perimental researches that have been made upon this subject, and which are due to M. Bertin.

M. Bertin's experiments all give what is called total rotation; that is to say, the angle formed by the two planes of polarisation, that is obtained by directing the electric current first in one direction and then in another; it is clear that this angle is double that which we have hitherto called the angle of rotation of the plane of polarisation; but this is of little consequence, since we are here concerned in relations alone. Furthermore, he determines the rotation of the two planes of polarisation by means of the two tints of passage, one observed when the current is travelling in a certain direction, and the other when it is travelling in a contrary direction. This mode of observation is superior in accuracy to that which is generally employed, because it gives a greater angle, and is independent of the determination, which is always very uncertain, of the zero, that is, of the position of the analyser, for which the light is extinct before the passage of the current. M. Bertin, in his researches, employed an electro-magnet, furnished with the system of armatures employed by M. Ed. Becquerel, or with greater advantage still, of an arrangement contrived by M. Rumkorff. This arrangement consists in placing the two poles of the electro-magnet facing each other. They are two cylinders of soft iron 1∙18 in. in diameter, and 3.54 in. in length, surrounded by a copper wire, 078 in diameter, covered with silk. These two cylinders, fixed horizontally by means of a double frame of cast iron, so that their axis is on the same right line, are pierced with a round hole, 39 in. wide, in the direction of the axis, in order to allow a ray of light to pass freely (Fig. 167.); the two poles facing each

Fig. 167.

each other are 39 in. apart; and in this space allow of the interposition either of a solid body, or of a small tube con

taining a liquid, and terminated by two flat surfaces. The two prisms-the polariser and the analyser, are fixed respectively upon each upright of the frame at the centre of the hole, so that the light meets one on entering, and the other on coming out.

M. Bertin also employed advantageously several bobbins placed successively along the same axis, and containing an iron core pierced along its axis by a cylindrical hole; so that the ray of light might freely travel in the direction of the axis from one end to the other. If several glasses are placed in the intervals by which the bobbins are separated, care being taken that the electric current traverses them all in the same direction, we find, as might have been expected, that the rotations produced by all these glasses are added together; for they all occur in the same direction, which is that of the direction of the current, according to the law discovered by Faraday. By this means, we may indefinitely multiply the action of a substance, and, consequently, render this action visible, however feeble it may be. A file of bobbins 3.93 in. wide, and each containing an iron cylinder 1·18 in. in diameter pierced along its axis, were centred in succession, one after the other, in a wooden trough. This file presented five intervals, including the extremities, in which the substances could be placed that were submitted to the magnetism; and the following are the very remarkable results of an experiment made with five small vessels filled with sulphuret of carbon, each presenting a stratum 393 in. in thickness:

With 5 vessels placed in the five intervals

(the two extremes are removed) ·

8° 5' of rotation.

3

6° 25'

1 only (the middle ones are removed)

Various similar experiments made with other substances equally show that the increase of rotation is less due to the increase of thickness of the body, that is submitted to the magnetic influence, than it is to the distribution of its different strata in the intervals of the bobbins.

Some philosophers had thought, at the origin of Faraday's discovery, that all solutions had the same rotatory power. The

facts observed by M. Bertin demonstrate, in the most peremptory manner, that this is an error. The following, for instance, are the numbers that he found for the rotation produced by different anhydrous liquid substances, and for water :

If we pass on to solutions, we find for them a less rotatory power than that of anhydrous liquids, especially for alcoholic solutions, which are inferior in this respect to aqueous solutions.

Thus,

M. Bertin, desirous of establishing, for each substance, its coefficient of magnetic polarisation, was compelled with this view to try and determine the laws that are followed by the double opposed influence that is exercised upon the intensity of magnetic polarisation by the increase of the thickness of the substance, and that of the distance of the magnetic poles, by which it is necessarily accompanied. He found, as the result of numerous experiments, by employing a single bobbin, and successively removing from it a piece of flint-glass which had at first been in contact by one of its extremities with this bobbin, that if the distances of the flint glass increase in arithmetical progression, the rotations of the plane of polarisation decrease in geometrical progression. Then, by putting the piece of flint-glass between two bobbins, he succeeded in determining the action of each of them; an action that is compounded of the mutual influence, variable with their relative distance, which is exercised upon each other by the opposite magnetic poles, between which the flint-glass is placed. However, he succeeded in obtaining a formula in which a term, dependent only on the nature of the body submitted to

experiment, and, consequently, independent of its thickness, of its distance from the poles, and of the force of the latter, might be determined by means of data furnished by experiment. He called this term the coefficient of magnetic polarisation. It represents the rotation that a plate in. in thickness would produce, supposing it was in contact with the pole. The following table contains these co-efficients for different substances compared with Faraday's heavy glass:

Examination of the Nature and of the Cause of the rotatory Power that is acquired by Bodies under magnetic Influence.

The results that we have been describing show us that magnetic rotatory power is a specific property of bodies; that is to say, that it depends essentially for each on their chemical and physical nature.

But what is the relation that exists between this property and the double nature of the body? Before attempting, not to resolve, which, in the present state of science, would be impossible, but simply to attack this question, let us endeavour to establish satisfactorily the character itself of the phenomenon of circular magnetic polarisation, and its relations of resemblance or non-resemblance with that of natural circular polarisation.

These two phenomena are of the same order; they present themselves under the same form: and we have seen from

Faraday's experiments that, in the same liquid, natural rotation and magnetic rotation are added together or are deducted from each other, according as they are in the same or in opposite directions.

An experiment of M. E. Becquerel's comes also entirely to confirm this mode of viewing it. Having obtained a deviation of 16° with Faraday's heavy glass, he prepared a solution of sugar, which, when placed in a glass tube of a suitable length, produced the same deviation. Then, by making the polarised ray pass successively through this solution and the heavy glass, while submitted to magnetic influence, he obtained a deviation of 32°, or a null deviation, according as the two rotatory powers act in the same direction or in contrary directions.

But, although the two classes of phenomena may be of the same order, there is a fundamental difference between them; it is relative to the direction of the rotation. In circular magnetic polarisation this direction is absolute; it only depends on the direction of the magnetisms or the currents: the polarised ray always turns in the same direction as that according to which the electric currents travel, that are acting either directly or by the intervention of magnetism upon the substance submitted to experiment. In natural circular polarisation, the direction is always relative to the position of the observer in respect to the polarised ray and the substance it is traversing. Thus, if we call one of Nichol's prisms a, and the other b, each being able to serve indifferently as polariser or analyser, the following is what happens in the case of circular magnetic polarisation. If the north pole is on the side of a, and the south pole on the side of b, or, what comes to the same thing, if the electric currents circulate from left to right around the transparent substance, the plane of polarisation will be deviated to his right; but if the observer transfers himself to b, and the polarised ray goes from b to a, instead of going from a to b, all other circumstances remaining the same, the plane of polarisation is indeed deviated in the same manner; but, as far as the observer is concerned, inhis new position this deviation occurs toward his left, and no