Page:Popular Science Monthly Volume 5.djvu/421

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405
THE PHYSICS OF ICE.

aggregate in six-rayed figures, but in endless diversity of patterns. Captain Scoresby figured 96 of these, beautiful illustrations of which are shown in the plate. And Nature is profuse of these "frozen flowers." On mountains, and amid solitudes of the North unseen by man, she scatters them as she does those which waste their perfume in the desert. The delicate lace-like figures which follow the touch of frost on the window-pane are of the stellate type, but, being modified by disturbing influences, develop into gorgeous patterns.

Ice, when decayed, is weakened and becomes soft throughout, scarcely more compacted than snow. By rains and mild weather of spring, ice on our Northern lakes is thus impaired. In this condition it is said to be "honey-combed;" and, while yet many inches in thickness, and apparently solid, is unsafe to travel over. The foot of a horse will pass through it, displacing merely the portions beneath, and without fracture of the surrounding parts. This arises from the prismatic structure already noticed; and it is along the lines of adhesion of the prisms that the ice first yields to the invasion of heat. When thus weakened, it will sometimes disappear from the surface of a lake by a few hours of heavy storm, or, if any portion remain, it will be in the form of crystals, thoroughly permeated by water. So rapidly has it vanished in many instances from lakes, that its sinking was insisted on, but it is now known that it disintegrates and disappears by internal liquefaction.

A most interesting and important property of ice remains to be noticed. We refer to that by which it may be moulded into almost any form by pressure. Cubes of solid ice have been pressed into balls, cups, rings, and other shapes, showing its extraordinary plasticity. At a temperature of 32°, ice is by no means a rigid substance, but readily yields to pressure. Placed in the cavity of a mould (Fig. 6), it is broken into innumerable fragments as pressure is applied. It has

Fig. 6.
PSM V05 D421 Mould and ice ball formed by compression.jpg
Mould and Ball of Ice formed by Compression.

been shown, however, that, unless the crushing be sudden, the ice is not reduced to a granular or powdery mass, but maintains its cohesion while it undergoes change of form. During the process a portion of the ice becomes liquefied, and the water escapes, carrying with it the heat liberated in the liquefaction, but the portions remaining are moist, and at each point of contact directly adhere together by freezing of the moisture. This refreezing takes place throughout the mass,