Faraday to William Whewell   22 November 1845

Brighton | Novr. 22 1845

My dear Sir,

I write at the moment of receiving yours¹. I am here to rest and work for I have hold of such things that I must not be still; and have no time to describe much. I have not seen the Athenaeum²; & if it were here should, perhaps, not look at it; for the light paper³ seems, for the moment, old & passed to me because of that which I am engaged in. But do not say, even, that, you are aware I am so engaged. I do not want men's minds to be turned to my present working until I am a little more advanced.

As to the light, which I shall resume as soon as I can, the chief fact is very simple. Suppose a polarized ray and a magnetic curve (as the lines of magnetic force are often called), passing through a transparent solid or liquid body, having no double refraction, and parallel to each other:- then the ray is rotated. If the magnetic line passes in one direction the rotation is one way; if it passes in the other, the rotation is the other way. Or suppose the polarized ray is passing through such bodies as those described above, & an electric current be sent, by a helix, round the substance & the ray; then again it is rotated: One way with the current in one direction & the other way with the current in the other direction. It so happens, that, with our conventional understanding of the direction in which an electric current is passing, i.e. from the zinc across the acid to the platinum or copper, - that the rotation is in the same direction as the current; (using Biots mode of expressing the rotation.)

The air, gases, & vapours do not, as yet, shew this property. Neither do those crystalline bodies which have double refraction: - all other transparent solid bodies & all fluid bodies do. Those fluids which rotate per se. have the magneto or electric rotation added to, or taken from, their own[.]

But the change in bodies which is induced by the Magnetic or Electric forces, I can trace by other properties than those on light, into bodies crystalline, opaque, metallic even, & gaseous. But of this say no word at present. The paper on light &c was read at the R.S last Thursday⁴, I believe: Another or two on the new matter will go in as soon as I can find the mere time to make the multitude of limiting experiments which are needful & write the paper⁵[.]

I am sure that when you have this discovery you will understand and appreciate it.

Ever My dear Sir | Yours faithfully | M. Faraday

Rev. Dr. Whewell | &c &c