in an oxidizing electrolyte and testing for the current generated, and another (B) by the use of condenserplates in the usual way adopted by contact theorists, the two series will be found curiously similar. The simplest conclusion appears to be that the so-called "contact" excitement is due to the presence of a gaseous film § containing water, carbon dioxide, or other oxygen compounds between the plates, which film may be considered as having all the properties of an oxidizing electrolyte except its conductivity.

If in forming series A we use an electrolyte containing some other active ion such as sulphur, we obtain a totally dif ferent series, which, as Professor Fleeming Jenkin remarks]], is "quite anomalous and inconsistent with the simple potential theory." But if the chemical theory be true, then in forming series B, if we substitute for the ordinary atmosphere, containing watery vapour and other oxygen compounds, an

*Communicated by the Author.

Sir William Thomson, 'Electrostatics and Magnetism,' § 400; Tait, 'Recent Advances,' p. 305 et seq.

Phil. Trans. 1834, p. 436.

$ Wiedemann, Galvanismus, p. 12. Electricity and Magnetism, p. 217.

atmosphere containing a suitable sulphur compound, the anomaly should disappear, and we should obtain effects with the condenser which would place the metals in the same potential order as when immersed in sulphur electrolytes. In order to verify this the following experiment was made. Starting with the fact that iron is positive to copper in an oxidizing electrolyte (as water), while copper is positive to iron in a solution containing potassium sulphide or other similar sulphur compound, I made a condenser with disks 4 inches in diameter, one of copper, the other iron, well ground together. The iron disk was screwed on the lower end of an iron rod sliding in a brass tube fixed with shellac in a wooden cover fastened on the neck of a gas-jar. The jar stood on a wooden stand, through the middle of which rose a similar insulated rod carrying the copper disk. Means were provided for adjusting the disks parallel to one another, and also for filling the jar which enclosed both disks with any required gas.

To measure the charge excited by the "contact" of the plates a quadrant-electrometer was employed, which gave a deflection of 5 millims. for the potential of a bichromate cell. When the condenser-plates were placed together (in ordinary atmosphere) connected with opposite pairs of quadrants and then separated, the index light moved over 1 centim., the iron being positive, as was to be expected. Hydrogen sulphide was then allowed to flow into the gas-jar; and on repeating the connexion and separation of the plates the iron proved to be negative, the light-spot moving over about 3 centims. in the direction opposite to its first motion. This was repeated several times; and on examining the plates after the experiment, the copper was found to be of a deep blue colour, while the iron was scarcely altered. It will be observed here that the only alteration in the circumstances of the experiment was the change in the atmosphere surrounding the plates. The contacts all remained the same; and when the atmosphere contained a sulphur compound, the plates assumed the same electric relation as they would in an electrolyte containing a sulphur compound. Even the proportionate degree of tension between the plates in air and in hydrogen sulphide is similar to the ratio of their electromotive force in water and in potassium sulphide solution.

The next experiment appears to confirm in a marked way the view that the difference of potential between two metals in contact is due principally, if not altogether, to the difference of their affinities for one of the elements of some compound gas in the atmosphere surrounding them. The experiment is a modification of one devised by Sir William Thomson, and

described in his 'Papers on Electrostatics and Magnetism,' p. 317:-"A metal bar insulated so as to be movable about an axis perpendicular to the plane of a metal ring made up half of copper and half of zinc, the two halves being soldered together, turns from the zinc towards the copper when vitreously electrified, and from the copper towards the zinc when resinously electrified." Instead of a copper and zinc ring I used a copper and iron one, CI, 3·1 inches

diameter outside, with a 1 inch hole in centre. It was supported on a tripod inside a case with plate-glass sides, and which could be connected by rubber tubing with an apparatus for generating hydrogen sulphide. A piece of lead-paper was placed inside the case

to detect the first entrance of the gas. From the top of the case rose a vertical glass tube with a torsion head, from which depended a platinum wire ⚫0025 in. in diameter and about 19 in. long, carrying the needle or bar, n, of thin sheet aluminium, 1 in. long by 1 in. wide, a mirror M of about 4 ft. focus, and a glass weight, W, which dipped in a vessel of water to steady it. The tripod carrying the ring rested on the points of three screws passing up through the bottom of the case, by means of which the plane of the ring could be adjusted so as to get equal deflections on each side of the

M

W

n

zero-line. The needle hung at a distance of 1 or 2 millims. above the ring, as nearly as possible over the junction of the metals, and having its suspension-wire in the centre of the ring. It was electrified by connecting it with the positive or negative conductor of a Winter's plate machine.

In a preliminary experiment with a copper-zine ring, deflections of 5 centims. on each side of zero on the scale were readily obtained. With the copper-iron ring, however, the deflections were only to 1 centim., the iron being as zinc to the copper. As the potential of the needle could not be maintained constant by the means employed, the deflection was continually varying in amount; but when the machine was carefully worked these variations were slight, and did not interfere with the result. What follows is from my notes of the third time of going over the experiment. The needle being

negatively electrified and deflection about centim. towards iron, the case was connected with the hydrogen-sulphide bottle, and sulphuric acid poured in to generate the gas. At 2 minutes afterwards the lead-paper began to darken at edge; and in half a minute more the needle crossed the zero-line and turned towards the copper half of ring-deflection about centim. The needle was then connected with positive conductor and immediately turned towards iron; connected again to negative it turned towards copper; and so on, till in about 10 minutes after admitting the gas the deflections became undecided, the copper having become covered with sulphide, which has no affinity for sulphur.

Edenderry House, Belfast,
January 1878.

XX. Notices respecting New Books.

Astronomical Observations made at the Royal Observatory, Edinburgh, Vol. XIV., for 1870-1877. By PIAZZI SMYTH, F.R.S.E. &c. Published by order of Her Majesty's Government. Edinburgh: Neill and Co., 1877.

IN

N this volume Professor Smyth has given his attention to a most important feature of Sidereal Astronomy, viz. "Stellar Proper Motions." At present we know but little either of the distribution of the stars in space or of the directions in which they are moving. Proctor has shown that groups of stars separated, as seen by the unassisted eye, many degrees from each other, possess a community of motion, the logical inference being that they are in some way connected. The spectroscope reveals to us the fact that many stars are receding from us in the line of sight, and that others are approaching us in the same line. Proctor's deduction of "Star Drift" is based, if we mistake not, on the Proper Motions as given in various catalogues which he found necessary to study in constructing his star-maps; but neither the present recorded Proper Motions nor the recorded motions in the line of sight give us any information as yet as to the distribution of the stars in space; and if Professor Smyth's suggestion be carried out, of filling up the lacunæ purposely left in his catalogue for the reception of observations of R.A. and N.P.D. from other sources to which he has not had access, it is extremely probable that many corrections may be made to the numerical values of Proper Motions now on record. Whether the Astronomer Royal for Scotland has succeeded in truly correcting the values on record or not, he has at all events drawn the attention of astronomers to the subject, and that in a way which cannot fail of contributing to its advancement.

It is considered, by astronomers competent to give expression to Phil. Mag. S. 5. Vol. 6. No. 35. Aug. 1878.

L