the direction from pillar to pillar the axial direction, and that perpendicular thereto, the equatorial. In some cubes the plane of most eminent cleavage formed two of the opposite sides, and in some the said plane was inclined at an angle of 30° or 60° to two opposite sides. When two of the former were so placed that the cleavage throughout both stood either axial or equatorial, no current was observed on heating. When the cleavage of one cube was axial and that of the other equatorial, there was a deflection of 45°. When a pair of the other cubes were placed so that the cleavage of each made an angle of 30° with the plane of the horizon, a current of 30° was observed; when the angle with the horizon was 60°, the deflection was 19°7. Bismuth was also found to change its thermo-electric power in contact with other metals, when the position of the plane of most eminent cleavage in relation to the plane of contact of both metals was altered. These results appear to stand in intimate connexion with those of M. Magnus.

Application of the results of M. Magnus to certain effects observed by M. Regnault.

An exceedingly interesting memoir, 'On the Measurement of Temperatures by Thermo-electric Currents,' by M. Regnault, appears in the Philosophical Magazine' for June 1850. In the course of experiment some very perplexing and indeed unexplainable phenomena presented themselves, the solution of which appears to be furnished by the experiments of M. Magnus. This does not appear to have been noticed by the latter philosopher, as he is silent on the subject. I have carefully plotted the seven series of results given by M. Regnault; taking the difference of temperature of iron and platinum as abscissæ, and the difference between bismuth and antimony as ordinates, and using a horizontal scale of twenty, and a vertical scale of ten divisions to an inch. In the curves formed by the plotting of the last three series, where every pains was taken to remove all possible causes of disturbance, the anomalies are most striking. Laying the datum line of one upon that of another, and commencing at a common point, the curves ought to superpose; but

they do not; that derived by plotting the 5th series falls considerably below those obtained by plotting the 6th or 7th. A mere inspection of the table exhibits the same in particular cases. For example, a difference of temperature of 268°•64 between iron and platinum, corresponds in the third series to a difference of 13°-71 between bismuth and antimony; whereas in the 6th series, a difference of 268° 66 between the former corresponds to a difference of 17° 77 between the latter; and in the 7th series, a difference of 268° 56 is equivalent to one of 18° 60. It hence appears that the thermo-electric force of iron and platinum is relatively greater in the 6th and 7th series than in the 5th. We shall now endeavour to account for this hitherto inexplicable result. Turning to the table at page 85 of this Report, we observe that the current formed at the junction of hard and soft in an iron wire passes from hard to soft, which proves that the iron is rendered more negative when it is softened by heat. Let us now devote a moment's attention to the result with platinum wire at page 87. In the case of two homogeneous wires, the current passes from warm to cold, causing a deflection of 24° when both wires are hard. When a hard and soft wire are taken, and the former is heated, the current passes as before from warm to cold, causing, however, a deflection of only 13°. It thus appears that the soft wire is less negative, or, what is the same, more positive than the hard wire. Consistently with this, if the heated wire be the soft one, the fact of its being hot and soft at the same time ought to make the current developed a maximum-this is the case. The deflection observed under these circumstances is 36°.

The general facts being thus established, that iron, when softened by heat, becomes more negative, and that platinum, when softened by heat, becomes more positive, let us apply them to the case before us. M. Regnault commenced his 5th series with a fresh couple of iron and platinum, increasing the difference of temperatures between the hot and cold junctions gradually until it reached 273'46. The absolute temperature of the hot junction at this point was in all probability 300°. After the couple had been thus heated, it was allowed to cool, and the 6th series. was commenced: here the anomaly before alluded to at once presented itself; a certain difference of temperature produced a stronger current than in the 5th series, a result which might

be inferred à priori from the foregoing considerations. For the iron by being once heated to 300° has become more negative, as before proved, while the platinum has become more positive; the thermo-electric force of the couple has, in short, been increased, and a more powerful current is the necessary consequence. This is still more strikingly exhibited in the 7th series, where M. Regnault commences with a difference of 103°•80, and goes on increasing to 282° 18; then, without interrupting the series, allows the difference to sink again to 148°.97. The bismuth and antimony equivalent for this is 12°.30; whereas for a difference of 152°.29 between the iron and platinum, before the difference of temperature had reached the above amount (282°-18), the antimony and bismuth equivalent is only 11°·69. This fluctuation in the 7th series causes the curve derived from plotting to present somewhat of the appearance of a railway section over undulating ground, whereas in all the other cases it presents a gradual and almost uniform ascent. The 'sudden leaps' noticed by M. Regnault, whose cause he considered it impossible to ascertain, appear to be thus capable of satisfactory explanation.

XIX.-ON KOHLRAUSCH'S

VERIFICATION OF THE

THEORY OF OHM.*

THE following quotation bears so pertinently upon the subject of the present review, that an apology for its introduction here is scarcely necessary. It is extracted from a discourse by Professor Dove, before the Berlin Society for Scientific Lectures.

'As the (then considered) essential portions of a galvanic circuit were two metals and a fluid, innumerable combinations were possible, from which the most suitable had to be chosen. This gigantic task was undertaken by Ritter, an inhabitant of a village near Leignitz, who almost sacrificed his senses to the investigation. He discovered the peculiar pile which bears his name, and opened that wonderful circle of actions and reactions which, through the subsequent discoveries of Ersted, Faraday, Seebeck and Peltier, drew with ever-tightening band the isolated forces of nature into an organic whole. But he died early, as Günther did before him, exhausted by restless labour, sorrow, and disordered living. It was soon found that many experiments succeeded better with a single pair of large plates than with several small ones; and, in short, that every apparatus exhibited certain actions better than all others. Here men of science long groped in darkness, when in the year 1827, the theory of galvanism by Ohm, then of Berlin, now of Nürnberg, rose like a pole-star to brighten the obscurity. He showed that, as the apparatus itself was composed solely of conductors, the electric current must proceed not only along the connecting wire from pole to pole, but also through the apparatus itself; that the resistance offered to the passage of the current consisted therefore of two portions, one exterior to the apparatus and one

*Philosophical Magazine, vol. iii. p. 321.

within it. At a stroke, the difficulties which up to this time had beset the subject, and which were thought insuperable by those who had confined their attention to the exterior resistance only, crumbled away.

6

"Ohm brought forward his discovery in the simple earnest language which distinguishes the true investigator of nature. A theory, he says, which lays claim to immortality must not depend upon the idle garniture of words for the proof of its noble origin, but must show in all its parts, by its simple and complete correspondence with facts, and without the aid of eloquence, its affinity to that spirit which animates nature. The manner in which this theory was received was different in different lands. Henry of Princeton, North America, who at once saw its infinitely practical importance, observes, When I first read Ohm's theory, a light arose within me like the sudden illumination of a dark room by lightning.' The Royal Society of London awarded him the Copley Medal, the highest prize given by the Society for physical investigation. In France also the discovery met with the greatest recognition which a foreign investigator could expect there. But what reward did Ohm reap in Germany? While the most laborious empirical enquiries were instituted, among which those of Fechner in Leipsic deserve especial mention, to bring the theory in all possible ways to the touchstone of experience, that science whose function it is to think the great thoughts of the Creator over again, glanced down with divine satisfaction from her Olympic throne upon these sublunary occupations. In the Berlin Jahrbücher für wissenschafliche Kritik, Ohm's theory was named a web of naked fancies, which can never find the semblance of support from even the most superficial observation of facts; "he who looks on the world," proceeds the writer, "with an eye of reverence, must turn aside from this book as the result of an incurable delusion, whose sole effort is to detract from the dignity of nature.”

The investigations, of which I now purpose giving a review, occupy themselves with the experimental verification of the entire theory of Ohm. A portion of that theory has been already tested by physicists of all lands, and found true: this portion, which on account of its superior importance is called