Faraday to John Frederick William Herschel   13 November 1845

Brighton | 13 Novr. 1845

My dear Sir John

Your letter¹ was a great pleasure and encouragement to me and I hasten to acknowledge it though so immersed in work & I think I may say discovery that (being only a slow worker) I have hardly a moment to spare. The paper which I have sent to the Royal Society contains an account of that which you have rightly understood². I was not aware that there was any thing in the Athenaeum³ until I received your letter. I have not seen it & do not know who could put it in except as a notion might be gathered up at the Institution from my brief notice to our own members on Monday week⁴. That you did not obtain any result in your experiment⁵ is not to be wondered at for in fact you could not with the apparatus you describe even though you had had the strongest battery to act on & in it for I have not yet been able to discover the effect in air or any gas[.] Several years ago I made similar experiments and even used Electrolytes when electric currents were passing through them but obtained nil results. These I published in the Phil Transactions for 1834 Exp Researches 951-955⁶ and Wartman[n]⁷ repeating them came to & published similar conclusions⁸. It was only the very strongest conviction that Light, Mag & Electricity must be connected that could have led me to resume the subject & persevere through much labour before I found the key. Now all is simplicity itself.

The results briefly are these[.] If certain transparent substances be placed between the poles of a magnet so that that line of power commonly called the Magnetic curve pass through them & a ray of polarized light be also passed through parallel to the line of magnetic force then the ray is rotated: if the line of Magnetic force pass one way the rotation is in one direction: if it pass the other way the rotation is in the contrary direction[.] If these same substances be placed in a helix & the electric current be sent round them whilst a polarized ray passes along the axis of the helix, it is in these substances rotated one way for one direction of the current the other way for the other direction[.]

Glass water and thousands of bodies I suppose shew this property but as yet not gases. But this rotation does not as yet coincide with that of quartz oil of turpentine &c though no doubt it will when we understand the whole subject as for instance let a represent a square cell filled with oil of turpentine and b c a ray of light passed through it[.] Let the circles d & e represent rotation round the oil of turpentine in the one or other direction and finally suppose that the fluid has naturally right handed rotation. Then if the ray be passed from b to c & observed at c the natural rotation will be right handed or according to the circle d. If the ray pass from c to b & be observed at b the rotation will still be righthanded to the observer at b & therefore according to the circle e. Now send an electric current round the fluid in the constant direction of the circle d. It will induce rotation of the ray to the right hand or as the circle d the observer being at c but leave the current unchanged and change the place of the observer from c to b now the rotation of the ray will be to the observers left hand not as the circle e but still as the circle d and the same holds good for the rotation by magnetic force[.] I would rather not pretend to say more about this at present because I am still at work on the point[.]

As to quartz I cannot say as much for it for a reason you will see presently[.]

As to crystals; being persuaded that their crystalline structure was connected with electrical forces I worked in one direction on the subject, I think now five years ago & gave an account of my negative results in Exp Researches Paragraphs 1692-1695⁹. Quartz will not exhibit any effect on the polarized ray i.e. by the mere influence of magnetic & electric forces though rotating fluids do easily shew the phenomena - so that I cannot say as much for it as for oil of turpentine sugar or tartaric acid[.] As far as I have gone there is this striking relation between crystals & the Electric & Magnetic forces namely that no doubly refracting crystal will exhibit the effect of these forces on the ray, but only singly refracting crystals as common salt, alum and fluor spar, all of which do so¹⁰.

I have however (guided by my views of the action of magnets across dimagnetics¹¹: Exp Researches 1709-1736¹²) discovered other means of examining the extraordinary state into which bodies generally are thrown by the magnetic forces, than their effect on a ray of light, and not only do doubly refracting crystals but opaque bodies - metals & I conclude all bodies come into subjection¹³. I am so hard at work on this & other matters that I must not take a moment more: for I am here now both to rest the head & work it at the same time but my next paper to the R.S will contain an account of new magnetic actions & conditions of matter¹⁴. In the mean time let me ask of you to say nothing about this part for I want to make it out as quietly as I can[.]

Ever My dear Sir | Yours faithfully | M. Faraday

Sir J.F.W. Herschell Bart | &c &c &c

Address: Sir J.F.W. Herschell Bart | &c &c &c | Collingwood | Hawkhurst | Kent