From John Frederick William Herschel   July 27 1856

Collingwood | July 27 1856

Dear Sir

Let me thank you for several instances of obliging attention in forwarding me copies of your scientific papers, on Diamagnetism, and quite recently of your lecture on the Slaty cleavage,¹ which appears to me a singularly happy explanation of that odd Phaenomenon. I may mention an illustration of the same nature with some others you have introduced, which occurs in the manufacture of the Coin of The Realm.² One part of the process consists in ‘Ringing’³ each piece of Gold or Silver Coin (after receiving the impression) every piece which will not ring on dashing it against a stone is rejected and re-melted, and such are numerous. Now the cause of this can be no other than minute air-blobs in the Bars out of which the lamina of metal are rolled by a flattening mill – each blob being flattened out into an infinitesimally thin film of air parallel to the face of the coin and so producing a solution of continuity and of course destroying the harmonic vibrations. –

There are one or two points suggested by your views of the slaty cleavage which harmonise so entirely with certain ideas I have long held about geological stratification in general that I cannot forbear mentioning them. I refer to the contortions of strata which are usually accounted for (as Poulett Scrope⁴ has done) by the Protrusive violence of igneous rocks crushing together and doubling up the strata. To me it has always seemed that this feature might be more tranquilly produced thus⁵

Suppose A the ocean’s surface and B B B its bed forming a basin having irregularities C C in it. Let sediment be deposited over the whole – uniformly – till the irregularities are filled in and the bottom is reduced to a generally speaking basin-like depression.

Then up to a certain point the friction of the bed will retain the deposited mud on its slope – and in this way strata nearly of uniform thickness (and parallel) will be formed.

But as the thickness of the whole increases – the weight will overcome the friction and the still soft strata will yield inwards from all sides toward the central portion of the bed – increasing the thickness of the strata there, and at the same time crumpling up the strata into contortions &c.

The whole affair may go on with any degree of slowness. Now as this process goes on the middle thickness will increase – and that by a general lateral compression and vertical dilatation of the whole central mass. And here we have the condition which your theory requires for the Slaty cleavage which (be it observed) does not absolutely require violent force (for mere force cannot produce any rearrangement of molecules) – but only intermolecular movement in a given fixed direction. Thus we see the blobs in slow moving lava currents drawn out in length and thus we see liquids in which excessively fine spicular crystals are floating, assume a silky appearance when stirred or agitated (I have a most beautiful case of the kind under my eye at this moment) by the tendency of the particles to arrange themselves when in motion all in one direction according to the laws not of pressure but of friction a distinction which is quite necessary to be borne in mind. Infinitesimal films of mica disseminated through amorphous mud would afford a precise parallel to this case.

Your account of the fibrous structure in tough Iron is quite beautiful and perfectly accounts for the exceeding toughness and tenacity of bar Iron formed by welding together smaller bars &c. – In fact such a bar is a rope of Iron wire held together as the strands of a hempen rope are by lateral friction & each portion each potion being per se of trivial length compared with the whole.

I am sure you will excuse me for writing to you in this desultory manner which perhaps I am not entitled to do but the fact is that since reading a great many years ago Wells’s little tract on dew⁶ I have not met with anything which seems to bring so many loose-lying phœnomena under a general and very simple principle.

I remain | D[ea]^r Sir | Yours truly | JFW Herschel

J. Tyndall, FRS

RI MS JT/1/H/92⁷