book I

have referred to the production of slaty cleavage by pressure; and at a future page I hope to show that the lamination of the ice of glaciers is due to the same cause; but, as justly observed by Mr. John Ball, there is no tendency to cleave in the _sound_ ice of glaciers; in fact, this tendency is obliterated by the perfect regelation of the severed surfaces.

[Sidenote: RECENT EXPERIMENTS OF FARADAY.]

Mr. Faraday has recently placed pieces of ice, in water, under the strain of forces tending to pull them apart. When two such pieces touch at a single point they adhere and move together as a rigid piece; but a little lateral force carefully applied breaks up this union with a crackling noise, and a new adhesion occurs which holds the pieces together in opposition to the force which tends to divide them. Mr. James Thomson had referred regelation to the cold produced by the liquefaction of the pressed ice; but in the above experiment all pressure is not only taken away, but is replaced by tension. Mr. Thomson also conceives that, when pieces of ice are simply placed together without intentional pressure, the capillary attraction brings the pressure of the atmosphere into play; but Mr. Faraday finds that regelation takes place _in vacuo_. A true viscidity on the part of ice Mr. Faraday never has observed, and he considers that his recent experiments support the view originally propounded by himself, namely, that a particle of water on a surface of ice becomes solid when placed between two surfaces, because of the increased influence due to their joint action.

CRYSTALLIZATION AND INTERNAL LIQUEFACTION.

(24.)

[Sidenote: HOW CRYSTALS ARE "NURSED."]

In the Introduction to this