still rendered an insulator. This thought was suggested to me by an experiment of Mr. Faraday, which I will here describe. Referring to certain effects obtained in his investigations on supposed diamagnetic polarity, he writes thus: If the effect were produced by induced currents in the mass, division of the mass would stop these currents and so alter the effect; whereas, if produced by a true diamagnetic polarity, division of the mass would not affect the polarity seriously or in its essential nature. Some copper filings were therefore digested for a few days in dilute sulphuric acid to remove any adhering iron, then wellwashed and dried, and afterwards warmed and stirred in the air, until it was seen by the orange colour that a very thin film of oxide had formed upon them; they were finally introduced into a glass tube and employed as a core. It produced no effect whatever, but was as inactive as bismuth.' (Exper. Resear. 2658.)

Now when bismuth is powdered and exposed to the action of the air, it very soon becomes tarnished, even without heating. A quantity of such powder was prepared, and its conducting power for electricity tested. The clean ends of two copper wires proceeding from a battery of Grove were immersed in the powder; but though the wires were brought as near as possible to each other, short of contact, not the slightest action was observed upon a galvanometer placed in the circuit. When the wires touched, the needle of the galvanometer flew violently aside, thus proving that the current was ready, but that the powder was unable to conduct it. Two glass tubes were filled with the der and submitted to experiment. The following results were obtained :

pow

These deflections are the same in kind as those obtained with the cylinders of massive bismuth. We have here no cessation of action. The division of the mass does not affect the result seriously or in its essential nature, and hence the deportment exhibits the characteristics of a true diamagnetic polarity.'

In summing up the results of his inquiry on this subject,

N

Mr. Faraday writes thus:- Finally, I am obliged to say that I can find no experimental evidence to support the hypothetical view of diamagnetic polarity, either in my own experiments, or in the repetition of those of Weber, Reich, and others. . . . It appears to me also, that, as magnetic polarity conferred by iron or nickel in small quantity, and in unfavourable states, is far more easily indicated by its effects upon an astatic needle, or by pointing between the poles of a strong horseshoe magnet, than by any such arrangement as mine or Weber's or Reich's, so diamagnetic polarity would be much more easily distinguished in the same way.' I was struck, on reading this passage, to find how accurately the surmise has been fulfilled by the instrument with which the foregoing experiments were made. In illustration of the powers of this instrument, as compared with that made use of by Mr. Faraday, I may be permitted to quote the following result from his paper on supposed diamagnetic polarity so often referred to:-A thin glass tube, 5 inches by threequarters of an inch, was filled with a saturated solution of protosulphate of iron, and employed as an experimental core; the velocity given to the machine at this and all average times was such as to cause five or six approaches and withdrawals of the core in one second; yet the solution produced no sensible indication on the galvanometer.' Referring to Table XXV., it will be seen that the instrument made use of in the present inquiry has given with a solution of protosulphate of iron a deflection amounting to no less than one hundred divisions of the scale. Mr. Faraday proceeds :-'A tube filled with small crystals of protosulphate of iron caused the needle to move about 2o. . . . . Red oxide of iron produced the least possible effect.' In the experiments recorded in the foregoing pages, the crystallised sulphate of iron gave a deflection of nearly two hundred divisions of the scale, while the red oxide gave a deflection as wide as the helices would permit, which corresponds to about eight hundred divisions of the scale. The correctness of Mr. Faraday's statement regarding the inferiority of the means first devised to investigate this subject, is thus strikingly illustrated. It might be added, that red ferroprussiate of potash and other substances, which have given me powerful effects, produced no sensible impression in experiments made with the other instrument.

Thus have we seen the objections raised against diamagnetic polarity fall away one by one, and a body of evidence accumulated in its favour, which places it among the most firmly established truths of science. This I cannot help thinking is mainly to be attributed to the bold and sincere questioning of the principle when it seemed questionable. The cause of science is more truly served, even by the denial of what may be a truth, than by the indolent acceptance of it on insufficient grounds. Such denials drive us to a deeper communion with Nature, and, as in the present instance, compel us through severe and laborious inquiry to strive after certainty, instead of resting satisfied, as we are prone to do, with mere probable conjecture.

Royal Institution, November 1855.

FARADAY'S LETTER TO MATTEUCCI ON DIAMAGNETIC POLARITY.

THE following beautiful letter, extracted from Dr. Bence Jones's Life and Letters of Faraday,' shows the views of diamagnetic polarity entertained by Faraday in 1855. It was written prior to the publication of the foregoing memoir, but I have no reason to believe that the views here expressed were ever changed.

'November 2, 1855.

'My dear Matteucci,-When I received your last of October 23, I knew that Tyndall would return from the country in a day or two, and so waited until he came. I had before that told him of your desire to have a copy of his paper, and I think he said he would send it to you; I have always concluded he did so, and therefore thought it best to continue the same open practice and show him your last letter, note and all.

'As I expected, he expressed himself greatly obliged by your consideration, and I have no doubt will think on, and repeat, your form of experiment; but he wished you to have no difficulty on his account. I conclude he is quite assured in his own mind, but does not for a moment object to counter views, or to their publication; and I think feels a little annoyed that you should imagine for a moment that he would object to or be embarrassed by your publication. I think in that respect he is of my mind, that we are all liable to error, but that we love the truth, and speak only what at the time we think to be truth; and ought not to take offence when proved to be in error, since the error is not intentional; but be a little humbled and so turn the correction of the error to good account. I cannot help thinking that there are many apparent differences amongst us, which are not differences in reality. I differ from Tyndall a good deal in phrases, but when I talk with him I do not find

that we differ in facts. That phrase polarity in its present undefined state is a great mystifier.

'Well! I am content, and I suppose he is, to place our respective views before the world, and there leave them. Although often contradicted, I do not think it worth while reiterating the expressions once set forth or altering them, until I either see myself in the wrong or misrepresented, and even in the latter case I let many a misrepresentation pass. Time will do justice in all these cases.

'One of your letters asks me, what do you conceive the nature of the lines of magnetic force to be? I think it wise not to answer that question by an assumption, and therefore have no further account to give of such physical lines than that already given in my various papers. See that referred to already in the "Philosophical Magazine" (3301-3305); and I would ask you to read also 3299, the last paragraph in a paper in the "Philosophical Magazine," June 1852, which expresses truly my present state of mind.

'But a physical line of force may be dealt with experimentally without our knowing its intimate physical nature. A ray of light is a physical line of force; it can be proved to be such by experiments made whilst it was thought to be an emission, and also by other experiments made since it has been thought to be an undulation. Its physical character is not proved either by the one view or the other (one of which must be, and both may be wrong), but it is proved by the time it takes in propagation, and by its curvatures, inflections and physical affections. So with other physical lines of force, as the electric current; we know no more of the physical nature of the electric lines of force than we do of the magnetic lines of force; we fancy, and we form hypotheses, but unless these hypotheses are considered equally likely to be false as true, we had better not form them; and therefore I go with Newton when he speaks of the physical lines of gravitating force (3305 note), and leave that part of the subject for the consideration of my readers.

'The use of lines of magnetic force (without the physical) as true representations of nature, is to me delightful, and as yet never failing; and so long as I can read your facts, and those of Tyndall, Weber, and others by them, and find they all come into one harmonious whole, without any contradiction, I am