From the researches of M. Becquerel it appears, that under the combined influence of heat and pressure, slow actions acquire new activity and produce interesting effects in a physico-chemical and geological point of view.-Comptes Rendus, May 11, 1857, p. 938.

NOTE ON THE OPTICAL PROPERTIES OF MAGNETIC BODIES. BY M. VERDET.

In a note read some time ago before the Academy of Sciences*, the author made known some experiments showing that under the influence of magnetism the salts of iron exert an action upon polarized light opposite to that of water, glass, sulphuret of carbon and some other transparent substances. On studying the compounds of the other magnetic metals, he has found that a certain number of them act upon light in the same way as the compounds of iron.

He gave the name of magnetic rotatory power to the property of rotating the plane of polarization temporarily developed by magnetism in transparent substances. The magnetic rotatory power of most transparent substances he called direct, and that of the salts of iron, inverse. He now proposes to substitute for these terms the expressions positive and negative, which indicate the direction of the rotation. Water, sulphuret of carbon, glass and the other transparent substances of which the rotatory power is positive, rotate the planes of polarization in the direction in which positive electricity traverses the conducting wire of the electro-magnet; the salts of iron rotate it in the direction of movement of negative electricity.

The magnetic metals of which the author has investigated the transparent compounds, are iron, nickel, cobalt, manganese, chromium, titanium, and cerium. All these metals are attracted by electromagnets, and form compounds endowed with the same property. There are other metals, such as platinum and its analogues, which appear to be magnetic, but all the compounds of which are diamagnetic; the magnetic character of these metals is therefore not absolutely certain t.

Iron. Protosalts of iron are endowed with a negative magnetic rotatory power, evidenced by the weakness of the action which the aqueous solutions of these salts exert upon polarized light. This action is always weaker than would be that of the water contained in the solution, but in the same direction, and the author has not met with any protosalt of iron possessing a negative power sufficient to destroy entirely the positive power of the water. To make quite sure of this phænomenon, the author prepared solutions of sulphate of iron of different degrees of concentration, and found that the rotations produced agreed exactly with the hypothesis, that these solutions are mixtures of two hodies endowed with opposite rotatory powerst.

*See Phil. Mag. S. 4. vol. xii. p. 483.

On the distinction between the magnetic and diamagnetic metals, see the 21st Series of Experimental Researches in Electricity,' by Professor Faraday.

The experiments relating to these variously concentrated solutions presented a remarkable peculiarity. Considering them as formed of water

The negative magnetic rotatory power of the salts of peroxide of iron is much greater than that of the protosalts. An aqueous solution of perchloride of iron containing 40 per cent. of salt, exerts a negative action upon polarized light six or seven times greater than the action of water, and nearly equal to that of Faraday's heavy glass. Ætherial and alcoholic solutions give the same results. But woodspirit appears to be the most suitable solvent; it can take up a considerable quantity of iron-salt, remaining much more transparent than water, æther or alcohol with a similar quantity of salt. By dissolving 55 parts of perchloride of iron in 45 parts of wood-spirit, a liquid is obtained, which, from its transparency, is adapted for exact observations, and the action of which upon polarized light is nearly double that of heavy glass, but in the opposite direction. The author made use of this liquid to see whether the magnetic rotatory power of the salts of iron varied according to the same laws as that of ordinary transparent substances. He compared the rotation produced by 1 centim. of the solution, with the opposite rotation of the same thickness of sulphuret of carbon, and caused the amount of the rotation to vary, by altering the intensity of the electro-magnet, the size and form of the armatures, or their distance from the transparent substance. The proportion of the two rotations was always the same, so that the negative rotation produced by the salts of iron varies in accordance with the same laws as the positive rotation caused by transparent substances in general.

From the experiments of Plücker and Faraday, it is known that the ferrocyanide of potassium is diamagnetic, and the ferridcyanide slightly magnetic. The author found that the rotatory power of the ferrocyanide is positive and pretty considerabie; that of the ferridcyanide negative and very great. 15 parts of ferridcyanide, dissolved in 85 parts of water, furnish a liquid the rotatory power of which is twice as great as that of water.

Nickel.-All the salts of nickel have a positive rotatory power, so that their solutions exert a greater action upon polarized light than that of the water which they contain. This positive rotatory power is well marked, and comparable to that of the salts of zinc and tin.

Cobalt. The magnetic rotatory power of the salts of cobalt is positive, but weaker than that of the salts of nickel. It is rather difficult to be shown, as no salt of cobalt can be dissolved in considerable quantity in water without diminishing the transparency of the liquid.

Manganese. The protosalts of manganese possess a positive

and anhydrous sulphate, the phænomena observed could be represented numerically, by attributing to the water and the anhydrous sulphate contrary actions proportionate to the density which these two bodies possess in the solution. On the other hand, this was impossible, supposing the solutions to contain water and crystallized sulphate of iron with 7 atoms of water. Hence it appears probable that the dissolved salt is not the crystallized, but the anhydrous sulphate; and the same kind of experiment may perhaps be applied to the solution of some analogous chemical questions.

rotatory power, comparable to that of the salts of cobalt; the salts of the sesquioxide possess too much colouring power to be adapted for these experiments. Nothing, however, is easier than to prove the magnetism of the three preceding metals and their salts.

Chromium.-The protosalts of chromium are difficult to prepare ; those of the sesquioxide have so much colouring power that they cannot be dissolved in very small quantity in water without destroying all its transparency; but chromic acid and some chromates are very convenient for experiments. Neutral chromate of potash has a weak negative rotatory power, which, however, it is impossible to mistake; the rotatory power of the bichromate is also negative, and greater than that of the neutral salt; chromic acid has a negative rotatory power comparable to that of the protosalts of iron. Chromic acid and bichromate of potash are magnetic, whilst the neutral chromate is diamagnetic. On comparing this circumstance with the observations relative to the ferrocyanide of potassium, we shall be led to conclude that the positive rotatory power of the latter is not due to its being diamagnetic, but to the physical properties of the iron being as completely masked as its chemical properties in this compound*.

Titanium.-Bichloride of titanium, which is liquid at ordinary temperatures, has a negative magnetic rotatory power, a little superior in absolute value to the magnetic rotatory power of water. The author could not ascertain with certainty whether it is magnetic or diamagnetic; pure titanium is magnetic. Titanium is generally regarded as allied to tin, and the bichloride in particular as analogous to bichloride of tin. It is remarkable, that, under the influence of magnetism these two bodies exert contrary actions upon polarized light.

Cerium.-A concentrated solution of sulphate of cerium and a solution of chloride of cerium, appeared to possess a magnetic rotatory power a little less than that of water. It is therefore probable that the rotatory power of the salts of cerium is negative. The magnetism of these salts is as evident as that of the salts of chromium or manganese.

Thus, by the properties which they communicate to their transparent compounds, the magnetic metals are divided into two series, -one containing iron, chromium, titanium, and probably cerium, the other including nickel, cobalt and manganese. It is worthy of remark, that the most strongly magnetic metals, iron and nickel, are the types of these two series, and that the less magnetic metals, as it were, establish the transitions.-Comptes Rendus, June 8, 1857, p. 1209.

In studying the optical properties of the strongly-coloured solutions furnished by the persalts of iron, the salts of nickel, cobalt and chromium, and the chromates, it is essential to take into account the influence of the coloration upon the position of the tint of passage. Serious errors will be committed if the necessary correction be neglected.

LONDON, EDINBURGH AND DUBLIN

PHILOSOPHICAL MAGAZINE

AND

JOURNAL OF SCIENCE.

[FOURTH SERIES.]

1.

AUGUST 1857.

IX. Researches in Statical Electricity.
By Sir W. SNOW HARRIS, F.R.S.*

[With a Plate.]
No: I.

1. Phænomena of an electrified hollow globe, and the general nature of electrical charge.

2. On the indications of the proof-plane.

3. On Coulomb's experiment with a hollow globe and circular plate of twice the diameter of the globe.

4. Empirical expression representing the electrical charge of insulated conductors.

IN

N my paper on some elementary laws of electricity, honoured by a place in the Transactions of the Royal Society for 1836, I ventured to call attention to certain phænomena of the proof-plane commonly employed to determine the electrical distribution in different points of a charged conductor. This question has recently again engaged my attention; I have been hence led to further investigate the general nature and operation of statical electrical force. In the course of my several inquiries certain facts have presented themselves, calculated, as it appears to me, to materially affect our views of electrical action. In order, however, to make these inquiries, as submitted in this present communication, clear and intelligible, it is desirable for me to briefly treat the question, as it were, ab initio in all its general elementary detail, so as to complete that continuous chain of reasoning requisite to the full development of every sound philosophical inquiry,-a privilege which will, I trust, be freely granted me.

* Communicated by the Author. Phil. Mag. S. 4. Vol. 14. No. 91. Aug. 1857.

G

2. I omit for the present all especial account of the several processes and electrical instruments of research employed in these inquiries; they are for the most part such as I have described in former papers in the Royal Society's Transactions, viz. the Hydrostatic Electrometer, the Scale-beam Electrometer, Unit-jar, Bifilar or Torsion Balance, Quantity-jar, &c.* I will merely observe, that I have, since my first announcement of these instruments, greatly improved them, and carried processes of quantitative measurement to a remarkable degree of precision, so that given and measured quantities of electricity may be deposited with perfect certainty on insulated conducting surfaces, the intensity or reactive force accurately deduced under a great variety of circumstances, of form, extent of surface, variation of distance, or any other element essential to the inquiry. I have further given most especial attention to the perfection of the several insulations upon which the accuracy of important deductions mainly depends. Wherever it is admissible, the conducting bodies are insulated by suspension filaments of strong silk-gut, carefully varnished with a solution of shell-lac or naphtha, and sustained by varnished glass supports, so that an extremely small surface is exposed in such kind of insulation. The glass rods and other solid insulators employed were also carefully varnished, and as slender as the nature of the experiment would admit, and were made perfectly dry when employed, by the dry heat of a curved iron heated to redness. By these means, and with attention to the air of the room, I have been enabled to retain the quantity-jar and needle of the balance in a charged state for two and sometimes three days together, and that, too, without much deterioration of charge. Suspended insulated conductors of some considerable extent have been observed to perfectly retain the quantity of electricity communicated to them, and far exceeding the time requisite for the experiment under examination; so that all calculation of loss of charge by atmospheric influence, and which is commonly a very precarious matter, became altogether eliminated in the inquiry.

3. Preliminary views.-If an insulated neutral conductor, N, Plate I. fig. 1, be immediately opposed to an insulated charged conductor P, then, as is well known, a peculiar and very extraordinary species of action ensues,-an action apparently of a sympathetic kind, and which at first seems to be an action exerted between the two bodies at a distance. The result of this action, whatever it be, is to change the actually existing electrical condition of the two bodies; e. g. the neutral body N, without any direct communication of electricity, exhibits a state of electrical excitation and becomes attractive of surrounding matter, * Transactions of the Royal Society for 1834, 1836, 1839.