Faraday to Arthur-Auguste De La Rive   31 January 1846

Royal Institution 31 Jany 1846

My dear & kind friend,

I received your letter¹ yesterday and wonder how in the midst of all your avocations and the multitude of your own beautiful and successful labours in the fields of science, bringing forth fresh harvests as you do from day to day you can have so much thought for me[.] I know that your kindness towards me is very great: but I wonder at your physical ability for so much thought directed as it is to such exalted objects and that you can continue it simultaneously with thoughts for every body else. It reminds me of my own imperfection arising I hope not willingly but from want of memory - but for that your kindness is a great compensation.

I was not aware that Dumas had caused any part of my letter² to him to appear in the Comptes Rendus³[.] In fact in that very letter I had to tell him that the Royal Society were very jealous of the appearance of a paper or the matter of a paper elsewhere than in the Transactions and that has been my difficulty in sending you any account, for the papers themselves were all sent in last year. I have applied to the Secretary and found the rule to be very strict so strict that a paper by Fox Talbot was kept out of the Transactions because he had sent the matter of it to the Academie des Sciences where it appeared in the Comptes Rendus⁴. I hope they will now publish quicker - at all events though I must not send you the proof sheets, as soon as ever I can have leave to send you the papers I will not lose a moment in doing so. In the mean time as a part of my letter to Dumas has appeared, (I have not seen it), I will write you a sort of account of my papers here with which you may do as you please[.]

The papers recently sent to the Royal Society form the XIX, XX and XXI Series of the Experimental Researches⁵. The XIXth has for its title On the Magnetization of light and the illumination of magnetic lines of force, and is divided into three parts. The first part is on the action of magnets on light and in it I shew by experiments that when any transparent liquid or solid body (not having the power of double refraction) is placed between the poles of a powerful electro-magnet and a polarized ray of light is transmitted through it in a direction parallel to the lines of magnetic force then withdrawing the magnetic force the transparent substance has no particular extra action on the light but on establishing the magnetic force the ray passing through the substance is rotated in a direction entirely dependant upon that of the magnetic force, and to a degree proportionate to the power of the magnet and the nature and extent of the substance[.] The law of rotation is that if a magnetic line of force be going from a north pole or coming from a south pole along the path of a polarized ray coming to the observer it will rotate that ray to the right hand or that if such a line of force be coming from a north pole or going from a south pole it will rotate such a ray to the left hand. The rotation continues as long as the magnetic force continues and ceases with it. Fluids possessing the power of rotating the ray can have this magnetic rotation impressed on light in them but then the rotation is in extraordinary contrast with their own rotation being added to it in one direction but at the same moment taken from it in the other. Silicated borate of lead or borate of lead are the substances which most readily shew this exertion of power. Part 2 of the paper shews by experiment the action of electric currents in helices producing the same phenomenon. The law of action is really the same but its expression becomes more simple for it so happens from our conventional expression of the direction of an electric current; that when a transparent substance is put either into a powerful or a long helix and the electric current sent round it the rotation of the ray is in the same direction as the course of the current: for the current which goes round the helix clock fashion makes the ray rotate clock fashion &c. Part 3 of the paper consists of general considerations and contains amongst other points my reasons for believing that the action is a true magnetic action on light.

The XXth and XXIst series of Researches are entitled On new magnetic actions and on the magnetic condition of all matter. These are divided into Seven parts. Part 1 contains a brief description of the powerful Electro magnets used and defines the meaning of a few terms amongst others axial and equatorial direction. Axial means that when an elongated body is placed between the contrary poles of a magnet its longest dimension points in the line joining the two poles: equatorial position means that the longest dimension of a body placed between the two poles is perpendicular to the line joining the two poles. Part 2 is devoted to the action of magnets on the heavy glass in which the effect on light was first observed. It is shewn that a prism of this heavy glass points equatorially and further that this position is due to a simple relation of the magnet & the glass dependant on their power of repelling each other. Either pole of an active magnet will permanently repel a piece of heavy glass: the repulsion is proportionate to the vicinity of the acting bodies and to the intensity of magnetic power. The intervention of metals glass or other substances usually considered not magnetic has no influence in altering the effect. - Part 3. is on the action of Magnets on other substances not being metals and here it is shewn that all bodies not being magnetic as Iron are magnetic as heavy glass or as I have for reason given called it, are diamagnetic i.e a portion of any shape will be repelled by the magnet and an elongated portion will point equatorially between the poles. It is very curious to see a tube of water or a piece of meat or a slice of apple or a plane of wood in this way repelled & pointing equatorially between the poles. Phosphorous stands high in this respect. Sulphur & India rubber very well. Division of the mass does not make any difference provided the general form & mass of the substance be retained. Part 4 is on the action of Magnets on metals generally and in the first place I find reason to believe that Platinum Palladium & Titanium are truly magnetic metals & must stand in the list with Iron Nickel and Cobalt the two first platinum & palladium taking a very low place. After that I find that all the metals not magnetic in the manner of Iron, are magnetic as heavy glass or are diamagnetic for they all point equatorially & are repelled by either pole of the magnet. Bismuth possesses this property in a very high degree equalling and perhaps surpassing phosphorous & heavy glass. The repulsion of bismuth I now find was known to Brugmans⁶ as far back as the year 1778⁷. Certain striking phenomena due to Magneto-dynamic induction and produced more especially by copper, silver, and the good conductors amongst the metals are then developed & explained which if left unaccounted for would interfere greatly with the diamagnetic results. - Part 5. takes up the action of magnets on the magnetic metals and other compounds. First I find that heat does not entirely destroy the magnetic power of Iron Nickel & Cobalt but that though they fall in force (which takes place not suddenly but by a gradation) they still remain magnetic, being always attracted by the magnet & pointing axially. Next I find that all the salts of Iron Nickel and Cobalt are magnetic and even all their solutions so here we obtain the power of preparing media which within a certain degree of strength are magnetic & adjustible in strength, are transparent, and above all are liquid so that the relations magnetic and diamagnetic of other bodies wholly immersed in them can be ascertained. In these solutions the power of the magnetic matter as the iron salt and the power of the water which is a diamagnetic are opposed to each other, so it is easy to prepare a solution which in air presents no appearance of being either magnetic or diamagnetic but is perfectly indifferent. Yet such a solution put (in a tube) into water points axially & is attracted shewing then its magnetic powers whereas if put into a solution a little more ferruginous than itself it points equatorially & is repelled precisely like a diamagnetic substance. The magnetism of all the compounds of Iron Nickel & Cobalt, led me to conclude that I could detect the magnetism of metals by the character of their salts, & by examination I find that compounds of Titanium, Manganese, Cerium & Chromium are abundantly magnetic and for these reasons in addition to others I believe them to be magnetic metals. Part 6 is devoted to the Action of Magnets in air and gases. To appearance this action is nul i.e. there is no difference between one gas and another; nor between a gas in its most rare & its densest state. When a tube of air is immersed in mercury or in water oil alcohol &c then it appears to be well magnetic i.e. it points axially & is attracted. When it is immersed in solutions of Iron Nickel Cobalt Manganese, Cerium, Chromium, &c &c of a certain strength then it appears as a diamagnetic pointing equatorially and being repelled. Its place in fact is between the magnetic & diamagnetic class, but whether it constitute a real zero point or have only a differential relation is not I think as yet absolutely decided. Part 7 contains general considerations arising from all the former results and amongst other points their relation to terrestrial phenomena & conditions, & their probable bearing upon the great question of the magnetism of the earth.

Thus my dear friend I have sent you a summary of my experiments or rather of the papers up to the present time. I have not a moment to do more now, and hope you will accept it with all its imperfections as a thank offering for your friendship & kindness to me. Our united remembrances to Madame De la Rive and Yourself. I wish I could once more look at the summit of Mont Blanc in your company[.]

Ever | Most faithfully Yours | M. Faraday