log k= 3.87943 log = 3.65263 log m = 3.94185. In any particular medium then, taking the logarithms of the differences of the three indices, we easily obtain the fourth by the above formula. For the corresponding relation of the other rays we must take a formula analogous to (21.), which will be as follows: (μc-B) (To2-TB2) (TE2—TB2) (Tõ2—TE2) -(-E-KB) (TG-5*) (cr) (r) +(μg−PB) (TE2—TB2) (Tc2—TB2) (TE2-TC2) which for brevity may be written as before, k' 0 = (CPB) - (μE —μB) l' + (μg −μÅ) m'. The coefficients 'l'm' may be found exactly as before from Fraunhofer's values of TTC TE To which give TG 26.437, T2 = 39.705. = 17'045, TE2 = 26·437, T By means of these we obtain log m' = 2.28794. I will only add at present that I am now engaged in determining by observation the indices for various media, especially those of a highly dispersive nature: and in the few attempts as yet made to verify the theory in these cases, (in which it is manifestly put to a more severe test than in any of the cases hitherto given,) I have found the method just explained by far the most preferable. The approximate method in any form appears to me at once more troublesome and less satisfactory. The values of the constants applicable to all media above given, may be useful to those who may engage in such calculations, or in verifying those already performed by other methods. Oxford, June 19th, 1836. XXVII. On certain Improvements in the Construction of Magneto-electrical Machines, and on the Use of Caoutchouc for Insulation in Voltaic Batteries. By FRED. W. MULLINS, Esq., M.P., F.S.S. To the Editors of the Philosophical Magazine and Journal. GENTLEMEN, I THINK it important to call the attention of the scientific readers of your valuable Journal, to some improvements recently made by me in the construction of the magneto-electric machine, which go far to demonstrate the still very imperfect state of these instruments, and form a foundation for alterations infinitely more important both in their mode of construction and application. The machine whose power I had an opportunity of testing was constructed on the most approved principle, and consists of two sets of bar-magnets arranged vertically, each set consisting of a dozen bars, and the upper poles of one set being unconnected with those of the other. I had previously seen and examined horizontal horse-shoe machines, and so far as I was enabled to institute a comparison considered the other mode of construction to be preferable. After trial, however, it struck me that the power of all magneto-electric machines was very imperfectly developed, and that it might be possible to obtain considerably greater effects from the same number of magnetic bars by establishing a magnetic connexion between the poles of the latter, and this without much difficulty or increased expense. With this view I procured two magnetized arcs of the shape given in the annexed figure, and of the same width and thickness as the bars of the machine. I then applied them, one to the opposite poles of the outside pair of bars, and one to those of the inside, and on giving the shock to a gentleman who was present, and who had tried the power of the instrument when the poles were unconnected, the effect was so much increased that he refused to repeat it, and on trying it on myself I found the power to be fully double what it had previously been. I was aware that connecting pieces of soft iron were sometimes used, but that their utility was said to be very questionable, and having myself tried them, I can safely say that soft iron as a mode of connexion is useless; it is evident, therefore, that the increase of power does not depend upon connexion, unless when the substance forming the connexion is in a peculiar state, and thereby capable of exerting a certain influence on the S N poles of each set of magnets, which influence, it can be shown, does not depend upon the size of the connecting magnets, for I have tried large horse-shoe magnetic bars, single and in sets, without any increase of power beyond that obtained from the small magnetic arcs represented in the figure. Induction is certainly a cause, but not the sole cause of the increased power; there are other causes, as yet unexplained, which I trust may appear satisfactory to those who may peruse a paper which I am now preparing on this highly interesting subject: suffice it here to say, that in the future construction of the instruments in question, magnetic arcs in connexion with vertical bar-magnets should decidedly be used in preference to any other form or mode of construction at present known; and I would strongly advise any person who happens to have a machine of the horse-shoe form to cut off the bend as indicated in the annexed figure and reapply the same or other pieces of the same size magnetized, for by so doing it will be found that a vast increase of power will be obtained. I have thrown out these hints in the hope that they may lead to still greater improvements in the mode of develop ing the powers of combined magnets. In concluding this subject it may be well to observe that with my improved magnetic machine I have charged a Leyden jar, and obtained by the same means various other results similar to those obtained from the action of the common electrical machine. In conclusion I would add, that in the various experiments I have made in regard to the best modes of developing and sustaining voltaic electricity, I have found that caoutchouc, or Indian-rubber, may be used with great advantage for insulation. I have applied it in place of glass in my intensitysustaining battery; and as it can be made to adhere to the copper and may be laid on as thin as common letter-paper, a combination of plates or cylinders may be brought so close together as to occupy only a third of the space filled by a similar combination in the batteries at present used. In my intensity-battery, from the advantages derived from bringing the metallic cylinders as close as possible, this mode of insulation is most convenient and satisfactory. I am, Gentlemen, yours, &c. House of Commons, July 1, 1836. FRED. W. MULLINS. Third Series. Vol. 9. No. 52. Aug. 1836. P XXVIII. Letter from Mr. FARADAY to Mr. Brayley on some former Researches relative to the peculiar Voltaic Condition of Iron reobserved by Professor SCHOENBEIN, supplementary to a Letter to Mr. Phillips, in the last Number. I Royal Institution, July 8, 1836. MY DEAR SIR, AM greatly your debtor for having pointed out to me Sir John F. W. Herschel's paper on the action of nitric acid on iron in the Annales de Chimie et de Physique; I read it at the time of its publication, but it had totally escaped my memory, which is indeed a very bad one now. It renders one half of my letter (supplementary to Professor Schoenbein's) in the last Number of the Philosophical Magazine, p. 57, superfluous; and I regret only that it did not happen to be recalled to my attention in time for me to rearrange my remarks, or at all events to add to them an account of Sir John Herschel's results. However, I hope the Editors of the Phil. Mag. will allow my present letter a place in the next Number; and entertaining that hope I shall include in it a few references to former results bearing upon the extraordinary character of iron to which M. Schoenbein has revived the attention of men of science. "Bergman relates that upon adding iron to a solution of silver in the nitrous acid no precipitation ensued*.” Keir, who examined this action in the year 1790†, made many excellent experiments upon it. He observed that the iron acquired a peculiar or altered state in the solution of silver; that this state was only superficial; that when so altered it was inactive in nitric acid; and that when ordinary iron was put into strong nitric acid there was no action, but the metal assumed the altered state. Westlar, whose results I know only from the Annales des Mines for 1832, observed that iron or steel which had been plunged into a solution of nitrate of silver lost the power of precipitating copper from its solutions; and he attributes the effect to the assumption of a negative electric state by the part immersed, the other part of the iron having assumed the positive state. Braconnot in 1833§ observed, that filings or even plates of iron in strong nitric acid are not at all affected at common temperatures, and scarcely even at the boiling-point. Sir John Herschel's observations are in reality the first which refer these phænomena to electric forces; but Westlar's, ↑ Ibid., pp. 374, 379. Annales des Mines, 1832, vol. ii. p. 322; or Mag. de Pharm. 1830. Annales de Chimie et de Physique, vol. lii. p. 288. * Phil. Trans. 1790, p. 374. which do the same, were published before them. The results obtained by the former, extracted from a private journal dated August 1825, were first published in 1833*. He describes the action of nitric acid on iron; the altered state which the metal assumes; the superficial character of the change; the effect of the contact of other metals in bringing the iron back to its first state; the power of platina in assisting to bring on the altered or prepared state; and the habits of steel in nitric acid: he attributes the phænomena to a certain permanent electric state of the surface of the metal. I should recommend the republication of this paper in the Philosophical Magazine. Professor Daniell, in his paper on Voltaic Combinations † (Feb. 1836), found that on associating iron with platina in a battery charged with nitro-sulphuric acid the iron would not act as the generating metal, and that when it was afterwards associated with zinc it acted more powerfully than platina itself. He considers the effect as explicable upon the idea of a force of heterogeneous attraction existing between bodies, and is inclined to believe that association with the platina cleanses the surface of the iron, or possibly causes a difference in the mechanical structure developed in this particular position. In my letter, therefore, as published in the Philosophical Magazine for the present month (July), what relates to the preserving power of platina on iron ought to be struck out, as having been anticipated by Sir John Herschel, and also much of what relates to the action of silver and iron, as having been formerly recorded by Keir. The facts relating to gold and carbon in association with iron; the experimental results as to the electric currents produced; the argument respecting the chemical source of electricity in the voltaic pile; and my opinion of the cause of the phænomena as due to a relation of the superficial particles of the iron to oxygen, are what remain in the character of contributions to our knowledge of this very beautiful and important case of voltaic condition presented to us by the metal iron. I am, my dear Sir, yours very truly, E. W. Brayley, Esq. London Institution. M. FARADAY. * Annales de Chimie et de Physique, 1833, vol. liv. p. 87. |