Fragments of science for unscientific people : a series of detached essays, lectures, and reviews / by John Tyndall.

Date:: 1871

Credit: Fragments of science for unscientific people : a series of detached essays, lectures, and reviews / by John Tyndall. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

432/492 (page 420)

$merit was made by my friend Dr. Percy, without any reference to the question of cleavage. Break a piece of ordinary iron and you have a granular fracture ; beat the iron, you elongate these granules, and finally render the mass fibrous. Here are pieces of rails along which the wheels of locomotives have slidden; the granules have yielded and become plates. They exfoliate or come off in leaves; all these effects belong, I believe, to the great class of phenomena of which slaty cleavage forms the most prominent example.1 [I would now lay more stress on the lateral yielding, referred to in the note at the bottom of page 418, accompanied as it is by tangential sliding, than I was prepared to do when this lecture was given. This sliding is, I think, the principal cause of the planes of weakness, both in pressed wax and slate rock. J. T. 1871.] 1 For some further observations on this subject by Mr. Sorby and myself, see Philosophical Magazine for August 1856.$