John Tyndall to Faraday   24 October 1850

Marburg | Hesse Cassel | Oct. 24th. 1850

Dear Sir.

A short time after I had the pleasure of seeing you last June I took the liberty of sending you a few specimens of calcareous spar which from their appearance I judged to be magnetic¹. I had at the time no means of proving whether they were so or not. Since my arrival in Marburg I have tested specimens of the same spar - the chemical analysis proves the absence of iron and the magnet shews them to be diamagnetic. The wish to furnish you with the means of observing the complementary action of the magnetic and diamagnetic specimens of the crystal being thus far disappointed I have great pleasure in now sending you a sample of the proper kind - a small rhomboid of magnetic spar. You will find that the optic axis of the rhomboid will set from pole to pole, in contra distinction to the diamagnetic crystals of the same form which as you are aware set their axes equatorial.

These effects appear to be capable of the fullest explanation by reference to a principle which you were the first to hint at, that is to say “the action of contiguous particles”. Wheat flour for instance is pretty strongly diamagnetic. If we take a little ball of dough made from the flour and squeeze it flat the plate thus formed will when suspended vertically in the magnetic field set its shortest dimension equatorial, thus behaving like a magnetic body. Now this is evidently due to the peculiar arrangement of the diamagnetic particles, and if we imagine a similarly formed magnetic mass suspended between the poles it is reasonable to suppose that the arrangement which in the former case caused the repulsion of the line of greatest compression would now cause its attraction. This conjecture is verified, for if instead of flour we use in the composition of our dough a precipitate of oxide of iron, the squeezed plate formed from the latter will sets its shortest dimension axial. This action appears to be strictly analogous to that exhibited by the spar. In the diamagnetic specimen the shortest dimension stands equatorial; in the magnetic specimen the same dimension stands axial. The molecules of both crystals are similarly arranged, but in the one case we have to deal with magnetic molecules and the other with diamagnetic.

I remain dear Sir | Most truly and respectfully yours | John Tyndall.

Dr. Faraday. | etc. etc.

Endorsed: Transmitted through Francis Don’t crush!