surface of the plate, and therefore stands axial. It ought, however, to stand equatorial, for the crystal is negative. Sulphate of Magnesia. - Suspending the crystalline prism from its end, and, following the method applied in the case of sulphate of zinc, we discover the ring systems and the position of the middle line. This line stands axial, but the crystal is nevertheless negative. Topaz.-This being one of the crystals pronounced by M. Plücker as peculiarly suited to the illustration of his new law, it is perhaps on that account deserving of more than ordinary attention. In the letter to Mr. Faraday, before alluded to, M. Plücker writes: 'The crystals most fitted to give the evidence of this law are diopside (a positive crystal), cyanite, topaz (both negative), and others crystallising in a similar way. In these crystals the line (A), bisecting the acute angles made by the two optic axes, is neither perpendicular nor parallel to the axis (B) of the prism. Such a prism, suspended horizontally, will point neither axially nor equatorially, but will take always a fixed intermediate direction. This direction will continually change if the prism be turned round its own axis (B). It may be proved by a simple geometrical construction, which shows that during one revolution of the prism round its axis (B), this axis, without passing out of two fixed limits c and D, will go through all intermediate positions. The directions C and D, where the crystal returns, makes, either with the line joining the two poles, or with the line perpendicular to it, on both sides of these lines, angles equal to the angle included by a and в; the first being the case if the crystal be a positive one, the last if a negative one. Thence it follows that if the crystal, by any kind of horizontal suspension, should point to the poles of a magnet, it is a positive one; if it should point equatorially, it is a negative one.' In experimenting with this crystal, we have found the greatest care to be necessary. Its diamagnetic force is so weak, that the slightest local impurity, contracted by handling or otherwise, is sufficient to derange its action. The crystals as they come from the mineralogist are unfit for exact experiment. * Phil. Mag., vol. xxxiv. p. 450. We have found it necessary to boil those we have used in muriatic acid, and to scour them afterwards with fine white sand, reduced to powder in a mortar. These precautions taken, we have been unable to obtain the results described by M. Plücker. We have examined five specimens of topaz from Saxony, the axial dimension of some of them exceeding the dimension perpendicular thereto by one-half; the axis, notwithstanding, stands in all cases from pole to pole. Two specimens of Brazilian topaz, the one an amber colour, the other almost as clear as distilled water, gave the same results; the axes of the crystals stand from pole to pole, and turning round makes no difference. On a first examination, some of the crystals exhibited an action similar to that described by M. Plücker; after boiling and scouring, these irregularities disappeared, and the axes one and all stood axial. One crystal in particular caused us considerable embarrassment. Its action was irregular, and the irregularity remained after the adoption of the methods described to ensure purity. On examination, however, a splinter from one of its sides was found to be attracted, a splinter from the side opposite was found to be repelled. To the naked eye the crystal appeared clean and clear. On examination, however, under a powerful microscope, the side of the crystal from which the magnetic splinter was taken was found dotted with small black particles imbedded in its mass; the other side of the crystal was perfectly transparent. On cleaving away the impurities, the irregularity vanished, and the crystal stood as the others. In the letter quoted, diopside is pronounced by M. Plücker to be a positive crystal. On examination with circular polarized light, as recommended by Dove,* we find the crystal to be negative. The same method pronounces topaz positive, instead of negative, as affirmed by M. Plücker. The specimens we have examined in this way are from Brazil and Saxony. Aberdeen topaz we have not examined, but it also is classed by Brewster among positive crystals. The obliquity of the middle line of topaz does not exist in the specimens which have come under our notice; it is exactly perpendicular to the planes of principal cleavage, and consequently exactly parallel to the axis of * Poggendorff's Annalen, vol. xl. pp. 457, 482. the prism. This agrees with the results of Brewster, who found the optic axes to be equally inclined to the plane of cleavage.' In experimenting with weak diamagnetic crystals, the greater the number of examples the better; as, if local impurity be present, it is thus more liable to detection. Our results with heavy spar have been confirmed by ten different crystals; with cœlestine, by five; and with topaz, as has been stated, by seven. The suspending fibre, in these and similar instances, was a foot in length and of an inch thick, or about one eighth of the diameter of a human hair. Sugar. It is well known that this crystal forms a prism with six sides, two of which are generally very prominent, the principal cleavage being parallel to these two, and to the wedge-like edge which runs along the end of the prism. The plane of the optic axes is perpendicular to the axis of the prism, and their ends may be found by cutting out a plate parallel to that axis, and inclined to the principal cleavage at an angle of about 20°. Such a plate exhibits both ring systems symmetrically, while a plate parallel to the principal cleavage exhibits one system only. Suspended between the excited poles, with the axis of the prism horizontal, and the principal cleavage vertical, the plane of the optic axes sets axial. According to the law of M. Plücker, it ought to stand equatorial, for the crystal is negative. Rock-crystal (Quartz).-This crystal has undergone more than one examination by M. Plücker, its deportment being, 'contrary to all expectation,' very weak-a result, it may be remarked, difficult of explanation on the hypothesis of an 'optic axis force.' M. Plücker's first experiments with this crystal were apparently made with great exactitude, the crystal being reduced to a spherical shape, and the influence of mere form thus annulled. These experiments proved the optic axis to be repelled. Later researches, however, induced the philosopher to alter his opinion, and accordingly, in his last memoir,† we find quartz ranked with those crystals whose optic axes are attracted, with the remark 'weak' added parenthetically. We have not been able to obtain this deportment. *Lardner's Encyclopædia, Optics, p. 204. After the washing and scouring process, the finest and most transparent crystals we could procure confirmed the first experiments of M. Plücker, and therefore contradict the new modification of his law. It is almost incredible how slight an impurity is sufficient to disturb the action of this crystal. A specimen with smaller crystals attached to it, or growing through it, is suspicious and ought to be rejected. Clear isolated crystals are alone suitable. We must remark that a fine cube, with faces half an inch square, suspended with the optic axis horizontal, showed no directive action; either one or the other of the diagonals set itself from pole to pole, though the axis ran parallel to four of the faces. As far as it has been practicable, we have cut and cloven, and examined the optical properties of the crystals which have passed through our hands ourselves, testing, in every possible case, the results of others by actual experiment. Most of the crystals in Brewster's list have been gone through in this way. Iceland spar, quartz, mica, arragonite, diopside, lepidolite, topaz, saltpetre, sugar, sulphate of zinc, sulphate of magnesia, and others have been examined and verified. In two cases, however, our results differed from the list, these being sulphate of nickel and borax. A prism of sulphate of nickel was suspended from its end between the poles; on exciting the magnet it took up a determinate position. When it came to rest, a line parallel to the magnetic axis was marked thereon, and a plate taken from the crystal parallel to this line and to the axis of the prism. Such a plate, ground thin, exhibited in the polariscope a pair of very beautiful ring systems. The ring systems of borax were found in a similar manner. The middle line, therefore, in both cases stood equatorial, and, according to the list, would contradict the law of M. Plücker, for both are there set down as positive. A careful examination with circular polarised light led us to the opposite conclusion. We thought it worth while to send specimens of each to Berlin, so as to have them examined by Professor Dove, the author of the method by which we examined them. The crystals have been returned to us with a note certifying that they are negative, thus confirming our observations. This certificate has reached us in the form of a private note, but we believe Professor Dove will not charge us with imprudence for thus availing ourselves of the high authority of his opinion. Yellow Ferrocyanide of Potassium.-This crystal does not stand in the list of Brewster, and we have sought for it in other lists in vain. In one German work on physics we find Blutlaugensalz set down as a negative crystal with one optic axis, but whether the red or yellow salt is meant, the author does not explain. We have examined the crystal ourselves, and find it positive with two optic axes. The middle line stands perpendicular to the principal cleavage. Suspended with this line horizontal, on closing the circuit it sets itself equatorial. Another exception to the law under consideration is here exhibited. M. Plücker recommends the magnet as a practical means of determining whether a crystal is positive or negative; this method being attended with the peculiar advantage that it can be applied in the case of opaque crystals, where all the ordinary methods fail. We find accordingly, in his last memoir on this subject, metallic and other opaque crystals with optical properties attributed to them. Antimony is negative with one optic axis; bismuth and arsenic are positive with one optic axis. The foregoing experiments demonstrate the insecurity of the basis on which this classification rests. By looking back upon the results described, it will be seen that we have drawn from each respective class of crystals one or more examples which disobey the law of M. Plücker. Of positive crystals with one axis, we have quartz; of positive crystals with two axes, we have heavy spar, cœlestine and ferrocyanide of potassium. Of negative crystals with one axis, we have carbonate of lime and iron, and several others; of negative crystals with two axes, we have dichroite, sugar, sulphate of zinc, and sulphate of magnesia. It is due, however, to M. Plücker to state that, in a considerable number of cases, we have found his law confirmed. Tourmaline, idocrase, beryl, Iceland spar, saltpetre, arragonite, and many others, all confirm it. Singularly enough, these are the very crystals with which M. Plücker has experimented. It is therefore not to be wondered at, that he should be led by such a mass of concurring evidence to pronounce his law general. Had his experiments embraced a sufficient number of cases, they would |